TGS Revision Year 10 Chemistry

Revsion for end of year exams for TGS students doing Chemistry

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  • Created on: 19-05-13 13:54

Solids, liquids and gases

a) Matter can be classified in terms of the three states of matter, which are inter-convertible.

Candidates should be familiar with the states of matter and be able to name each inter-conversion  process. They should be able to describe and explain their inter-conversion in terms of how the particles are arranged and their movement.  Candidates should understand the energy changes that accompany changes of state. 

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Atoms

a) All substances are made of atoms. A substance that is made of only one sort of atom is called an  element. There are about 100 different elements. Elements are shown in the periodic table. The  groups contain elements with similar properties.

Candidates should know where metals and non-metals appear in the periodic table. 

b) Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen,  e.g  Na represents an atom of sodium

c) Atoms have a small central nucleus, which is made up of protons and neutrons, and around which  there are electrons. 

d) The relative electrical charges are as shown: 

Name of particle Charge 
Proton +1 
Neutron 0 
Electron –1 

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Atoms

e) In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no  overall electrical charge. 

f) The number of protons in an atom of an element is its atomic number. The sum of the protons and  neutrons in an atom is its mass number.

Candidates will be expected to calculate the numbers of each sub-atomic particle in an atom from its  atomic number and mass number. 

g) All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

h) Atoms of the same element can have different numbers of neutrons; these atoms are called  isotopes of that element. 

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Atoms

i) Atoms can be represented as shown in this example: 

(Mass number) 23 

 Na 

(Atomic number) 11 

j) Electrons occupy particular energy levels. Each electron in an atom is at a particular energy level  (in a particular shell). The electrons in an atom occupy the lowest available energy levels (innermost available shells). 

k) The relative masses of protons, neutrons and electrons are: 

Name of particle Mass 
Proton 1 
Neutron 1 
Electron Very small 

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Atoms

l) The relative atomic mass of an element (Ar) compares the mass of atoms of the element with the  12C isotope. It is an average value for the isotopes of the element.

Candidates will not be expected to calculate relative atomic masses from isotopic abundances. 


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Chemical reactions and related calculations

a) When elements react, their atoms join with other atoms to form compounds. This involves giving, taking or sharing electrons to form ions or molecules to attain the electron arrangement of the nearest noble gas. 

b) The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

c) The relative formula mass of a substance, in grams, is known as one mole of that substance. 

Candidates are expected to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa.

d) The percentage by mass of an element in a compound can be calculated from the relative atomic  mass of the element in the formula and the relative formula mass of the compound. 

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Chemical reactions and related calculations

e) The empirical formula of a compound can be calculated from the masses or percentages of the elements in a compound.

Candidates should be able to calculate empirical formulae.

f) Chemical reactions can be represented by word equations or by symbol equations.

Candidates should be able to write word and balanced symbol equations for reactions in the specification. 

g) Information about the states of reactants and products can be included in chemical equations. 

Candidates should be able to use the state symbols (g), (l), (s) and (aq) in equations where appropriate.

h) No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants. 

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Chemistry reactions and related calculations

i) The masses of reactants and products can be calculated from balanced symbol equations. 

Candidates should be able to calculate the mass of a reactant or product from information about the masses of the other reactants and products in the reaction and the balanced symbol equation.

j) Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:

  • the reaction may not go to completion because it is reversible 
  • some of the product may be lost when it is separated from the reaction mixture 
  • some of the reactants may react in ways different from the expected reaction. 

k) The amount of a product obtained is known as the yield. When compared with the maximum  theoretical amount as a percentage, it is called the percentage yield. 

Candidates will be expected to calculate percentage yields of reaction products.

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Chemical reactions and related calculations

l) In some chemical reactions, the products of the reaction can react to produce the original reactants. 

Such reactions are called reversible reactions and are represented: 

A + B (http://kerala.skoool.in/uploadedImages/reversible_symbol.gif) C + D 

For example: 

ammonium chloride (http://kerala.skoool.in/uploadedImages/reversible_symbol.gif) ammonia + hydrogen chloride 

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Bonding

a) Compounds are substances in which atoms of two or more elements are chemically combined. 

b) Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels (shells) of atoms in order to achieve the electron arrangement of a noble gas. 

c) When atoms form chemical bonds by transferring electrons, they form ions. Atoms that lose electrons become positively charged ions. Atoms that gain electrons become negatively charged ions. Ions have the electron arrangement of a noble gas (Group 0). Compounds formed from metals and non-metals consist of ions. 

Candidates should know that metals form positive ions, whereas non-metals form negative ions.

d) The elements in Group 1 of the periodic table, the alkali metals, all react with non-metal elements to form ionic compounds in which the metal ion has a single positive charge.

Knowledge of the chemical properties of alkali metals is limited to their reactions with non-metal elements and water. 

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Bonding

e) The elements in Group 7 of the periodic table, the halogens, all react with metals to form ionic compounds in which the halide ions have a single negative charge.

f) An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding. Sodium Chloride

g) When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong. Some covalently bonded substances, such as H2, Cl2, O2, HCl, H2O, NH3 and CH4, consist of simple molecules. Others, such as diamond and silicon dioxide, have giant covalent structures (macromolecules). 

h) Compounds formed from non-metals consist of molecules. In molecules, the atoms are held together by covalent bonds. 

Candidates should be able to represent the covalent bonds in molecules such as water, ammonia, hydrogen, hydrogen chloride, methane and oxygen.Candidates should be able to recognise other simple molecules and giant structures from diagrams that show their bonding

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Structure and how it influences the properties and

a) Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces of attraction in all directions between oppositely charged ions. 

These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds. 

b) When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current. 

Knowledge of the structures of sodium chloride is not required.

c) Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points. 

d) Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils. Intermolecular forces are weak compared with covalent bonds.

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Structure and how it influences the properties and

e) Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge. 

f) Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points. 

Candidates should be able to recognise other giant structures or macromolecules from diagrams showing their bonding. 

g) In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard. 

h) In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other because there are no covalent bonds between the layers and so graphite is soft and slippery. Candidates should be able to explain the properties of graphite in terms of weak forces between the layers. 

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Structure and how it influences the properties and

i) In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.

Candidates should realise that graphite is similar to metals in that it has delocalised electrons. 

j) Carbon can also form fullerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, eg in tennis rackets.

Candidates are only required to know that the structure of fullerenes is based on hexagonal rings of carbon atoms. 

k) Nanoscience refers to structures that are 1–100 nm in size, of the order of a few hundred atoms. Nanoparticles show properties different from those for the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors, stronger and lighter construction materials, and new cosmetics such as suntan creams and deodorants. 

Questions may be set on information that is provided about these materials and their uses.

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The periodic table

a) The periodic table is arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups. The table is called a periodic table because similar properties occur at regular intervals. 

b) Elements in the same group in the periodic table have the same number of electrons in their highest energy level (outer electrons) and this gives them similar chemical properties. 

c) The elements in Group 0 of the periodic table are called the noble gases. They are unreactive because their atoms have stable arrangements of electrons. 

Candidates should know that the noble gases have eight electrons in their outer energy level, except for helium, which has only two electrons. 

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Trends within the periodic table

a) The elements in Group 1 of the periodic table (known as the alkali metals): 

  • are metals with low density (the first three elements in the group are less dense than water) 
  •  react with non-metals to form ionic compounds in which the metal ion carries a charge of +1. 
  • The compounds are white solids that dissolve in water to form colourless solutions 
  • react with water, releasing hydrogen 
  • form hydroxides that dissolve in water to give alkaline solutions. 

b) In Group 1, the further down the group an element is, the more reactive the element. 

c) Compared with the elements in Group 1, transition elements: 

  • have higher melting points (except for mercury) and higher densities 
  • are stronger and harder 
  • are much less reactive and so do not react as vigorously with water or oxygen. 
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Trends within the periodic table

d) Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts. 

e) The elements in Group 7 of the periodic table (known as the halogens) react with metals to form ionic compounds in which the halide ion carries a charge of –1. 

f) In Group 7, the further down the group an element is: 

  • the less reactive the element 
  • the higher its melting point and boiling point. 

g) A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. 

h) The trends in reactivity within groups in the periodic table can be explained because the higher the energy level of the outer electrons: 

  • the more easily electrons are lost 
  • the less easily electrons are gained
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The reactivity series

a) Metals can be arranged in an order of their reactivity from their reactions with water and dilute acids. 

Candidates should be able to recall and describe the reactions, if any, of potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper with water or dilute acids, where appropriate, to place them in order of reactivity. 

b) Displacement reactions involving metals and their compounds in aqueous solution establish positions within the reactivity series. 

Candidates should be able to describe displacement reactions in terms of oxidation and reduction, and to write the ionic equations. Candidates should be aware that copper can be obtained from solutions of copper salts by displacement using scrap iron. 

c) The non-metals hydrogen and carbon are often included in the reactivity series based on the reactions of metals with dilute acid, and of metal oxides with carbon. 

Candidates should know that a lighted spill can be used to test for hydrogen.

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Extracting metals

a) Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal. 

b) Metals that are less reactive than carbon can be extracted from their oxides by reduction with carbon: for example, iron oxide is reduced in the blast furnace to make iron. 

Details of the blast furnace are not required, but candidates should know the raw materials used and explain the simple chemistry involved, including the use of equations. 

Knowledge of the details of the extraction of other metals is not required. Examination questions may provide information about specific processes for candidates to interpret or evaluate. 

c) Metals that are more reactive than carbon, such as aluminium, are extracted by electrolysis of molten compounds. The use of large amounts of energy in the extraction of these metals makes them expensive.

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Extracting metals

d) Copper can be extracted from copper-rich ores by heating the ores in a furnace (smelting). The copper can be purified by electrolysis. The supply of copper-rich ores is limited and is being depleted. Traditional mining and extraction have major environmental impacts. smelting processes 

e) New ways of extracting copper from low-grade ores are being researched to limit the environmental impact of traditional mining. Copper can be extracted by phytomining, or by bioleaching. 

Candidates should know and understand that: 

  • phytomining uses plants to absorb metal compounds and that the plants are burned to produce ash that contains the metal compounds 
  • bioleaching uses bacteria to produce leachate solutions that contain metal compounds. 

f) Copper can be obtained from solutions of copper salts by electrolysis. 

Candidates should know the electrode material and be able to write the ionic half equations for the reactions occurring at both electrodes.

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Extracting metals

g) Copper can be obtained from solutions of copper salts by displacement using scrap iron. 

Candidates should be able to describe this in terms of oxidation and reduction, and to write the ionic equation. 

h) We should recycle metals because extracting them uses limited resources, and is expensive in terms of energy and in terms of effects on the environment. 

Candidates are not required to know details of specific examples of recycling, but should understand the benefits of recycling in the general terms specified here.

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Structure and bonding in metals and alloys

a) Metals consist of giant structures of atoms arranged in a regular pattern. 

b) The electrons in the highest occupied energy levels (outer shell) of metal atoms are delocalised and so free to move through the whole structure. This corresponds to a structure of positive ions with electrons between the ions holding them together by strong electrostatic attractions. 

c) Metals conduct heat and electricity because of the delocalised electrons in their structures. 

Candidates should know that conduction depends on the ability of electrons to move throughout the metal. 

d) The layers of atoms in metals are able to slide over each other. This means metals can be bent and shaped. 

e) Alloys are usually made from two or more metals. The different sizes of atoms in the metals distort the layers in the structure, making it more difficult for them to slide over each other. This makes alloys harder than pure metals. 

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Structure and bonding in metals and alloys

f) Most metals in everyday use are alloys. Pure copper, gold, iron and aluminium are too soft for many uses and so are mixed with small amounts of other metals to make them harder for everyday use. 

Candidates should be familiar with these specified examples but examination questions may contain information about alloys other than those named in the subject content to enable candidates to make comparisons.

g) Shape memory alloys can return to their original shape after being deformed. An example is Nitinol, which is used in dental braces. 

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Properties and uses of metals

a) The elements in the central block of the periodic table are known as transition metals. Like other metals, they are good conductors of heat and electricity and can be bent or hammered into shape. They are useful as structural materials and for making things that must allow heat or electricity to pass through them easily. 

Knowledge of the properties of specific transition metals other than those named in this specification is not required.

b) Iron from the blast furnace contains about 96% iron. The impurities make it brittle and so it has limited uses. 

c) Most iron is converted into steels. Steels are alloys since they are mixtures of iron with carbon. Some steels contain other metals. Steels can be designed to have properties for specific uses. Low-carbon steels are easily shaped, high-carbon steels are hard, and stainless steels are resistant to corrosion.

  •  

 

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Properties and uses of metals

d) Copper has properties that make it useful for electrical wiring and plumbing. 

Candidates should know and understand that copper: 

  • is a good conductor of electricity and heat can be bent
  • but is hard enough to be used to make pipes or tanks 
  • does not react with water.
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Rates of reaction

a) The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed over time:

 Rate of reaction = amount of reactant used
                                          time 

 Rate of reaction = amount of product formed
                                           time 

Candidates need to be able to interpret graphs showing the amount of product formed (or reactant used up) with time, in terms of the rate of the reaction.

b) Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.

c) Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of reaction.

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Rates of reaction

e) Increasing the concentration of reactants in solutions increases the frequency of collisions and so increases the rate of reaction.

f) Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

g) Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts.

h) Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce costs.

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Crude oil

a) Crude oil is a mixture of a very large number of compounds. 

b) Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only. 

c) The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by evaporating the oil and allowing it to condense at a number of different temperatures. This process is called fractional distillation. 

Candidates should know and understand the main processes in continuous fractional distillation in a fractionating column. 

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Hydrocarbons

a) Most of the hydrocarbons in crude oil are saturated hydrocarbons called alkanes. The general formula for the homologous series of alkanes is CnH2n+2. 

Candidates should know that in saturated hydrocarbons all the carbon–carbon bonds are single covalent bonds. 

Candidates should be able to recognise alkanes from their formulae in any of the forms, but do not need to know the names of specific alkanes other than methane, ethane and propane. 

c) Some properties of hydrocarbons depend on the size of their molecules. These properties influence how hydrocarbons are used as fuels. 

Knowledge of trends in properties of hydrocarbons is limited to: 

  • boiling points 
  • viscosity 
  • flammability.
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Fuels

a) Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur. The gases released into the atmosphere when a fuel burns may include carbon dioxide, water (vapour), carbon monoxide, sulfur dioxide and oxides of nitrogen. Solid particles (particulates) may also be released. 

Candidates should be able to relate products of combustion to the elements present in compounds in the fuel and to the extent of combustion (whether complete or partial). 

No details of how the oxides of nitrogen are formed are required, other than the fact that they are formed at high temperatures. Solid particles may contain soot (carbon) and unburnt fuels. 

b) The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. 

c) Sulfur dioxide and oxides of nitrogen cause acid rain, an increase in carbon dioxide results in climate change, and solid particles cause global dimming. 

Candidates should know at least one effect of, but are not required to know details of any other causes of, acid rain or climate change. 

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Fuels

d) Sulfur can be removed from fuels before they are burned, eg in vehicles. Sulfur dioxide can be removed from the waste gases after combustion, eg in power stations. 

e) Biofuels, including biodiesel and ethanol, are produced from plant material, and are possible alternatives to hydrocarbon fuels. 

Candidates should know and understand the benefits and disadvantages of biofuels in terms of: 

  • use of renewable resources 
  • their impacts on land use 
  • their carbon footprint. 

Candidates should know that ethanol for use as a biofuel is produced from a dilute solution of ethanol obtained by the fermentation of plant materials at a temperature between 20 °C and 35 °C. 

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Fuels

f) Hydrogen can be burned as a fuel in combustion engines or can be used in fuel cells that produce electricity to power vehicles. 

Candidates should be able to compare the advantages and disadvantages of the combustion of hydrogen with the use of hydrogen fuel cells from information that is provided. Candidates should know and understand the benefits and disadvantages of hydrogen fuel in terms of: 

  • storage and use 
  • products of combustion. 
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Obtaining useful substances from crude oil

a) Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules. This process involves heating the hydrocarbons to vaporise them. The vapours are either passed over a hot catalyst or mixed with steam and heated to a very high temperature so that thermal decomposition reactions then occur. 

b) The products of cracking include alkanes and unsaturated hydrocarbons called alkenes. The general formula for the homologous series of alkenes is CnH2n.

Candidates should know that in unsaturated hydrocarbons some of the carbon–carbon bonds are double covalent bonds. 

Candidates should be able to recognise alkenes from their names or formulae, but do not need to know the names of individual alkenes other than ethene and propene.

d) Alkenes react with bromine water, turning it from orange to colourless. 

e) Some of the products of cracking are useful as fuels. 

f) Ethanol can be produced by reacting ethene with steam in the presence of a catalyst.

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Polymers

a) Alkenes can be used to make polymers such as poly(ethene) and poly(propene). In polymerisation reactions, many small molecules (monomers) join together to form very large molecules (polymers). 

For example: 

(http://www.scienceaid.co.uk/chemistry/organic/images/polymerisation.jpg)

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Polymers

b) The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low-density (LD) and high-density (HD) poly(ethene) are produced using different catalysts and reaction conditions. 

c) Thermosoftening polymers consist of individual, tangled polymer chains. Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when they are heated. 

d) Polymers have many useful applications and new uses are being developed. Examples include:  new packaging materials, waterproof coatings for fabrics, dental polymers, wound dressings,  hydrogels, and smart materials (including shape memory polymers). 

Candidates should consider the ways in which new materials are being developed and used, but will not need to recall the names of specific examples.

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Polymers

e) Many polymers are not biodegradable, ie they are not broken down by microbes. This can lead to problems with waste disposal. 

Knowledge of specific named examples is not required, but candidates should be aware of the problems that are caused in landfill sites and in litter. 

f) Plastic bags are being made from polymers and cornstarch so that they break down more easily. 

Biodegradable plastics made from cornstarch have been developed. 

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Alcohols

a) Alcohols contain the functional group –OH. Methanol, ethanol and propanol are the first three members of a homologous series of alcohols.

Candidates should be able to recognise alcohols from their names or formulae, but do not need to know the names of individual alcohols other than methanol, ethanol and propanol. 

b) Methanol, ethanol and propanol: 

  • dissolve in water to form a neutral solution 
  • react with sodium to produce hydrogen 
  • burn in air 
  • are used as fuels and solvents, and ethanol is the main alcohol in alcoholic drinks. 

c) Ethanol can be oxidised to ethanoic acid, either by chemical oxidising agents or by microbial action. Ethanoic acid is the main acid in vinegar. 

Candidates should be aware that vinegar is an aqueous solution that contains ethanoic acid.

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Carboxylic acids

a) Ethanoic acid is a member of the homologous series of carboxylic acids, which have the functional group –COOH.

Candidates should be able to recognise carboxylic acids from their names or formulae, but do not need to know the names of individual carboxylic acids other than methanoic acid, ethanoic acid and propanoic acid.

b) Carboxylic acids: 

  • dissolve in water to produce acidic solutions 
  • react with carbonates to produce carbon dioxide 
  • react with alcohols in the presence of an acid catalyst to produce esters 
  • do not ionise completely when dissolved in water and so are weak acids 
  • aqueous solutions of weak acids have a higher pH value than aqueous solutions of strong acids with the same concentration. 

Candidates are expected to write balanced chemical equations for the reactions of carboxylic acids.

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Esters

a) Ethyl ethanoate is the ester produced from ethanol and ethanoic acid. Esters have the functional group –COO–.

Candidates will not be expected to give the names of esters other than ethyl ethanoate, but should be able to recognise a compound as an ester from its name or its structural formula.

b) Esters are volatile compounds with distinctive smells and are used as flavourings and perfumes. 

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Exothermic and endothermic reactions

a) When chemical reactions occur, energy is transferred to or from the surroundings.

Knowledge of delta H (ΔH) conventions and enthalpy changes, including the use of positive values for endothermic reactions and negative values for exothermic reactions, is required.

b) An exothermic reaction is one that transfers energy to the surroundings. Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation. Everyday uses of exothermic reactions include self-heating cans (eg for coffee) and hand warmers.

c) An endothermic reaction is one that takes in energy from the surroundings. Endothermic reactions 

include thermal decompositions. Some sports injury packs are based upon endothermic reactions.

d) If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case. 

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Calculating and explaining energy changes

a) The relative amounts of energy released when substances burn can be measured by simple calorimetry, eg by heating water in a glass or metal container. This method can be used to compare the amount of energy produced by fuels. 

Candidates should be able to calculate and compare the amount of energy released by different fuels given the equation: 

Q = mc  T

b) Energy is normally measured in joules (J). For comparison purposes, energy values could be given in kJ or calories for a given mass or amount of substance, eg calories per gram, kJ per mole or kJ per gram. 

If candidates are required to convert from calories to joules, the conversion factor will be given in the question.

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Calculating and explaining energy changes

c) The amount of energy produced by a chemical reaction in solution can be calculated from the measured temperature change of the solution when the reagents are mixed in an insulated container. This method can be used for reactions of solids with water or for neutralisation reactions.

d) Simple energy level diagrams can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction. 

Candidates will be expected to understand simple energy level diagrams showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved arrow to show the energy as the reaction proceeds. Candidates should be able to relate these to exothermic and endothermic reactions.


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Calculating and explaining energy changes

e) During a chemical reaction: energy must be supplied to break bonds  energy is released when bonds are formed. Candidates should be able to calculate the energy transferred in reactions and interpret simple energy level diagrams in terms of bond breaking and bond formation (including the idea of activation energy and the effect on this of catalysts). 

f) In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. 

g) In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

h) Catalysts provide a different pathway for a chemical reaction that has a lower activation energy. Energy Diagrams

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Electrolysis

a) When an ionic substance is melted or dissolved in water, the ions are free to move about within the liquid or solution.

b) Passing an electric current through ionic substances that are molten, eg lead bromide, or in solution breaks them down into elements. This process is called electrolysis and the substance broken down is called the electrolyte.

c) During electrolysis, positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode).

d) Oxidation and reduction can be defined as the loss and gain of electrons respectively.

e) At the cathode, positively charged ions gain electrons; at the anode, negatively charged ions lose electrons.

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Electrolysis

f) Reactions at electrodes can be represented by half equations, for example:

2Cl– → Cl2 + 2e–
or 
2Cl– – 2e– → Cl2 

Candidates should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations. 

g) If there is a mixture of ions: 

  • at the cathode, the products formed depend on the reactivity of the elements involved 
  • at the anode, the products formed also depend on the relative concentrations of the ions present. 

h) Electrolysis is used to electroplate objects. This may be for reasons such as appearance, durability and prevention of corrosion. It includes copper plating and silver plating

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Electrolysis

i) Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite. Aluminium forms at the negative electrode and oxygen at the positive electrode. The positive electrode is made of carbon, which reacts with the oxygen to produce carbon dioxide.

Candidates should understand why cryolite is used in this process. 

Candidates should be aware that large amounts of energy are needed in the extraction process.

j) The electrolysis of sodium chloride solution produces hydrogen and chlorine. Sodium hydroxide solution is also produced. These are important reagents for the chemical industry, eg sodium hydroxide for the production of soap and chlorine for the production of bleach and plastics.

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Analysing substances

a) Flame tests can be used to identify metal ions. Lithium, sodium, potassium, calcium and barium compounds produce distinctive colours in flame tests: 

  • lithium compounds result in a crimson flame 
  • sodium compounds result in a yellow flame 
  • potassium compounds result in a lilac flame 
  • calcium compounds result in a red flame 
  • barium compounds result in a green flame. 

Flame colours of other metal ions are not required knowledge. 

b) Aluminium, calcium and magnesium ions form white precipitates with sodium hydroxide solution but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution. 

c) Copper(II), iron(II) and iron(III) ions form coloured precipitates with sodium hydroxide solution. Copper(II) forms a blue precipitate, iron(II) a green precipitate and iron(III) a brown precipitate.

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Comments

Shami Bhatti

Maybe lacking a bit on detail in some places but a good resource.

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