atoms, compounds and isotopes
- compounds are formed when atoms of two or more elements are chemically bonded together. e.g carbon dioxide is a compound formed when there is a chemical reaction between carbon and oxygen.
- its difficult two separate the two elements back out again.
ISOTOPES ARE DIFFERENT ATOMIC FORMS OF THE SAME ELEMENT, WHICH HAVE THE SAME NUMBER OF PROTONS BUT A DIFFERENT NUMBER OF NEUTRONS.
- isotopes have the same atomic number but different mass numbers.
- if they had different atomic numbers they would be different elements.
- all the atoms on the left hand side of the periodic table have just one or two electrons in their outer shell. They are keen to get rid of the electron so that the atom can have full shells. this leaves the atom as a ion and because of this they get attracted to an ion with a opposite charge.
- atoms on the right hand side of the periodic table have nearly full outer shells, they want to gain one or two electrons so they can have a full outer shell. so they become ions and latch on to another ion that will give up its electron.
- ionic compounds have giant ionic lattices, they form a closely packed regular lattice arrangement. they are very strong electrostatic forces of attraction between oppositely charged ions.
- ionic compounds all have similar properties:
- they have high melting points and high boiling points due to the strong attraction between the ions. it takes a large amount of energy to overcome this attraction, when the ionic compounds melt the ions are free to move and they carry an electrical current.
-they dissolve easily in water, the ions separate and are free to move in the solution, they carry an electrical current.
-A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the outer shells of the atoms. An atom that shares one or more of its electrons will complete its outer shell.
- Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.
- Atoms may form multiple covalent bonds - they share not just one pair of electrons but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.
Covalently bonded substances fall into two main types:
- simple molecules
- giant covalent structures
- Simple molecules contain only a few atoms held together by covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.
- However, although the covalent bonds holding the atoms together in a simple molecule are strong, the intermolecular forces between simple molecules are weak.
properties of single molecular substances:
- Low melting and boiling points - this is because little energy is needed to break the weak intermolecular forces.
Do not conduct electricity - this is because they do not have any free electrons or an overall electric charge.
When one of these substances melts or boils, it is these weak intermolecular forces that break, not the strong covalent bonds. At room temperature, simple molecular substances are gases, or liquids or solids with low melting and boiling points.
giant covalent structures:
- Giant covalent structures contain very many atoms, each joined to adjacent atoms by covalent bonds. The atoms are usually arranged into giant regular lattices - extremely strong structures because of the many bonds involved.
- Very high melting points – this is because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600°C.
- Variable electrical conductivity - diamond does not conduct electricity, whereas graphite contains free electrons so it does conduct electricity. Silicon is a semi-conductor – it is midway between non-conductive and conductive.
- Graphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond.
- It is used in pencils, and as a lubricant. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.
- Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure.
- diamond is very hard and has a high melting point. This explains why it is used in cutting tools. It does not conduct electricity.
- Silica (or silicon dioxide), which is found in sand, has a similar structure to diamond, so its properties are similar to diamond. It is hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms.
- metals also consist of a giant structure. metallic bonds involve free electrons which produce all the properties of metals. the delocalised (free) electrons come from the outer shell of every metal atom in the structure.
- these electrons are free to move through the whole structure and so metals are good conductors of heat and electricity. these electrons hold the atoms together in a regular structure,
- there are strong electrostatic attraction between the positive metal ions and the negative electrons. they allow the layers of atoms to slide over each other allowing metals to be bent and shaped.
alloys are harder than pure metals:
- pure metal are not always right for the job. so you mix them with other metals creating an alloy with the properties they want.
- different elements have different sized atoms, when a metal is mixed with a pure metal, the new metal atoms will mess with the other metal atoms. this makes it More difficult for them to slide over each other so the alloys are harder.
- nano-particles contain a few hundred atoms. they contain fullerene, these are molecules
- Nanoparticles have a very large surface area compared with their volume. So they are often able to react very quickly. This makes them useful as catalysts to speed up reactions. For example, they can be used in self-cleaning ovens and windows.
- Nanoparticles also have different properties to the same substance in normal-sized pieces. For example, titanium dioxide is a white solid used in house paint and certain sweet-coated chocolates.
- Titanium dioxide nanoparticles are so small they do not reflect visible light, so cannot be seen. They are used in sunblock creams to block harmful ultraviolet light without appearing white on the skin.
Nanoscience may lead to the development of:
- new catalysts
- new coatings
- new computers
- stronger and lighter building materials
- sensors that detect individual substances in tiny amounts.
- Polymers have properties which depend on the chemicals they are made from, and the conditions in which they are made. For example, poly(ethene) can be low-density or high-density depending upon the catalyst and reaction condition used to make it.
- Thermosoftening polymers soften when heated and can be shaped when hot. The shape will harden when it is cooled, but can be reshaped when heated up again.
- Poly(ethene) is a thermosoftening polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material.
- Thermosetting polymers have different properties to thermosoftening polymers. Once moulded, they do not soften when heated and they cannot be reshaped.
- Vulcanised rubber is a thermoset used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily.
- the starting material and conditions will the affect the polymers properties.
chemical analysis and instrumental methods
- Paper chromatography is used to analyse coloured substances, such as the coloured pigments in plants and artificial colours used as food additives.
- Paper chromatography works because some of the coloured substances are better at dissolving in the liquid than they are at bonding with the paper, so they travel further up the paper.
-Two substances are likely to be the same if they have the same colour and they travel the same distance up the paper.
Instrumental methods of analysis:
Instrumental methods of analysis rely on machines. There are several different types of instrumental analysis. Some are suitable for detecting and identifying elements, while others are better suited to compounds. In general, instrumental methods of analysis are:
- Accurate (they reliably identify elements and compounds)
- Sensitive (they can detect very small amounts of a substance in a small amount of sample)
- Gas chromatography allows a mixture of compounds to be separated. The GC machine consists of a long glass tube packed with a powdered solid material, which is fitted into an oven. The tube is called the column, even though it is usually wound into a coil so that it fits into the oven.
- The sample is dissolved in a solvent, then injected into one end of the column.
An unreactive gas - usually nitrogen - carries the sample through the column. Different substances in the sample travel through the column at different speeds and so become separated from each other.
- The separated substances leave the column one after the other. As they leave, they are detected by a detector.
- The time taken for a substance to travel through the column is called its retention time. A detector produces a graph where each substance is represented by a peak:
- The number of peaks shows the number of compounds present in the sample
- The position of each peak shows the retention time for each compound.
- The mass spectrometer can be used to identify substances quickly and accurately, and in very small amounts. It can also provide the relative formula mass of the substances separated by gas chromatography.
- The peak furthest to the right in a mass spectrum is called the 'molecular ion peak'. Its relative mass is the relative formula mass of the substance being analysed.
rates of reaction
rates of reaction:
- The rate of a reaction can be measured by the rate at which a reactant is used up, or the rate at which a product is formed.
- The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts, are all factors which affect the rate of a reaction.
- Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.
measuring the rate of reaction:
- Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction.
measuring rate of reaction
There are two ways to measure the rate of a reaction:
- Measure the rate at which a reactant is used up
- Measure the rate at which a product is formed
- The method chosen depends on the reaction being studied. Sometimes it is easier to measure the change in the amount of a reactant that has been used up; sometimes it is easier to measure the change in the amount of product that has been produced.
The measurement itself depends on the nature of the reactant or product:
- The mass of a substance - solid, liquid or gas - is measured with a balance
- The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette
- rates of reaction = the amount of reactant used up or the amount of reation formed divided by time taken
- For example, if 24 cm3 of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12 cm3 hydrogen / min.
- In a chemical reaction, the reactant particles can only react with each other when they bump into one another. According to collision theory when molecules collide bonds between their atoms can break, and then new bonds can form to make new molecules.
- The molecules in gases and liquids are moving constantly, and millions of collisions take place every second. But only a small number of these collisions lead to the formation of product. For a collision to be 'successful', the particles involved must possess enough energy, called the activation energy, to break some of the existing bonds.
- Any change that increases the number of collisions per second, or increases the energy of the particles that are colliding, will increase the rate of reaction.
- If the temperature is increased, there will be more energy in the collisions. More collisions will have the activation energy, resulting in an increase in the rate of reaction.
- If the concentration of one or more of the reactants is increased, there will be more collisions per second, resulting in an increase in the rate of reaction.
- Exothermic reactions transfer energy to the surroundings. Endothermic reactions take in energy from the surroundings.
- Reversible reactions are where the products can react to remake the original reactants. If the forward reaction is exothermic, the reverse reaction is endothermic.
- When a chemical reaction occurs, energy is transferred to or from the surroundings - and there is often a temperature change.
- Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter.
The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:
- Combustion (burning)
- Many oxidation reactions, for example rusting
- Neutralisation reactions between acids and alkalis
- Exothermic reactions can be used for everyday purposes. For example, hand warmers and self-heating cans for drinks (such as coffee) use exothermic reactions.
- These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder.
- The temperature decrease can also be detected using a thermometer.
Some examples of endothermic reactions are:
- The reaction between ethanoic acid and sodium carbonate
- The thermal decomposition of calcium carbonate in a blast furnace
In reversible reactions, the reaction in one direction will be exothermic and the reaction in the other direction will be endothermic.
- The decomposition of ammonium chloride is a reversible reaction:
- Ammonium chloride decomposes when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens.
- The reaction between anhydrous copper sulfate and water is reversible:
- Water is driven off from hydrated copper sulfate when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens. This is easily observed. When water is added to anhydrous copper sulfate, enough heat is released to make the water bubble and boil.
acids and alkalis
- Acids have a pH of less than 7. Bases have a pH of more than 7. When bases are dissolved in water, they are known as alkalis.
- Salts are made when an acid reacts with a base, carbonate or metal. The name of the salt formed depends on the metal in the base and the acid used. For example, salts made using hydrochloric acid are called chlorides.
- Substances with a pH of less than 7 are acids. The stronger the acid, the lower the pH number. Acids turn blue litmus paper red. They turn universal indicator red if they are strong, and orange or yellow if they are weak.
- Substances that can react with acids and neutralise them to make a salt and water are called bases. They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.
Bases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. Sodium hydroxide is an alkali because it does dissolve in water.
Alkaline solutions have a pH of more than 7. The stronger the alkali, the higher the pH number. Alkalis turn red litmus paper blue. They turn universal indicator dark blue or purple if they are strong, and blue-green if they are weak.
- Neutral solutions
Neutral solutions have a pH of 7. They do not change the colour of litmus paper, but they turn universal indicator green. Water is neutral.
acids and alkalis
When acids react with bases, a salt and water are made. This reaction is called neutralisation. In general:
- acid + metal oxide → salt + water
- acid + metal hydroxide → salt + water
- Remember that most bases do not dissolve in water. But if a base can dissolve in water, it is also called an alkali.
When acids react with carbonates, such as calcium carbonate (found in chalk, limestone and marble), a salt, water and carbon dioxide are made. In general:
acid + metal carbonate → salt + water + carbon dioxide
- Notice that an extra product - carbon dioxide - is made. It causes bubbling during the reaction, and can be detected using limewater. You usually see this reaction if you study the effects of acid rain on rocks and building materials.
- Reactive metals
Acids will react with reactive metals, such as magnesium and zinc, to make a salt and hydrogen. In general:
acid + metal → salt + hydrogen
- The hydrogen causes bubbling during the reaction, and can be detected using a lighted splint. You usually see this reaction if you study the reactivity series of metals.
oxides, hydroxides and ammonia
- When an alkali is added to an acid the pH of the mixture rises. This is because the alkali reacts with the acid to form neutral products.
- The reverse situation also happens too: when an acid is added to an alkali the pH of the mixture falls. This is because the acid reacts with the alkali to form neutral products.
A reaction in which acidity or alkalinity is removed is called neutralisation. A neutralisation involving an acid and a base (or alkali) always produces salt and water.
acid + base → salt + water
Hydrogen ions and pH:
- In all solution, all acids contain hydrogen ions, H+. The greater the concentration of these hydrogen ions, the lower the pH.
- The name of the salt produced in a neutralisation reaction can be predicted. The first part of the name is ‘ammonium’ if the base used is ammonia. Otherwise, it is the name of the metal in the base.
The second part of the name comes from the acid used:
- chloride, if hydrochloric acid is used
- nitrate, if nitric acid is used
- sulfate, if sulfuric acid is used
- phosphate, if phosphoric acid is used.
oxide, hydroxides and ammonia
- The table shows some examples.
hydrochloric acid. + copper oxide → copper chloride + water
sulfuric acid. +. sodium hydroxide. → sodium sulfate + water
nitric acid + calcium hydroxide → calcium nitrate + water
phosphoric acid +. iron(III) oxide →. iron(III) phosphate + water
Carbonates and acids:
- Carbonates also neutralise acids. As well as a salt and water, carbon dioxide is also produced. The name of the salt can be predicted in just the same way.
- hydrochloric acid + potassium carbonate → potassium chloride + water + carbon dioxide.
- A salt is any compound formed by the neutralisation of an acid by a base. The name of a salt has two parts. The first part comes from the metal, metal oxide or metal carbonate. The second part comes from the acid.
You can always work out the name of the salt by looking at the reactants:
- nitric acid always produces salts that end in nitrate and contain the nitrate ion, NO3-
hydrochloric acid always produces salts that end in chloride and contain the chloride ion, Cl-
sulfuric acid always produces salts that end in sulfate and contain the sulfate ion, SO42-
For example, if potassium oxide reacts with sulfuric acid, the products will be potassium sulfate and water.
The table shows some more examples:
Metal Acid Salt
Sodium hydroxide reacts with Hydrochloric acid to make Sodium chloride
Copper oxide reacts with Hydrochloric acid to make Copper chloride
Sodium hydroxide reacts with Sulfuric acid to make Sodium sulfate
Zinc oxide reacts with Sulfuric acid to make Zinc sulfate
- Note that ammonia forms ammonium salts when it reacts with acids. For instance, ammonia reacts with hydrochloric acid to make ammonium chloride.
To make an insoluble salt, two soluble salts need to react together in a precipitation reaction.
The table shows soluble and insoluble salts:
Soluble Insoluble All nitrates None
All common sodium, potassium and ammonium salts None
Most common sulfates Calcium sulfate and barium sulfate
Most common chlorides Silver chloride
Sodium, potassium and ammonium Most common carbonates
- We can see from the table that silver chloride is an insoluble salt. It can be made by reacting a soluble silver salt with a soluble chloride salt.
- Silver nitrate and sodium chloride are both soluble. When their solutions are mixed together, soluble sodium nitrate and insoluble silver chloride are made:
silver nitrate + sodium chloride → sodium nitrate + silver chloride
AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s)
- The silver chloride appears as tiny particles suspended in the reaction mixture - this is the precipitate. The precipitate can be filtered, washed with water on the filter paper, and then dried in an oven.
- Soluble salts can be made by reacting acids with either soluble or insoluble bases.
Making a salt from an alkali.
- If you are using an alkali - which is a soluble base - then you need to add just enough acid to make a neutral solution (check a small sample with universal indicator paper). Warm the salt solution to evaporate the water. You get larger crystals if you evaporate the water slowly.
Making a salt from an insoluble metal oxide or carbonate:
- Copper oxide and other transition metal oxides or hydroxides do not dissolve in water. If the base is insoluble, then an extra step is needed to form a salt.
- You add the base to the warm acid until no more will dissolve and you have some base left over – this is called an ‘excess’. You filter the mixture to remove the excess base, and then evaporate the water in the filtrate to leave the salt behind.
- Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.
What is electrolysis:
- Ionic substances contain charged particles called ions. For example, lead bromide contains positively charged lead ions and negatively charged bromide ions.
- Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.
- For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or when melted. For example, if electricity is passed through molten lead bromide, the lead bromide is broken down to form lead and bromine.
Here is what happens during electrolysis:
- Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
- Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised. The substance that is broken down is called the electrolyte.
electrolysis of aluminium
- Aluminium is the most abundant (found in large quantities) metal on Earth. But it is expensive, largely because of the amount of electricity used up in the extraction process.
Aluminium ore is called bauxite. The bauxite is purified to yield a white powder - aluminium oxide - from which aluminium can be extracted.
- The extraction is done by electrolysis. But first the aluminium oxide must be melted so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2000°C) so it would be expensive to melt it. Instead, it is dissolved in moltencryolite - an aluminium compound with a lower melting point than aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.
- Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, a form of carbon.
- Aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off.
- Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.
- Electrolysis is used to electroplate objects. This is useful for coating a cheaper metal with a more expensive one, such as copper or silver.
How it works
- The negative electrode should be the object that is to be electroplated
- The positive electrode should be the metal that you want to coat the object with
- The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate
Electroplating with silver:
- The object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply. A piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.
Electroplating with copper:
- The object to be plated, such as a metal pan, is connected to the negative terminal of the power supply. A piece of copper is connected to the positive terminal. The electrolyte is coppersulfate solution.
- This arrangement can also be used to purify copper during copper manufacture. In this case, both electrodes are made from copper. The negative electrode gradually gets coated with pure copper as the positive electrode gradually disappears.
electrolysis of sodium chloride
Useful substances can be obtained by the electrolysis of sodium chloride solution.
Electrolysis of sodium chloride solution:
- chlorine gas forms at the anode (positive electrode)
- hydrogen gas forms at the cathode (negative electrode)
- a solution of sodium hydroxide forms.
- These products are reactive, so it is important to use inert (unreactive) materials for the electrodes.
- A half-equation shows you what happens at one of the electrodes during electrolysis. Electrons are shown as e–. These are the half-equations:
anode: 2Cl– – 2e– → Cl2 (oxidation)
cathode: 2H + 2e– → H2 (reduction).
- Oxidation happens at the anode because electrons are lost. Reduction happens at the cathode because electrons are gained. Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
- Sodium ions Na+ and hydroxide OH– are also present in the sodium chloride solution. They are not discharged at the electrodes. Instead, they make sodium hydroxide solution.