Chemistry Revision

Element= A substance which cannot be broken down into another substance

Compound= When two or more different elements react together

Mixture= When two or more substances are mixed but not chemically joined

Fullerenes are molecules of carbon atoms with hollow shapes. Their structures are based on hexagonal rings of carbon atoms joined by covalent bonds. Eg nanotubes.

Graphene is a single layer of graphite. Due to the strong covalent bonds, graphene has a high melting point and is very strong. iat also conducts electricity very well because it has delocalised electrons. Graphene is one atom thick.

Both fullerene and graphene are forms of carbon.

Displacement reactions are examples of redox reactions:

• the ions of the less reactive metal gain electrons and are reduced

• the atoms of the more reactive metal lose electrons and are oxidised

Ar: The relative atomic mass of an element is the mean mass of its atoms compared to the mass of an atom of the carbon-12 isotope.

Mr: The relative molecular mass of an element/compound is the mass of a molecule compared to 1/12 the mass of one atom of carbon-12.

Electrolysis is the process in which ionic substances are broken down into simpler substances when an electric current is passed through it.

Ionic substances form when a metal reacts with a non metal; they contain charged particles called ions. Theses ions must be free to move in a solution for electrolysis to work.

Why we use it: Electrolysis is used for manufactoring processes, such as electroplating.

Every alkali metal reacts vigorously with water. In each reaction, Hydrogen gas is released and metal hydroxide is produced. As you go down the group, the reaction increases and so does the reactivity.

Alkaline solutions are formed when the hydroxide produced dissolves, and these solutions turn the universal indicator purple. 

Thermosoftening                                          |    Thermosetting

1) Melt when heated                                    |     1) Does not melt when heated- chars/burns

2) Majority of everday plastics                     |     2) Resistant to much higher temps than 

3) Recyclable                                               |          thermosoftening plastics

4) Do not have covalent bonds-                   |     3) Used for electrical plugs

  molecules can move over each                 |      4) Strong covalent bonds

 other when heated                                      |

Dmitri Mendeleev created a version of the periodic table which proved to be a critical discovery. His version of the periodic table organized elements into rows according to their atomic mass and into columns based on chemical and physical properties. This allowed him to predict missing elements' properties and atomic masses.

A carboxylic acid is any molecule that has a COOH group. Carboxylic acids are considered weak acids because their solutions have a high concentration of hydrogen ions (H+) which gives them a low pH. 

Metal carbonates neutralise acids, despite being insoluble. 

Metal carbonate + acid ----> a salt+ water + carbon dioxide

This reaction fizzes whilst releasing carbon dioxide bubbles

• The Earth's early atmosphere was most likely formed from the gases released by volcanoes
• Probably mostly carbon dioxide- not much oxygen
• Small amounts of water vapour, ammonia & methane
• Water vapour condensed to form oceans

Exothermic                                                 |   Endothermic

1) Transfers energy to surroundings &      |  1) Takes in energy & the temp of surroundings

    the temp of surroundings increases.     |      decreases

   Eg, a hand warmer                                 |    Eg, an instant ice pack

2) Occurs in conbustion reactions,            |  2) Occurs in thermal decomposition reactions 

   oxidation reactions & neutralisation 

   reactions

Group 0- The Noble gases are unreactive because they have a full outer shell. Eg, helium, neon & argon.

Group 1- Alkali metals are very reactive so must be stored in oil. Examples are lithium, potassium, & sodium.

Group 7- Halogens are reactive non metals. They have both low melting and boiling points. Chlorine, bromine & iodine are all examples of group 7 elements.

Transition metals- The middle block of the periodic table. They have high melting points and densities, form coloured compounds and act as catalysts. 

Amino acids arejoined together through peptide bonds. Peptide bonds are covalent bonds and a chain of amino acids joined by peptide bonds created a polypeptide. Different amino acids can be joined to create a protein.

Mole= the unit for the amount of a substance

1 mol is the amount of substance that contains the same number of particles as there are atoms in 12.0 g of carbon-12.

The number of particles in a substance can be found using Avogadro's constant.

Avogadro constant= 6.022 x 10²³ atoms per mole

The pH scale is used to measure acidity/alkalinity of a substance. 

Acid + water= an acidic solution- also has a pH less than 7 

Base + water= an alkaline solution- also has a pH more than 7

Neither= a neutral solution- also has a pH of 7

Indicators are substances that change colour depending on whether a solution is acid or alkaline, thus showing their pH.

The substances in crude oil are extracted using fractional distillation. The crude oil is evaporated by putting it in a tall fractionating column and its vapours condense at different boiling points. As the vapour reaches the top it condenses and the newly separated liquids are led out of the column at different heights.

Alloy= a mixture of two or more elements- at least 1 is a metal.

Alloys tend to be stronger than a pure metal because solid metals have a regular lattice struture and so the atoms can slide off each other. However, when an alloy is added the structure becomes irregular so slides less and thus becomes stronger.

Alloy uses:

Car body parts, drill bits, washing machines, etc.

Increasing the temperature increases the reaction rate because the particles move quicker and so are much more likely to collide and react.

The modern atmosphere is mainly made up of nitrogen (78%) and oxygen (21%).

When alkali metals (group 1) react with water, they produce a metal hydroxide and hydrogen.

Element  Observations

Lithium, Li Fizzes steadily, slowly becomes smaller until it disappears

Sodium, Na Melts to form a ball, fizzes rapidly, quickly becomes smaller until it disappears

Potassium, K Quickly melts to form a ball, burns violently with sparks and a lilac flame, disappears rapidly, often with a small explosion

The elements get more reactive as you go down the group

Ammonium sulfate can be made in the lab using dilute ammonia solution and dilute sulfuric acid:

ammonia + sulfuric acid → ammonium sulfate

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

Flame Test

The flame test can be used to test for alkali metals (group 1). A solid piece of the metal is held over a flame and the colour of the flame tells us which metal it is.

More reactive metals are more likely to lose electrons and form positive ions.

Thermosoftening plastics melt when heated because they don't have covalent bonds (the molecues can move over each other) so can be recycled into something new. However, thermosetting plastics burn, although they are resistant to much higher temperatures compared to thermosoftening plastics.

Moles measure the amount of substance in a unit. It allows scientists to make predictions about the masses of different substances.

Number of moles = mass ÷ relative formula mass

This can be rearranged to find the mass:

Mass= moles x relative formula mass

•   Collect used items
•   Transport the used items to a recycling centre
•  Break up the items and sorting the different materials

•  Melt the metal and removing impurities from  the molten metal
• Solidify the metal in ingots (slabs of metal)

An ionic compound is formed in an arranged called an ionic lattice, which is regular and repeating. This lattice is formed because the ions attract to each other with the oppositely charged ions next to each other. This lattice is giant- eg a grain of sand could contain 1,200,000,000,000,000,000 ions. It is also 3D, which is why solid ionic compounds end up forming crystals.

The properties of giant ionic structures are:

• High melting point
• High boiling point

A chemical cell converts chemical energy into electrical energy. Most batteries are chemical cells. A chemical reaction takes place inside the battery and causes electric current to flow. There are two types of batteries: those that are rechargable and those that aren't. To recharge a battery, an electic current is passed backwards through it.

Once a compound has been separated by chromatography, the spots can be identified. This can then be compare with known compounds.  We can measure the Rf (retention factor) value of a sample and compare it to known data to identify unknown samples.

Retention factor= distance moved by substance ÷ distance moved by solvent

• Plum pudding model- J J Thompson discovered the electron. In the plum pudding model, the atom is a ball of positive charge with negative electrons embedded in it
• Nuclear model- In the gold foil experiment, positively charged alpha particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions. The nuclear model suggested that the mass of an atom is concentrated in the centre and the nucleus is positivly charged.

Chemical reactions can be summarised into a word equation, where the reactants are on the left side of the ------> and the products are on the right.

Eg: Sulfur + Oxygen --------> Sulfur Dioxide 

This can then be turned into a chemical equation:

S + O₂ → SO₂ 

It is important that this is balanced so that the same amount of each element is the same on each side of the equation. 

Reversible reactions= the products can react to produce the original reactants

Equilibrium= the concentrations of reactants and products do not change

<------  Rather than the usual ------> symbol used in an irreversibly         -------> reaction, a different symbol is used 

Half equations are used to show what happens at one of the electrodes during electrolysis. 

Electrons= e^-

To balance the equation you must add/subtract a number of electrons equal to the total number of charges on the ions in the equation. 

Cations half equations- They need to gain enough electrons to make them neutral. So an Al³⁺ ion needs to gain three electrons: ]

Al³⁺ + 3e^- → Al

Anion half equations- Add in two electrons to balance the charge so that both sides have the same charge. The two electrons need to go on the right-hand side, so that both sides have an overall charge of -2. For example:  2Cl^- → Cl₂ + 2e^-

Hydrogen

If a lighted wooden splint makes a popping sound when placed in a test tube then hydrogen is present

Oxygen

If a glowing wooden splint relights when placed in a test tube then oxygen is present

Carbon Dioxide

If limewater turns milky then carbon dioxide is present