chemistry revision gcse

Pure substance

A single element or compound not mixed with any other substance, which melts and boils at specific, fixed temperatures.

Chromatography $R_f$ value formula

$$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$$

Percentage yield equation

$$\text{Percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$

Concentration equation in $g\,dm^{-3}$

$\text{Concentration} = \frac{\text{mass of solute}}{\text{volume of solution}}$ where mass is measured in $g$ and volume in $dm^3$.

Alkane general formula

$$C_n H_{2n+2}$$

Cracking

A thermal decomposition reaction that breaks down long-chain alkane molecules into shorter, more useful alkanes and alkenes.

Electrolysis

The process that uses a direct electric current to decompose a molten or dissolved ionic compound.

Oxidation and reduction in terms of electrons

Oxidation is the loss of electrons, whereas reduction is the gain of electrons.

Pure substance

A single element or compound not mixed with any other substance, which melts and boils at specific, fixed temperatures.

Chromatography $R_f$ value formula

$$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$$

Percentage yield equation

$$\text{Percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$

Concentration equation in $g\,dm^{-3}$

$\text{Concentration} = \frac{\text{mass of solute}}{\text{volume of solution}}$ where mass is measured in $g$ and volume in $dm^3$.

Alkane general formula

$$C_n H_{2n+2}$$

Cracking

A thermal decomposition reaction that breaks down long-chain alkane molecules into shorter, more useful alkanes and alkenes.

Electrolysis

The process that uses a direct electric current to decompose a molten or dissolved ionic compound.

Oxidation and reduction in terms of electrons

Oxidation is the loss of electrons, whereas reduction is the gain of electrons.

Atomic number

The number of protons in the nucleus of an atom.

Mass number

The total number of protons and neutrons in the nucleus of an atom.

Isotope

Atoms of the same element with the same number of protons but a different number of neutrons.

Relative atomic mass ($A_r$)

The average mass of an atom of an element compared to $1/12\text{th}$ of the mass of an atom of carbon-12.

Relative formula mass ($M_r$)

The sum of the relative atomic masses of all the atoms present in a chemical formula.

Mole definition

The amount of substance containing $6.02 \times 10^{23}$ particles.

Equation for moles using mass and relative formula mass

$\text{Moles} = \frac{\text{mass}}{\text{relative formula mass}}$ where mass is in $g$.

Ionic bonding

The strong electrostatic attraction between oppositely charged ions.

Covalent bonding

A shared pair of electrons between two non-metal atoms.

Metallic bonding

The electrostatic attraction between positive metal ions and a delocalised sea of electrons.

Properties of giant ionic lattices

High melting and boiling points, and conduct electricity when molten or aqueous but not solid.

Properties of simple molecular structures

Low melting and boiling points due to weak intermolecular forces between molecules.

Structure and properties of diamond

Giant covalent structure where each carbon atom forms 4 strong covalent bonds, making it extremely hard with a high melting point.

Structure and properties of graphite

Giant covalent structure where carbon forms 3 bonds in hexagonal layers with weak forces between layers, allowing them to slide, and delocalised electrons to conduct electricity.

Structure and properties of graphene

A single layer of graphite, one atom thick, which is extremely strong, lightweight, and conducts electricity.

Fullerenes

Molecules of carbon atoms with hollow shapes, such as buckminsterfullerene ($C_{60}$) and carbon nanotubes.

Group 1 elements trend in reactivity

Reactivity increases down the group as the outer electron is further from the nucleus and lost more easily.

Group 7 elements trend in reactivity

Reactivity decreases down the group as it becomes harder to attract an electron into the outer shell.

Displacement reaction in halogens

A reaction where a more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt.

Group 0 elements (Noble gases)

Unreactive gases with full outer electron shells, making them exceptionally stable.

Catalyst

A substance that increases the rate of a chemical reaction without being used up, by providing an alternative reaction pathway with lower activation energy.

Activation energy

The minimum amount of energy required for reacting particles to collide successfully and react.

Exothermic reaction

A chemical reaction that releases thermal energy to the surroundings, causing the surrounding temperature to rise.

Endothermic reaction

A chemical reaction that absorbs thermal energy from the surroundings, causing the surrounding temperature to fall.

Reaction profile for an exothermic reaction

A diagram showing reactants at a higher energy level than products, reflecting a overall release of energy.

Reaction profile for an endothermic reaction

A diagram showing reactants at a lower energy level than products, reflecting a overall absorption of energy.

Acid definition in terms of pH and ions

A substance that produces aqueous hydrogen ions ($H^+_{(aq)}$) in solution and has a pH less than 7.

Base definition

A substance that neutralises an acid to produce a salt and water.

Alkali definition

A soluble base that produces aqueous hydroxide ions ($OH^-_{(aq)}$) in solution with a pH greater than 7.

Neutralisation reaction ionic equation

$$H^+<em>{(aq)} + OH^-</em>{(aq)} \rightarrow H<em>2O</em>{(l)}$$

General equation for acid + metal

$$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}$$

General equation for acid + metal carbonate

$$\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide}$$

Test for oxygen gas

Insert a glowing splint into the gas; if oxygen is present, the splint relights.

Test for hydrogen gas

Hold a burning splint near the gas; if hydrogen is present, it burns with a squeaky pop sound.

Test for carbon dioxide gas

Bubble the gas through limewater; if carbon dioxide is present, the limewater turns milky/cloudy.

Test for chlorine gas

Hold damp litmus paper in the gas; if chlorine is present, the paper bleaches white.

Anode in electrolysis

The positive electrode where negative ions undergo oxidation by losing electrons.

Cathode in electrolysis

The negative electrode where positive ions undergo reduction by gaining electrons.

Product at cathode during electrolysis of aqueous solutions

Hydrogen gas is produced if the metal is more reactive than hydrogen; otherwise, the metal element is deposited.

Product at anode during electrolysis of aqueous solutions

Oxygen gas is produced unless halide ions ($Cl^-$$, $Br^-$$, $I^-$$) are present, in which case the corresponding halogen gas forms.

Bioleaching

A extraction method using bacteria to extract metals from low-grade ores by producing leachate solutions containing metal ions.

Phytomining

An extraction method using plants to absorb metal compounds from soil, which are harvested and burned to leave ash rich in metal compounds.

Life Cycle Assessment (LCA)

An evaluation of the environmental impact of a product across all stages of its life, from raw material extraction through usage to final disposal.

Functional group of alcohols

$$-OH$$

Functional group of carboxylic acids

$$-COOH$$

Addition polymerisation

A reaction where unsaturated monomer molecules (alkenes) join together to form a long polymer chain with no other products.

Condensation polymerisation

A reaction where monomers with two functional groups join together, releasing small molecules such as water as by-products.

Flame test color for Sodium ($Na^+$)

Yellow-orange flame.

Flame test color for Potassium ($K^+$)

Lilac flame.

Flame test color for Calcium ($Ca^{2+}$)

Orange-red flame.

Flame test color for Copper ($Cu^{2+}$)

Green flame.

Flame test color for Lithium ($Li^+$)

Crimson flame.

Precipitate test for sulfate ions ($SO_4^{2-}$)

Add dilute hydrochloric acid followed by barium chloride solution; a white precipitate indicates sulfate ions.

Precipitate test for halide ions

Add dilute nitric acid followed by silver nitrate solution; chloride forms a white precipitate, bromide cream, and iodide yellow.

Le Chatelier's Principle

If a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to counteract that change.

Effect of increasing pressure on dynamic equilibrium

Shifts the equilibrium position towards the side of the reaction with fewer gas molecules.

Haber process equation

$$N<em>{2(g)} + 3H</em>{2(g)} \rightleftharpoons 2NH_{3(g)}$$

Alkenes general formula

$$C_n H_{2n}$$

Hydrocarbon test for unsaturation

Shake the hydrocarbon with bromine water; an unsaturated hydrocarbon (alkene) turns the orange solution colorless.

Atom economy formula

$$\text{Atom economy} = \frac{\text{relative formula mass of desired product}}{\text{total relative formula mass of all reactants}} \times 100$$

Collision theory

Chemical reactions occur only when reacting particles collide with energy greater than or equal to the activation energy and in the correct orientation.

Effect of increasing surface area on rate of reaction

Increases the collision frequency by exposing more reactant particles to collisions, thereby increasing the rate of reaction.

Strong acid vs weak acid

A strong acid completely ionises in aqueous solution, whereas a weak acid only partially ionises.

Potable water

Water that is safe and fit for human consumption.

Desalination methods

Processes like reverse osmosis or distillation used to remove dissolved salts from seawater to make it potable.

Greenhouse effect mechanism

Greenhouse gases absorb long-wavelength infrared radiation re-emitted by Earth's surface and re-radiate it, trapping heat in the atmosphere.

Carbon footprint

The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event.

Atmospheric composition of modern Earth

Approximately $78\%$ nitrogen, $21\%$ oxygen, $0.9\%$ argon, $0.04\%$ carbon dioxide, and trace amounts of other gases.

Ceramic material properties

Hard, brittle, high melting point, and excellent electrical and thermal insulators.

Composite material

A material made of a matrix/binder surrounding and binding together fibers or fragments of a reinforcement material.

Corrosion prevention methods

Applying protective coatings (painting, greasing, electroplating) or sacrificial protection using a more reactive metal.

NPK fertilisers

Formulations containing compounds of nitrogen, phosphorus, and potassium to improve agricultural crop growth.