Definitions for the 'C3: Chemical Reactions Definitions' topic in the GCSE OCR Gateway Chemistry Spec.
Easy to understand and fully detailed; perfect definitions for use in the exams.
Acid- produces hydrogen ions (H + ) in aqueous solutions with a pH range between 0 and 7.
Activation energy- the energy required for a reaction to occur.
Alkali- produces hydroxide ions (OH - ) in aqueous solutions with a pH range between 7 and
14.
Anion- a negatively charged ion.
Anode- the positive electrode. It is where negatively charged ions lose electrons in oxidation
reactions. It is the electrode where oxygen is produced unless the solution contains halide
ions - then the halogen is produced.
Avogadro’s constant- the number of atoms, molecules or ions in a mole of a given
substance.
Cathode- the negative electrode. It is where positively charged ions gain electrons in
reduction reactions. It is the electrode where hydrogen is produced if the metal in the
electrolyte is more reactive than hydrogen.
Cation- a positively charged ion.
Conservation of mass- a law which states that no atoms are lost or made during a chemical
reaction so the mass of the products equals the mass of the reactants.
Electrode- a solid conductive material through which electricity can flow. They are used in
electrolysis to conduct electricity.
Electrolysis- the splitting up of an ionic compound using electricity. The electric current is
passed through a substance causing chemical reactions at the electrodes which lead to the
decomposition of the materials.
Electrolyte- a solution containing free ions from the molten or aqueous ionic substance. The
ions are free to move to carry charge.
Endothermic reaction- a reaction in which energy is taken in from the surroundings so the
temperature of the surroundings decreases. Bond breaking is an endothermic process.
Exothermic reaction- a reaction in which energy is transferred to the surroundings so the
temperature of the surroundings increases. Bond making is an exothermic process.
Inert- unreactive.
Limiting reactant- the reactant that is completely used up since it limits the amount of
products formed.
Mole- unit of measurement for chemical amounts. The mole is the unit for amount of
substance and the symbol for the unit mole is mol.
Neutralisation- the reaction in which an acid and a base react to form a salt and water.
Oxidation- a reaction involving the gain of oxygen. Oxidation is the loss of electrons.
Oxidising agent- a species which brings about oxidation by gaining electrons. The oxidising
agent is itself reduced.
pH scale- a measure of the acidity or alkalinity of a solution ranging from 0 to 14. pH can be
measured using universal indicator or a pH probe.
Reaction profile- graph used to show the relative energies of reactants and products, the
activation energy and the overall energy change of a reaction.
Reducing agent- a species which brings about reduction by losing electrons. The reducing
agent is itself oxidised.
Reduction- a reaction involving the loss of oxygen. Reduction is the gain of electrons.
State symbols- the symbols used in chemical equations to denote the states of the
chemicals reacting: (s) - solid, (l) - liquid, (g) - gas, (aq) - aqueous solution.
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