Chemistry R1 & R2 definitions

Exothermic

A reaction in which energy is transferred from the system to the surroundings

Endothermic

A reaction in which energy is transferred from the surroundings to the system

Specific heat capacity

The amount of energy required to increase the temperature of a 1 kg substance by 1 degree K (or C)

Standard enthalpy change of combustion (2 marks!)

The enthalpy change when 1 mole of substance burns (combines vigorously with oxygen) under standard conditions

Standard enthalpy change of reaction (2 marks!)

The enthalpy change when molar quantities of reactants in their standard states react to form products in their standard states under standard conditions

Standard enthalpy change of formation (2 marks!)

The enthalpy change when 1 mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions

Standard enthalpy change of neutralisation (2 marks!)

The enthalpy change when solutions of an acid and alkali react together under standard conditions to produce 1 mole of water

Hess’ law

States that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same

Bond enthalpy

The energy required to break 1 mole of chemical bonds in the gaseous state

Average bond enthalpy

The enthalpy change when 1 mole of molecules is broken in the gaseous state and averaged for the bond in similar compounds

(In)complete combustion

A reaction is complete when it has reached equilibrium (when the concentrations of the reactants and products are no longer changing)

Standard enthalpy of atomisation

The enthalpy change when 1 mole of an element in its standard state is atomised to produce 1 mole of gaseous atoms

Standard enthalpy change of hydration

The enthalpy change when 1 mole of gaseous ions is dissolved in water to give 1 mole of aqueous ions and a solution of indefinite dilution

Standard enthalpy change of solution

The enthalpy change when 1 mole of an ionic substance is dissolved sufficient water to form an indefinite dilution

Entropy

A measure of the amount of disorder of the particles in a system.

Gibbs free energy

The energy associated with a chemical reaction that can be used to do work, and is equal to ΔH−TΔS where enthalpy is H, T is temperature in K, and S is the entropy of the system

Limiting reagent

The substance which is completely consumed in a chemical reaction and determines the yield of the product formed

Neutralisation

When an acid and base react together to form a salt and water

(e.g. HCl + NaOH → NaCl + H2O)

Lattice Enthalpy

The energy required to separate one mole of an ionic compound into one mole of gaseous ions.

Atom economy

A measure of reaction efficiency which compares the total mass of reactant atoms which end up as desired products, represented as a percentage

Mass of desired products / mass of total products x 100

Percentage yield

The experimental yield actually produced by the reaction compared to the theoretical yield, expressed as a percentage

Actual / theoretical x 100

Rate of reaction

The change over time of the concentration of a reactant or a product of a reaction

Collision theory

An explanation of how reactions take place, which states that for reactions to happen, the particles must collide with a minimum amount of energy, and the collisions must have the correct collision geometry

Activation energy (Ea)

The minimum amount of kinetic energy needed by particles for successful collisions to occur

Catalyst

A substance that lowers the activation energy by providing an alternative pathway. This changes the rate of reaction and is not used up or changed by it.

Average rate

The change over time of the concentration of a reactant or a product of a reaction

Instantaneous rate

The rate of reaction at a particular moment in time

Initial rate

The rate of reaction when t = 0

Dynamic equilibrium

When the rate of the forward reaction equals the rate of the reverse reaction in a closed reaction, and the concentrations of the products and reactants are constant

Order of reaction

The power to which the concentration of that reactant is raised to in the rate equation

Rate law / expression

The mathematical expression that describes the relationship between the rate of a chemical reaction and the concentration of its reactants

rate = k[A]x[B]y

Molecularity

The number of reactant species taking part in any reaction

Rate determining step (RDS)

The slowest step of a chemical reaction that determines the rate at which the overall reaction proceeds

Reaction quotient (Q)

The ratio between the concentration of products and reactants in a reversible reaction not at equilibrium

Standard change in entropy (2 marks!)

The entropy of 1 mole of a substance measured at standard conditions (298 K and 101 kPa). Units are JK