Ch5a - Chemical Energetics 1

Define enthalpy of a system.

The enthalpy of a system is a measure of its energy content and has the symbol H.

The higher the energy content, the more unstable is the system.

Define enthalpy change.

Enthalpy change is the change in energy content (absorbed or released) of a chemical reaction at constant pressure.

State whether endothermic or exothermic reactions are more energetically feasible.

Exothermic reactions are more energetically feasible than endothermic reactions.

Define activation energy.

The activation energy is the minimum amount of energy that reactant particles must possess before they can collide successfully to form products.

State the standard conditions.

1 bar and 298K

Define the standard enthalpy change of reaction.

The standard enthalpy change of reaction is the energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions.

Define the standard enthalpy change of formation.

The standard enthalpy change of formation of a substance is the energy change when 1 mole of the pure substance in a specified state is formed from its constituent elements in their standard states under standard conditions.

Define standard enthalpy change of combustion.

The standard enthalpy change of combustion of a substance is the energy released when 1 mole of the substance is burnt in excess oxygen under standard conditions.

Define standard enthalpy change of neutralisation.

The standard enthalpy change of neutralisation between an acid and a base is the energy change when the acid and base react to form 1 mole of water under standard conditions.

State whether how the standard enthalpy change of neutralisation differs for a weak acid compared to a strong acid.

Generally, a weak acid-strong base neutralisation reaction is less exothermic than that of a strong acid-strong base.

A weak acid undergoes partial dissociation in aqueous solution.

During neutralisation, some energy is taken in for the further dissociation of the weak acid to release H⁺(aq). This process is endothermic. As a result, the overall energy released from this weak acid-strong base reaction is less compared to that of a strong acid-strong base reaction.

Define standard enthalpy change of atomisation of an element.

The standard enthalpy change of atomisation of an element is the energy absorbed when 1 mole of gaseous atoms is formed from the element under standard conditions.

E.g. ½ Cl₂(g) → Cl(g)

Define the standard enthalpy change of atomisation of a compound.

The standard enthalpy change of atomisation of a compound is the energy absorbed when 1 mole of the compound is converted to gaseous atoms under standard conditions.

E.g. CH₄(g) → C(g) + 4H(g)

Define bond energy.

The bond energy (BE) of a X—Y bond is the average energy required to break 1 mole of X—Y bonds in the gaseous state.

Define specific heat capacity.

The specific heat capacity (c) of a substance is the quantity of heat required to raise the temperature of 1g of the substance by 1K.

Define heat capacity.

The heat capacity (C) of a substance is the quantity of heat required to raise the temperature of the substance by 1K.

Define lattice energy.

The lattice energy (LE) of an ionic compound is the energy released when 1 mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions.

State the factors affecting lattice energy.

• ionic charge

• ionic size

Explain how ionic charge affects lattice energy.

The bigger the cationic charge or anionic charge,

the greater the magnitude of the lattice energy / the more negative the lattice energy

Explain how ionic size affects lattice energy.

The smaller the cationic radius or anionic radius,

the smaller the inter-ionic distance,

the greater the magnitude of the lattice energy / the more negative the lattice energy

Define ionisation energy.

The first ionisation energy of an element is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly positively charged gaseous ions.

Define electron affinity.

The first electron affinity of an element is the energy change when 1 mole of gaseous atoms acquires 1 mole of electrons to form 1 mole of singly negatively charged gaseous ions.

Define standard enthalpy change of hydration.

The standard enthalpy change of hydration of an ion is the energy released when 1 mole of gaseous ions is dissolved in water to form 1 mole of aqueous ions under standard conditions.

Define standard enthalpy change of solution.

The standard enthalpy change of solution of a substance is the energy change when 1 mole of the substance is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions.