Unit 5 - Enthalpy changes
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What is ΔH?
Enthalpy Change
Describe an exothermic reaction
Heat energy is released to the surroundings so the temperature increases. ΔH is negative
Describe an endothermic reaction
Heat energy is absorbed from the surroundings so the temperature decreases. ΔH is positive
Define activation energy
The minimum energy that colliding particles must have to break bonds to start a chemical reaction
Explain an exothermic reaction in terms of bond breaking and bond making
More energy is released in bond making than is absorbed in bond breaking
Explain an endothermic reaction in terms of bond breaking and bond making
More energy is absorbed in bond breaking than is released in bond making
Describe how to calculate ΔH from an enthalpy profile diagram
ΔH = ΔH (products) - ΔH (reactants)
What are standard conditions and what is the symbol for this.
101 kPa and 298 K. The symbol to indicate standard conditions is θ.
Define standard enthalpy change of reaction, ΔHᵣθ
The enthalpy change… when the amounts of reactants shown in the stoichiometric equation react to give products… under standard conditions
Define standard enthalpy change of formation, ΔHfθ
The enthalpy change… when one mole of a compound is formed from its elements… under standard conditions
Define standard enthalpy change of combustion, ΔHcθ
The enthalpy change… when one mole of a substance is burnt in excess oxygen… under standard conditions
Define standard enthalpy change of neutralisation, ΔHₙₑᵤₜθ
The enthalpy change… when one mole of water is formed by the reaction of an acid with an alkali… under standard conditions
Define standard enthalpy change of atomisation, ΔHₐₜθ
The enthalpy change… when one mole of gaseous atoms is formed from its element… under standard conditions
Define Hess’ law
The enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place
Define exact bond energy
The energy needed to break a specific covalent bond in a named molecule in the gaseous state
Define average bond energy
The average energy needed to break a specific covalent bond averaged from a variety of molecules in the gaseous state