AQA A Level Chemistry: Thermodynamics Flashcards
What is enthalpy change (ΔH)?
The heat energy change measured under conditions of constant pressure.
What are the standard conditions for enthalpy changes?
- Temperature: 298 K (25°C)
- Pressure: 100 kPa
- All substances in their standard states
What is the enthalpy of formation (ΔHf°)?
The enthalpy change when 1 mole of a substance is formed from its elements in their standard states under standard conditions.
What is the enthalpy of combustion (ΔHc°)?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
What is Hess’s Law?
The total enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same.
What is bond enthalpy?
The enthalpy change when 1 mole of a bond in a gaseous molecule is broken.
What is mean bond enthalpy?
The average enthalpy change required to break 1 mole of a particular type of bond, averaged over different compounds.
How do you calculate enthalpy change using bond enthalpies?
ΔH = Σ(Bonds broken) – Σ(Bonds formed)
What is lattice enthalpy (ΔHLE)?
The enthalpy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions.
Is lattice enthalpy exothermic or endothermic?
Exothermic (always negative) because energy is released when the lattice forms.
What is the first ionisation energy?
The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.
What is the second ionisation energy?
The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.
What is the first electron affinity?
The enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of gaseous 1– ions.
What is the second electron affinity?
The enthalpy change when 1 mole of electrons is added to 1 mole of gaseous 1– ions to form 1 mole of gaseous 2– ions (usually endothermic).
What is the enthalpy of solution (ΔHsol)?
The enthalpy change when 1 mole of an ionic substance dissolves completely in water to form an infinitely dilute solution.
What is the enthalpy of hydration (ΔHhyd)?
The enthalpy change when 1 mole of gaseous ions dissolves in water to form hydrated ions.
What is the equation relating lattice enthalpy, hydration enthalpy, and enthalpy of solution?
ΔHsol = ΔHhyd (cation) + ΔHhyd (anion) – ΔHLE
What factors affect lattice enthalpy?
- Ionic charge: Higher charge → stronger attraction → more negative lattice enthalpy.
- Ionic radius: Smaller ions → stronger attraction → more negative lattice enthalpy.
What factors affect hydration enthalpy?
- Ionic charge: Higher charge → stronger attraction to water → more exothermic.
- Ionic radius: Smaller ions → stronger attraction → more exothermic.
What is entropy (S)?
A measure of disorder or randomness in a system.
What is the unit of entropy?
J K⁻¹ mol⁻¹
What increases entropy?
- Increase in the number of moles of gas.
- Change of state from solid → liquid → gas.
- Dissolving of a solid into ions.
What is the formula for entropy change?
ΔS = ΣS(products) – ΣS(reactants)
What is Gibbs free energy (ΔG)?
The energy available to do work and determine the feasibility of a reaction.
What is the Gibbs free energy equation?
ΔG = ΔH – TΔS
Where:
* ΔG = Free energy change (kJ mol⁻¹)
* ΔH = Enthalpy change (kJ mol⁻¹)
* T = Temperature (K)
* ΔS = Entropy change (J K⁻¹ mol⁻¹)
When is a reaction feasible?
When ΔG ≤ 0 (negative or zero).
What is the effect of temperature on feasibility?
- If ΔH is negative and ΔS is positive → reaction is always feasible.
- If ΔH is positive and ΔS is negative → reaction is never feasible.
- If both ΔH and ΔS are positive or negative → feasibility depends on temperature.
How do you calculate the temperature at which a reaction becomes feasible?
T = ΔH / ΔS
What is a Born-Haber cycle?
A thermochemical cycle used to calculate lattice enthalpies by applying Hess’s Law.
What is the importance of Gibbs free energy in industrial processes?
Helps predict if a reaction is thermodynamically feasible and minimises energy costs.
