1What is the definition of enthalpy change (ΔH)?
Enthalpy change (ΔH) is the heat energy change in a system measured at constant pressure.
2What are the standard conditions for enthalpy measurements?
100 kPa pressure and a temperature of 298 K (25°C).
3In thermochemistry, what are the system and the surroundings?
The system is the chemicals undergoing change, and the surroundings are everything outside the system.
4Why do exothermic reactions have negative ΔH?
Exothermic reactions release heat to the surroundings, causing the system to lose energy, making ΔH negative.
5Why do endothermic reactions have positive ΔH?
Endothermic reactions absorb heat from the surroundings, causing the system to gain energy, making ΔH positive.
6How do you construct an enthalpy level diagram for an exothermic reaction?
The reactants are at a higher energy level than the products, with a downward arrow labeled ΔH (negative).
7What is the standard enthalpy change of formation (ΔfH)?
ΔfH is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
8What is the standard enthalpy of combustion (ΔcH)?
ΔcH is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
9What is the standard enthalpy of neutralisation (ΔneutH)?
ΔneutH is the enthalpy change when 1 mole of water is formed from an acid and a base under standard conditions.
10Why is the enthalpy of neutralisation nearly the same for all strong acid-base reactions?
Because strong acids and bases fully ionize, the reaction always involves H⁺ (aq) + OH⁻ (aq) → H₂O (l), releasing nearly the same energy.
11What is the standard enthalpy of atomisation (ΔatH)?
ΔatH is the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.
12What is the equation used to calculate energy transfer in a calorimetry experiment?
q = mcΔT
13In the equation q = mcΔT, what does each variable represent?
q = energy transferred (J), m = mass of solution (g), c = specific heat capacity (J g⁻¹ °C⁻¹), ΔT = temperature change (°C).
14State 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.
15Why is Hess’s Law useful in enthalpy calculations?
It allows determination of enthalpy changes for reactions that cannot be measured directly by constructing enthalpy cycles.
16What are two major sources of error in calorimetry experiments?
Heat loss to the surroundings and incomplete reaction.
17What is mean bond enthalpy?
Mean bond enthalpy is the average energy required to break a specific bond, averaged over different compounds.
18Why do enthalpy changes calculated from bond enthalpies differ from experimental values?
Bond enthalpies are averages and do not account for specific molecular environments or intermolecular forces.
19How can bond enthalpy values predict reaction rates?
Bonds with lower bond enthalpy break more easily, meaning they require less energy input and lead to faster reactions.
20How can experimental errors in calorimetry be minimized?
Use an insulated container, stir the solution, measure temperatures accurately, and account for heat loss.
21What is enthalpy change of a reaction
Heat energy change when a reaction takes place in the molar quantities shown in chemical equation under standard conditions