Energetics AS Flashcards
What is Enthalpy Change (ΔH)?
ΔH is the heat energy transferred in a reaction at constant pressure, measured in kJ mol⁻¹.
What are the standard conditions for measuring enthalpy change?
100 kPa and a stated temperature.
Describe exothermic reactions.
In exothermic reactions, the chemical store decreases and the thermal store of surroundings increases. ΔH is negative.
Describe endothermic reactions.
In endothermic reactions, the chemical store increases and the thermal store of surroundings decreases. ΔH is positive.
What happens to energy when bonds are broken?
Energy is needed to break bonds, making the reaction endothermic.
What happens to energy when bonds are formed?
Energy is released when bonds are formed, making the reaction exothermic.
What is the Standard Enthalpy of Formation (ΔfH°)?
It is the enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions.
What is the Standard Enthalpy of Combustion (ΔcH°)?
It is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
What is the Standard Enthalpy of Neutralisation (ΔneutH°)?
It is the enthalpy change when 1 mole of water is formed in a reaction between an acid and an alkali under standard conditions.
What is the formula for calculating heat energy (q) in calorimetry?
q = mcΔT, where m = mass, c = specific heat capacity, ΔT = temperature change.
How do you calculate ΔH from q and n?
ΔH = q / n.
What does Hess’s Law state?
Hess’s Law states that the enthalpy change for a reaction is independent of the route taken.
How is ΔH calculated using Hess’s Law?
ΔH = [SUM of ΔfH products] - [SUM of ΔfH reactants].
What is mean bond enthalpy?
It is the average energy needed to break 1 mole of a particular bond in various compounds.
What is the energy change when bonds are broken?
Breaking bonds requires energy, making it an endothermic process (bond enthalpies are positive).
