Enthalpy changes Flashcards
What is the definition of enthalpy change (ΔH)
Enthalpy change (ΔH) is the heat energy change in a reaction at constant pressure. It is measured in kilojoules per mole (kJ mol⁻¹).
What does a negative enthalpy change (ΔH) indicate about a reaction?
A negative ΔH indicates an exothermic reaction, where heat is released to the surroundings, resulting in products having lower enthalpy than reactants.
What does a positive enthalpy change (ΔH) indicate about a reaction?
A positive ΔH indicates an endothermic reaction, where heat is absorbed from the surroundings, resulting in products having higher enthalpy than reactants.
Define standard enthalpy change of reaction (ΔH°r).
The standard enthalpy change of reaction (ΔH°r) is the enthalpy change when a reaction occurs in the molar quantities shown in the balanced equation under standard conditions
Standard conditions
- 298 K
- 1 atm
- 1 M concentration
Define standard enthalpy change of combustion (ΔH°c).
when one mole of a substance is completely burned in oxygen under standard conditions
What is the standard enthalpy change of formation (ΔH°f)?
when one mole of a compound is formed from its elements in their standard states under standard conditions.
What is Hess’s Law?
Hess’s Law states that the total enthalpy change of a reaction is the same, no matter the route taken, provided the initial and final conditions are the same.
How is the standard enthalpy change of formation (ΔH°f) calculated using Hess’s Law?
ΔH°f can be calculated by using the equation:
ΔH°reaction =
ΣΔH°f(products) - ΣΔH°f(reactants).
How does bond enthalpy relate to enthalpy changes ?
The enthalpy change of a reaction can be estimated by summing the bond enthalpies of bonds broken and bonds formed.
What is bond enthalpy ?
Bond enthalpy is the energy required to break one mole of a bond in a gaseous molecule.
What is the significance of the enthalpy change of neutralization (ΔH°neut)?
when one mole of water is formed from a neutralisation reaction, usually exothermic
Explain why enthalpy changes cannot be directly measured for some reactions.
Enthalpy changes cannot be directly measured for reactions that occur too slowly, have high activation energies, or involve intermediates. Instead, they are calculated using Hess’s Law or bond enthalpies.
What is the standard enthalpy change of atomization (ΔH°atom)?
when one mole of gaseous atoms is formed from the element in its standard state.
Standard enthalpy change of hydration
when one mole of gaseous ions dissolves in water under standard conditions
