3.2.1 Enthalpy changes Flashcards
What is the enthalpy change of reaction (ΔrH)?
The enthalpy change when a reaction occurs in the molar quantities shown in a stated equation, under standard conditions.
What is the enthalpy change of formation (ΔfH)?
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (100 kPa, 298 K).
What is the enthalpy change of combustion (ΔcH)?
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions (100 kPa, 298 K).
What is the enthalpy change of neutralisation (ΔneutH)?
The enthalpy change when one mole of water is formed from an acid-base reaction under standard conditions (100 kPa, 298 K).
What are standard conditions in enthalpy calculations?
100 kPa pressure and a stated temperature, usually 298 K.
What are standard states?
The physical state of a substance under standard conditions (100 kPa, 298 K).
What is the definition of activation energy?
The minimum energy required for a reaction to take place by breaking bonds in reactant molecules.
How does an enthalpy profile diagram represent an exothermic reaction?
The products have lower enthalpy than reactants, with a negative enthalpy change (ΔH negative). The activation energy is shown as a peak.
How does an enthalpy profile diagram represent an endothermic reaction?
The products have higher enthalpy than reactants, with a positive enthalpy change (ΔH positive). The activation energy is shown as a peak.
What is the equation used to determine enthalpy changes experimentally?
q = mcΔT, where q is heat energy (J), m is mass (g), c is specific heat capacity (J g⁻¹ K⁻¹), and ΔT is temperature change (K).
What is average bond enthalpy?
The average enthalpy change when one mole of a given bond is broken in gaseous molecules.
Why are actual bond enthalpies different from average bond enthalpies?
Bond enthalpies vary depending on molecular environment, so averages are taken across different compounds.
How do bond enthalpies relate to exothermic and endothermic reactions?
Bond breaking is endothermic (requires energy, ΔH positive), while bond making is exothermic (releases energy, ΔH negative).
What is Hess’s Law?
The total enthalpy change of a reaction is independent of the route taken, provided initial and final conditions are the same.
How can Hess’s Law be used to determine ΔrH from combustion data?
ΔrH = ΣΔcH(reactants) - ΣΔcH(products)
How can Hess’s Law be used to determine ΔrH from formation data?
ΔrH = ΣΔfH(products) - ΣΔfH(reactants)
What experimental techniques are used to determine enthalpy changes?
Calorimetry using q = mcΔT, combustion in a bomb calorimeter, and indirect determination using Hess’s Law and enthalpy cycles.
Explain why this equation represents the standard enthalpy change of combustion of butane.
One mole of butane completely combusts in oxygen
.
Allow: One mole forms CO₂ and H₂O only.
Define the term standard enthalpy change of formation.
(Enthalpy change) when one mole of a compound is formed from its elements
298 K / 25 °C AND 1 atm / 100 kPa / 101 kPa / 1 bar.
Allow: Energy required OR energy released.
Define bond enthalpy.
(Average enthalpy change) when one mole of gaseous covalent bonds is broken
Ignore: Energy required OR energy released.
Do not allow: Bonds formed
Suggest two reasons, apart from measurement uncertainties, why the experimental value for ΔcH is less exothermic than the data book value.
Incomplete combustion
Heat loss to the surroundings.
Non-standard conditions.
What is meant by the term average bond enthalpy?
(Average enthalpy change) when one mole of gaseous covalent bonds is broken
Do not allow bonds formed.
Explain, in terms of bond breaking and bond forming, why a reaction can be exothermic.
More energy is released by forming bonds than is required to break bonds
Allow: Bond breaking is endothermic, bond making is exothermic.
Define standard enthalpy change of combustion.
(Enthalpy change when) one mole of a substance completely combusts in oxygen .
298 K / 25 °C AND 1 atm / 100 kPa / 101 kPa / 1 bar.
Allow: Burns in excess oxygen.
