Chapter 4 Flashcards
(16 cards)
Define enthalpy change (ΔH).
The heat energy change measured under constant pressure.
What is meant by exothermic reaction?
A reaction that releases heat energy to the surroundings (ΔH negative).
What is meant by endothermic reaction?
A reaction that absorbs heat energy from the surroundings (ΔH positive).
Define standard enthalpy change of combustion.
Enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.
Define standard enthalpy change of formation.
Enthalpy change when one mole of a compound is formed from its elements in their standard states.
Explain Hess’s Law.
Total enthalpy change for a reaction is the same, no matter the route taken.
How do you calculate enthalpy change using bond enthalpies?
ΔH = bonds broken – bonds formed.
Why is bond breaking endothermic?
Energy must be supplied to break bonds.
Why is bond formation exothermic?
Energy is released when new bonds form.
Explain why experimental enthalpy values differ from theoretical values using bond enthalpies.
Bond enthalpies are average values from many compounds, so actual bonds may differ slightly.
What is the unit of enthalpy change?
Kilojoules per mole (kJ/mol).
Describe the reaction profile for an exothermic reaction.
Reactants higher in energy than products; energy released to surroundings.
Describe the reaction profile for an endothermic reaction.
Reactants lower in energy than products; energy absorbed from surroundings.
What is activation energy?
Minimum energy required for a reaction to occur.
How does a catalyst affect activation energy?
Lowers the activation energy without being consumed.
What is the equation for calculating ΔH from temperature change?
ΔH = m × c × ΔT ÷ n (where m = mass, c = specific heat capacity, ΔT = temperature change, n = moles).
