[ 1 & 2 ] 3.1.4 — Energetics Flashcards
What is an exothermic reaction?
A reaction that releases heat because the products are more stable and have lower potential energy than the reactants.
What happens to temperature in an exothermic reaction?
It increases due to increased kinetic energy.
What is an endothermic reaction?
A reaction that absorbs heat because the products are less stable and have higher potential energy than the reactants.
What happens to temperature in an endothermic reaction?
It decreases due to decreased kinetic energy.
What does ΔH represent?
The enthalpy change during a chemical reaction.
What is the sign of ΔH for an exothermic reaction?
Negative (ΔH = -ve)
What is the sign of ΔH for an endothermic reaction?
Positive (ΔH = +ve)
What are the standard conditions for measuring enthalpy changes?
100 kPa pressure and 298 K temperature.
What is the unit of enthalpy change?
kJ mol⁻¹
What is the standard enthalpy of formation?
ΔH when 1 mole of a compound forms from its elements in standard states.
What is the standard enthalpy of combustion?
ΔH when 1 mole of a substance burns in excess oxygen under standard conditions.
What is the standard enthalpy of formation of an element in its standard state?
Zero.
What is the formula for calculating heat energy (q)?
q = mcΔT or q = VρcΔT
What are the specific heat capacity and density of water?
c = 4.18 Jg⁻¹K⁻¹, ρ = 1.0 gcm⁻³
How do you determine ΔH from q?
Divide q by the number of moles of limiting reagent.
What is a mean bond enthalpy?
Average energy needed to break one mole of a specific bond in the gas phase.
Why are mean bond enthalpies only approximate?
Because bond strength varies with molecular environment and ignores intermolecular forces.
What is the formula using bond enthalpies to calculate ΔH?
ΔH = Σ(bonds broken) - Σ(bonds formed)
What is atomisation energy?
Energy required to convert one mole of a substance into gaseous atoms.
What is Hess’ Law?
The total enthalpy change of a reaction is the same, regardless of the path taken.
What is the Hess’ Law formula using formation enthalpies?
ΔH = ΣΔHf(products) - ΣΔHf(reactants)
What is the Hess’ Law formula using combustion enthalpies?
ΔH = ΣΔHc(reactants) - ΣΔHc(products)
Why is combustion usually exothermic?
Because burning substances release heat and form stable oxides.
