3.3 - chemical energy Flashcards
what is enthalpy?
a measure of the chemical energy in a substance
what are endothermic reactions?
a reaction which absorbs heat from its surroundings (temperature decreases)
what are exothermic reactions?
a reaction which gives out heat energy (temperature increases)
what is the symbol for enthalpy change?
ΔH
what is the formula for enthalpy change?
ΔH = Hp - Hr
what is an activated complex?
unstable intermediate: bonda re being broken and made
what is activation energy?
minimum energy required to form an activated complex
ΔH is negative?
exothermic reaction
ΔH is positive?
endothermic reaction
what does a catalyst do to the activation energy?
a catalyst will lower the value of the activation energy (Ea) but will not affect ΔH for the reaction
what are industrial considerations regarding endothermic and exothermic reactions?
- endothermic reactions can be expensive as heat energy must be supplied to maintain the reaction
- exothermic: heat may need to be removed to prevent thermal explosion
what is enthalpy of combustion?
enthalpy change when one mole of a substance completely combusts in oxygen
what are the three different routes which enthalpy can be calculated by?
- experimental date
- hess’s law
- bond enthalpy
how does a bomb calorimeter allow the enthalpy of combustion to be accurately determined?
- no incomplete combustion due to pure oxygen
- no loss of fuel through evaporation
- water completely surrounds burning fuel: no heat lost to surroundings
what does hess’s law state?
the enthalpy change for a chemical reaction is independent of the route taken
what does hess’s law state (in symbol form)?
ΔH1 = ΔH2 + ΔH3 = ΔH4 + ΔH5 + ΔH6
what is molar bond enthalpy?
the energy required to break or make one mole of bonds in a diatomic molecule
what is mean bond enthalpy?
the average energy required to break one mole of bonds, for a bond that occurs in a number of compounds
bond breaking?
endothermic (positive)
bond making?
exothermic (negative)
