Edexcel A level Chemistry Topic 8:
Energetics I questions and answers
What is enthalpy? - - the thermal energy that's stored in a system
- can't measure enthalpy of products & reactants directly so instead measure
amount of energy absorbed or released to the surroundings
what is enthalpy change? - - the heat change in a reaction at a constant pressure.
- generally equated as the H(products) - H(reactants)
What is ∆Hᶱ ? - enthalpy change under standard conditions and with elements in
their standard states
- units kJmol-1
What are the standard conditions? - - 100kPa
-298K
- solutions at a concentration of 1 moldm-3
Exothermic reactions? - - release heat energy to surroundings
- ∆H is -ve
- temp goes UP
, Edexcel A level Chemistry Topic 8:
Energetics I questions and answers
Endothermic reactions? - - absorb heat energy from the surroundings
- ∆H is +ve
- temp. goes DOWN
Describe enthalpy profile diagrams for exothermic rtns - - enthalpy of products <
enthalpy of reactants
- rtn releases heat so heat LOSS from system to surroundings
Describe enthalpy profile diagrams for endothermic rtns - - enthalpy of products >
enthalpy of reactants
- reaction absorbs heat
- heat gain from surroundings
Define ∆rHᶱ - enthalpy change when the molar quantities of reactants, as stated in
the chemical eqn, react under standard conditions
Define ∆fHᶱ - enthalpy change when one mole of a substance is formed from its
constituent elements in their standard states under standard conditions
Energetics I questions and answers
What is enthalpy? - - the thermal energy that's stored in a system
- can't measure enthalpy of products & reactants directly so instead measure
amount of energy absorbed or released to the surroundings
what is enthalpy change? - - the heat change in a reaction at a constant pressure.
- generally equated as the H(products) - H(reactants)
What is ∆Hᶱ ? - enthalpy change under standard conditions and with elements in
their standard states
- units kJmol-1
What are the standard conditions? - - 100kPa
-298K
- solutions at a concentration of 1 moldm-3
Exothermic reactions? - - release heat energy to surroundings
- ∆H is -ve
- temp goes UP
, Edexcel A level Chemistry Topic 8:
Energetics I questions and answers
Endothermic reactions? - - absorb heat energy from the surroundings
- ∆H is +ve
- temp. goes DOWN
Describe enthalpy profile diagrams for exothermic rtns - - enthalpy of products <
enthalpy of reactants
- rtn releases heat so heat LOSS from system to surroundings
Describe enthalpy profile diagrams for endothermic rtns - - enthalpy of products >
enthalpy of reactants
- reaction absorbs heat
- heat gain from surroundings
Define ∆rHᶱ - enthalpy change when the molar quantities of reactants, as stated in
the chemical eqn, react under standard conditions
Define ∆fHᶱ - enthalpy change when one mole of a substance is formed from its
constituent elements in their standard states under standard conditions
