Energy and enthalpy changes

Question 1

what is an open system ?

Answer 1

both energy and matter can exchange with the surroundings

Question 2

what is a closed system ?

Answer 2

energy but hot matter can exchange with the surroundings

Question 3

what is enthalpy ?

Answer 3

• 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

Question 4

what is enthalpy change ?

Answer 4

the overall change exchanged with the surroundings when a change happens at constant pressure and the final temperature is the same as the starting temperature
- units: kJ mol -1

Question 5

what is ∆Hᶱ ?

Answer 5

• the greek letter ‘delta’ means change in
• ᶱ means the substance was in standard state under standard conditions

Question 6

what are the standard conditions ?

Answer 6

• 100 kPa pressure
• 298K (25 degrees celsius)
  solution of a concentration with 1 mol dm -3

Question 7

exothermic reactions

Answer 7

• release heat energy to surroundings
• ∆H is -ve
• temp goes uo

Question 8

endothermic reactions

Answer 8

• absorb heat energy from the surroundings
• ∆H is +ve
• temp. goes down

Question 9

Describe enthalpy profile diagrams for exothermic reactions

Answer 9

• enthalpy of products < enthalpy of reactants
• reactions releases heat so heat LOSS from system to surroundings

Question 10

Describe enthalpy profile diagrams for endothermic reactions

Answer 10

• enthalpy of products > enthalpy of reactants
• reaction absorbs heat
• heat gain from surroundings