Energetics

Question 1

Define enthalpy change

Answer 1

Heat energy transferred in reaction at constant pressure

Question 2

Is breaking bonds endo/exo?

Answer 2

Endothermic

Question 3

Is making bonds endo/exo?

Answer 3

Exothermic

Question 4

Bond enthalpy equation

Answer 4

Reactants - Products

Question 5

Outline why enthalpy change calculated by bond enthalpies are less accurate than enthalpy change calculated using Hess’ law (2)

Answer 5

• Bond enthalpies are mean/average values
• From a range of compounds

Question 6

Define standard enthalpy of formation

Answer 6

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 7

Define standard enthalpy of combustion

Answer 7

Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

Question 8

Percentage apparatus error

Answer 8

(100 x margin of error) / amount measured

Question 9

Experimental error

Answer 9

(real ans - exp ans) / real ans

= ans x 100

Question 10

Suggest one improvement that would reduce errors due to heat loss in the student’s experiment (1)

Answer 10

• Add a lid
• Reduce distance between flame + beaker

Question 11

Define Hess’ law

Answer 11

Total enthalpy change of a reaction is independent of the route taken

Question 12

Enthalpy of formation equation

Answer 12

Products - Reactants

Question 13

Enthalpy of combustion equation

Answer 13

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