Enthalpy Change/ Bond enthalpies

Question 1

Enthalpy change definition?

Answer 1

• Heat energy change in a reaction measured under constant pressure.

Question 2

Units for enthalpy change.

Answer 2

kJmol⁻¹

Question 3

Symbol for standard enthalpy change. Explain different aspects of the symbol.

Answer 3

• ∆HƟ (underground sign will be superscript.)
• The underground sign means that the substance was in standard state/ under standard conditions.

Question 4

What are standard conditions?

Answer 4

• 298K (25⁰C)
• 100kPa

Question 4

What is meant by “standard enthalpy changes?”

Answer 4

• Changes under standard conditions.

Question 5

What are endothermic vs exothermic reactions?

Answer 5

• Endothermic: reactions that take in heat.
• Exothermic: reactions that release heat.

Question 6

Give 2 examples of endothermic reaction

Answer 6

• Thermal decomposition
• Photosynthesis

Question 7

Enthalpy def

Answer 7

• Measure of heat content of substance at constant pressure.

Question 8

Give 2 examples of exothermic reactions

Answer 8

• Neutralisation
• Combustion reactions

Question 9

Energy of products/ reactants in exothermic vs endothermic reactions.

Answer 9

• Exo: Products lower in energy than reactants.
• Endo: Products higher in energy than reactants.

Question 10

What is enthalpy change like in exo vs endothermic reactions?

Answer 10

• Exo: enthalpy change is negative.
• Endo: enthalpy change is positive.

Question 11

What process is bond making vs bond breaking?

Answer 11

• Bond making: exothermic
• Bond breaking: endothermic

Question 12

Definition of endothermic reaction in terms of making/ breakind bonds.

Answer 12

• More energy needed to break bonds than energy released when new bonds are formed, reaction is endothermic

Question 13

Definition of exothermic reaction in terms of making/ breakind bonds.

Answer 13

• More energy released when bonds are formed than what was needed to break the initial bonds.

Question 14

True or False

Bonds of the same type will always have the same enthalpy/ heat content.

Answer 14

• False.
• Same bond in different positions of molecule/ in different types of molecule –> different amount of energy required to break the bond.

Question 15

What do enthalpy change calculations use?

Answer 15

• Mean bond enthalpies

Question 16

What is mean bond enthalpy?

Answer 16

• Enthalpy change required to break 1 mole of covalent bonds- in gas- , averaged over a series of compounds that contain the bond.

Question 17

How do we work out enthalpy change of reaction?

Answer 17

Total energy to break bonds - energy released forming bonds.

Question 18

What is bond dissociation enthalpy?

Answer 18

• Enthalpy change required to break 1 mole of covalent bonds of substance in gaseous state.

Question 19

True or False.
N2 has a standard enthalpy of formation of 235kJmol-1

Answer 19

False.
N2 doesn’t have value for standard enthalpy of formation because it is an element (not a compound!)

Question 20

What is the standard enthalpy of combustion? Give its symbol.

Answer 20

• Standard enthalpy of combustion is the enthalpy change when 1 mole of substance completely burns in oxygen - with all substances in their standard states.
• ∆cHƟ (underground sign is superscript. )

Question 21

What is standard enthalpy of formation? Give its symbol.

Answer 21

• ∆fHƟ (underground sign is superscript. )
• Standard enthalpy of formation is enthalpy change required for formation of 1 mole of compound from its elements - elements = in their standard states.