3.2.1 - Enthalpy Changes

Question 1

What is enthalpy change?

Answer 1

The heat energy transferred in a reaction at constant pressure.

Question 2

What happens in exothermic reactions?

Answer 2

Energy is given out, ΔH is negative.

Question 3

Equation for exothermic reaction.

Answer 3

C6H12O6 + 6O2 -> 6CO2 + 6H20

Question 4

What happens in endothermic reactions?

Answer 4

Energy is absorbed, ΔH is positive.

Question 5

Equation for an endothermic reaction.

Answer 5

6CO2 + 6H20 -> 6O2 + C6H12O6

Question 6

In an endothermic reaction what is more stable?

Answer 6

Reactants

Question 7

In an exothermic reaction what is more stable?

Answer 7

Products.

Question 8

On an enthalpy profile diagram what are the names of the axis?

Answer 8

Reaction pathway and enthalpy / kJmol-1

Question 9

What are the standard conditions?

Answer 9

298K, 1atm, 1moldm-3

Question 10

Standard enthalpy change of formation?

Answer 10

1 mole of product must be formed.

Question 11

Standard enthalpy change of combustion?

Answer 11

1 mole of reactant must be combusted or burnt.

Question 12

Standard enthalpy change of neutralisation?

Answer 12

1 mole of water must be formed.

Question 13

Bond breaking is?

Answer 13

Endothermic

Question 14

Bond making is?

Answer 14

Exothermic

Question 15

q=

Answer 15

mcΔT

Question 16

What does q stand for?

Answer 16

Enthalpy change / heat lost or gained.

Question 17

Average bond enthalpy?

Answer 17

Breaking 1 mole of bonds in gaseous molecules.

Question 18

Why isn’t it always possible to measure the enthalpy change of a reaction in a lab?

Answer 18

• the reaction may have a high activation energy.
• the rate of reaction may be very low.
• more than one reaction could be taking place (side reaction)

Question 19

In enthalpy of combustion which way do the arrows point?

Answer 19

Downwards.

Question 20

What are the products of enthalpy of combustion?

Answer 20

CO2 and H2O.

Question 21

What cannot be combusted?

Answer 21

Water, CO2 and oxygen.

Question 22

Which way do the arrows point in enthalpy of formation?

Answer 22

Upwards.

Question 23

What doesn’t need to be considered during an enthalpy of formation?

Answer 23

Elements.

Question 24

What can be reacted during an unfamiliar enthalpy cycle?

Answer 24

HCl