(c) Chemical energy

Question 1

What is enthalpy?

Answer 1

A measure of the chemical energy in a substance.

Question 2

What is the enthalpy change? How is it denoted?

Answer 2

It is written as delta H

It is the difference in potential energy between reactants and products.

Question 3

What unit is used when calculating enthalpy?

Answer 3

Kilojoules per mole.

Question 4

What is the formula for enthalpy change?

Answer 4

Enthalpy of products- Enthalpy of reactants.

Question 5

What is delta H always in exothermic reaction?

Answer 5

Negative.

Question 6

Why is delta H always negative in exothermic reaction?

Answer 6

In exothermic reactions the energy content of the products is less than that of the reactants.

Question 7

What is delta H always in endothermic reactions?

Answer 7

Positive.

Question 8

Why is delta H always positive for endothermic reactions?

Answer 8

The energy content of the reactants is less than that of the products.

Question 9

What is the enthalpy of combustion?

Answer 9

The energy released when one mole of a substance burns completely in oxygen.

Question 10

How is enthalpy of combustion denoted?

Answer 10

Delta H.

Question 11

What is the method used to measure enthalpy of combustion?

Answer 11

Burn a substance and use the heat produced to heat water.

Question 12

What is the formula to work out enthalpy change?

Answer 12

Eh=c m DELTA T

Question 13

What does Eh represent in the formula for enthalpy change?

Answer 13

Energy in kJ.

Question 14

What does c represent in the formula for enthalpy change?

Answer 14

The specific heat capacity of water.

Question 15

What is the specific heat capacity of water?

Answer 15

4.18 kg-1 degreesC-1.

Question 16

What does m represent in the formula for enthalpy change?

Answer 16

Mass of water in kg.

Question 17

What does delta T represent in the formula for enthalpy change?

Answer 17

Temperature rise.

Question 18

What does Hess’s law state?

Answer 18

The enthalpy change for a reaction is the same irrespective of the route taken.

Question 19

What is the use of Hess’s Law

Answer 19

It can be used to calculate enthalpy changes which are difficult or impossible to determine by experiment.

Question 20

What is molar bond enthalpy?

Answer 20

The energy required to break one mole of bonds in a diatomic molecule.

Question 21

What is mean bond enthalpy?

Answer 21

The average energy required to break one mole of bonds, for a bond which occur in a number of compounds.

Question 22

Why do mean bond enthalpies have to be calculated?

Answer 22

The value of bonds will vary depending on what environment they are in.

Question 23

What can bond enthalpies be used for?

Answer 23

To estimate the enthalpy change occurring in a gas phase reaction.

Question 24

What type of process is bond making?

Answer 24

Exothermic.

Question 25

What type of process is bond breaking?

Answer 25

Endothermic.

Question 26

What will all values of bond breaking therefore be?

Answer 26

Positive.

Question 27

What will all values of bond making therefore be?

Answer 27

Negative.

Question 28

Why is the value of enthalpy of combustion in the data booklet much higher than the one obtained when in the lab?

Answer 28

1. Heat loss to surroundings.
2. Incomplete combustion.
3. Evaporation of alcohol.

Question 29

What are the products of incomplete combustion?

Answer 29

carbon monoxide and water.

Question 30

What are the products of complete combustion?

Answer 30

Carbon dioxide plus water.

Question 31

Why is the copper tube coiled when performing and enthalpy of combustion experiment?

Answer 31

To redistribute heat.

Question 32

What are the 5 values that need to be taken in an enthalpy of combustion experiment?

Answer 32

Mass of water
Mass of ethanol before burning
Mass of ethanol after burning
Temperature of water before
Temperature of water after.