3c) - Chemical Energy

Question 1

What does Hess’s law state

Answer 1

Hess’s law states that the enthalpy change for a chemical reaction is independent of the route taken

Question 2

What is the combustion of alcohols

Answer 2

Alcohols combust in oxygen to produce carbon dioxide and water

Question 3

Which type of reaction is combustion

Answer 3

Combustion is an exothermic reaction

Question 4

Complete combustion

Answer 4

Hydrocarbons and alcohols burn in a plentiful supply of oxygen to produce carbon dioxide and water

Question 5

Enthalpy

Answer 5

Enthalpy is a measure of the chemical energy in a substance

Question 6

Exothermic

Answer 6

A reaction or process that releases heat energy is described as exothermic. In industry, exothermic reactions may require heat to be removed to prevent temperature rising.

Question 7

Endothermic

Answer 7

A reaction or process that taken in heat energy is described as endothermic. In industry endothermic reactions may incur costs in supplying heat energy in order to maintain the reaction rate.

Question 8

What is the enthalpy of combustion

Answer 8

The enthalpy of combustion of a substance is the amount of energy given out when one mole of a substance burns in excess oxygen

Question 9

Draw a diagram labelled showing the experimental procedure for enthalpy of combustion of alcohols

Answer 9

-thermometer
-copper can
-water
-clamp
-draught shield
-spirit burner
-liquid fuel

Question 10

Possible source of error in the enthalpy of combustion of alcohols exp

Answer 10

• heat lost to surroundings
• stir thermometer to ensure temperature is the same

Question 11

Enthalpy of solution experiment

Answer 11

-thermometer
-water
-polystyrene cup (acts as an insulator/poor conductor)
-solute

Question 12

For combustion of hydrocarbons

Answer 12

Balance in alphabetical order (C,H,O)

Question 13

What is molar bond enthalpy

Answer 13

The molar bond enthalpy is the energy required to break one mole of bonds in a diatomic molecule

Question 14

What is the mean molar bond enthalpy

Answer 14

A mean molar bind enthalpy is the avergae energy required to break one mole of bonds for a bond that occurs in a number of compounds

Question 15

Which process is bond breaking

Answer 15

Bond breaking is an endothermic process

Question 16

Which process is bond making

Answer 16

Bond making is an exothermic process

Question 17

What is the energy required to Make one mole of bonds

Answer 17

The energy required to make one mole of bonds is the same as the bond enthalpy, but has a negative value

Question 18

What is the method of calculating enthalpy of combustion when also given hydrocarbon information

Answer 18

Calculate enthalpy of combustion using specific heat capacity, mass of water, temperature change
Calculate number of moles
Divide answer to first step by number of moles to get enthalpy combustion

Question 19

Enthalpy sublimation

Answer 19

C(s) —— C(g) bottom of data booklet