2.8.1 Enthalpy Changes

Question 1

What does ‘enthalpy’ mean?

Answer 1

Enthalpy is the term used to describe the heat energy content of a system

Question 2

What is H?

Answer 2

H is enthalpy change

Question 3

How can enthalpy change be calculated?

Answer 3

It can be calculated from measuring changes in temperature

Question 4

What is enthalpy change defined as?

Answer 4

‘The heat energy change under measured conditions of constant pressure’

Question 5

What does H equal?

Answer 5

H = H(products) - H(reactants)

Question 6

What sign is the H if the reaction is exothermic?

Answer 6

-H

Question 7

What sign is the H if the reaction is endothermic?

Answer 7

+H

Question 8

What is standard pressure?

Answer 8

100kPa

Question 9

What are the conditions for standard enthalpy change?

Answer 9

• 100kPa

- the substances are in their standard States

Question 10

What are the units for H?

Answer 10

KJmol-1

Question 11

What is the definition of standard enthalpy change of formation?

Answer 11

The enthalpy change when a molecule or compound is formed from its elements in their standard states

Question 12

What is the equation for the formation of magnesium oxide?

Answer 12

Mg(s) + 1/2O2(g) ➡️ MgO(s)

Question 13

What is the definition for enthalpy change of combustion?

Answer 13

The enthalpy change when one mole of substance is completely burned in oxygen under standard conditions

Question 14

What is the equation for the combustion of ethane?

Answer 14

C2H6(g) + 7/2O2(g) ➡️ 2CO2(g) + 3H2O(l)

Question 15

What do chemical reactions involve?

Answer 15

• the breaking of bonds between atoms

- the making of new bonds between atoms