3.2.1 Enthalpy Changes

Question 1

What is enthalpy?

Answer 1

A measure of the internal heat energy of a chemical system

Question 2

What is an exothermic change?

Answer 2

An energy transfer from the system to the surroundings

Question 3

What are the characteristics of an exothermic enthalpy change?

Answer 3

• Negative ΔH
• Temperature of surroundings increases

Question 4

What is an endothermic change?

Answer 4

An energy transfer from the surroundings to the system

Question 5

What are the characteristics of an endothermic enthalpy change?

Answer 5

• Positive ΔH
• Temperature of surroundings decreases

Question 6

What is activation energy?

Answer 6

The minimum energy input required for a reaction to take place

Question 7

What are the standard conditions?

Answer 7

• 298K
• 1atm (100-101kPa)
• 1moldm-3
• Standard state

Question 8

What is a substance’s standard state?

Answer 8

Its physical state under standard conditions

Question 9

What is enthalpy change of reaction?

Answer 9

Enthalpy change associated with a stated equation (ΔrH⦵)

Question 10

What is enthalpy change of formation?

Answer 10

Formation of 1 mol of a compound from its elements (ΔfH⦵)

Question 11

What is the enthalpy change of formation of an element?

Answer 11

Always 0

Question 12

What is enthalpy change of combustion?

Answer 12

Complete combustion of 1 mol of a substance (ΔcH⦵)

Question 13

What is enthalpy change of nuetralisation?

Answer 13

Formation of 1 mol of water from neutralisation (ΔneutH⦵)

Question 14

What is the standard value for enthalpy change of neutralisation?

Answer 14

H+ (aq) + OH- (aq) -> H2O (l)
-57kJmol-1

Question 15

What is the formula for energy transferred?

Answer 15

q = m x c x ΔT

Question 16

What is the formula for ΔH?

Answer 16

ΔH = kJ/mol

Question 17

How do you determine enthalpy change of reaction?

Answer 17

• React substances in a polystyrene foam cup with a thermometer
• Cup provides insulation
• Record mass of solution

Question 18

How do you determine enthalpy change of combustion?

Answer 18

• Use a spirit burner to heat a beaker of water with a thermometer
• Record change in mass of spirit burner
• Record mass of water

Question 19

Why is the experimental value for enthalpy change of combustion usually lower than the theoretical version?

Answer 19

• Heat loss to surroundings
• Incomplete combustion of fuel
• Evaporation of fuel from the wick
• Non-standard conditions

Question 20

How can the limitations of the spirit burner experiment be minimised?

Answer 20

• Use draught screens
• Input O2 gas

Question 21

What is average bond enthalpy?

Answer 21

The breaking of 1 mol of bonds in gaseous molecules

Question 22

What are the characteristics of bond enthalpy?

Answer 22

• Always endothermic
• +ΔH

Question 23

What is bond breaking?

Answer 23

• Endothermic
• Energy is required

Question 24

What is bond making?

Answer 24

• Exothermic
• Energy is released

Question 25

When will a reaction be exothermic in terms of bond energies?

Answer 25

When more energy is released forming bonds than is required to break bonds

Question 26

When will a reaction be endothermic in terms of bond energies?

Answer 26

When more energy is required to break bonds than is released forming bonds

Question 27

What is Hess' law?

Answer 27

Enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same