Module 3: Chapter 9 (Enthalpy)

Question 1

What is meant by enthalpy?

Answer 1

Enthalpy (H) is the measure of heat energy present in a chemical system

Question 2

What is meant by enthalpy change?

Answer 2

Enthalpy change (ΔH) is the amount of energy released taken in during a chemical reaction

Question 3

What is meant by an exothermic reaction? And draw the enthalpy profile for one

Answer 3

An exothermic reaction (-ΔH) releases energy into the surroundings

Question 4

What is meant by an endothermic reaction? And draw the enthalpy profile for one

Answer 4

An endothermic reaction (+ΔH) takes in energy from the surroundings

Question 5

What is meant by activation energy?

Answer 5

Activation energy (EA) is the minimum energy required for a reaction to take place. It is the energy input required to break bonds

Question 6

What is meant by standard conditions?

Answer 6

The ΔH of a reaction can vary depending on the conditions used. Data books and tables will always give the ΔH value obtained under standard conditions (⦵)

Question 7

What are the standard conditions?

Answer 7

• 100kPa (close to 1 atm pressure)
• 298 K (25 degrees celsius)
• standard state - the physical state of a substance under standard conditions

Question 8

What is meant by the enthalpy change of reaction?

Answer 8

Enthalpy change of reaction (ΔrH) is the enthalpy change associated with a stated equation in the molar quantities shown in the chemical equation (under standard conditions)

Question 9

What is meant by the enthalpy change of formation?

Answer 9

Enthalpy change of formation (ΔfH) is the enthalpy change associated with the formation of 1 mole of a compound from its elements (under standard conditions)

• the ΔfH for an element will always be 0 as no chemical change is actually taking place

Question 10

What is meant by the enthalpy change of combustion?

Answer 10

Enthalpy change of combustion (ΔcH) is the enthalpy change associated with the complete combustion of 1 moles of a substance (under standard conditions)

Question 11

What is meant by the enthalpy change of neutralisation?

Answer 11

Enthalpy change of neutralisation (ΔneutH) is the enthalpy change associated with the formation of 1 mole of water from a neutralisation reaction between an acid and a base

Question 12

What is always the ΔneutH for the reaction between a strong acid and a strong alkali?

Answer 12

-57 kJ mol^-1

Question 13

What is the equation for energy transferred?

Answer 13

q = m c ΔT

• the specific heat capacity of water is 4.18 Jg^-1k^-1
  MUST DIVIDE BY 1000 TO CONVERT J INTO KJ

Question 14

What is meant by average bond enthalpy?

Answer 14

The energy required to break one mole of a specified type of bond in gaseous molecules
- average bond enthalpies are calculated from actual bond enthalpies found in different environments

Question 15

Bond breaking is ….(?)

Answer 15

endothermic

Question 16

Bond making is ….(?)

Answer 16

exothermic

Question 17

How do you use bond enthalpies to calculate energy released?

Answer 17

1. draw structures of reactants and products in reaction
2. identify the bonds broken and made
3. calculate total energy released
4. add together to find ΔrH

Question 18

What does Hess’s law state?

Answer 18

If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route

Question 19

Draw an enthalpy cycle

Answer 19

(check in textbook)

Question 20

What is the rule if you are calculating enthalpy changes of formation?

Answer 20

Arrows point upwards

elements at the bottom

Question 21

What is the rule if you are calculating enthalpy changes of combustion?

Answer 21

Arrows point downwards

combustion products at the bottom

Question 22

Why are bond enthalpies always endothermic?

Answer 22

Bond enthalpies involve bond breaking which requires energy from the surroundings

Question 23

What is the calculation when using enthalpy changes of formation for (ΔrH =)?

Answer 23

ΔrH = ΣΔfH products - ΣΔfH reactants

Question 24

What is the calculation when using enthalpy changes of combustion (ΔrH =)?

Answer 24

ΔrH = ΣΔcH reactants - ΣΔcH products

Question 25

How do you calculate enthalpy change with bond enthalpies?

Answer 25

ΔrH = Σ(bond enthalpies of reactant) - Σ(bond enthalpies of products)

can also equal ΔfH or ΔcH

Question 26

What do you do if it asks to calculate the enthalpy change of formation with enthalpy of combustion values?

Answer 26

Ignore the value type and treat it as a regular formation way (arrows point up with elements at the bottom)

Question 27

Explain why a calculated enthalpy change may not be a standard enthalpy change?

Answer 27

• not done under standard conditions (100kPa, standard state or at 298K)