Bond Enthalpies - Thermodynamics Pt A (2.2)

Question 1

How do you measure enthalpy changes? Explain this technique

Answer 1

Calorimetry:

• Using a nested insulated cup with the reaction mixture inside and an insulated stopper (lid)
• Put the thermometer through the hole in the lid to measure the temp. before the reaction
• Have the stirrer also put through the hole of the lid
• Once you add all the reactants close the lid & stir
• At the end of the reaction check the temp. to work out temp. change

Question 2

What is specific heat capacity?

Answer 2

The energy required to raise the temperature of 1g of the liquid by 1 degree

Question 3

What symbol/letter represents specific heat capacity?

Answer 3

c

Question 4

Give the equation for enthalpy change using temperature change

Answer 4

ΔH=mcΔT

Question 5

Does the units of temperature in the enthalpy change equation matter?

Answer 5

No - since you are working out temperature change (it would be the same difference in either degrees or kelvin)

Question 6

Name the two methods of calculating theoretical enthalpy changes

Answer 6

1. Bond enthalpies

2. Enthalpy (Hess) cycles

Question 7

Is bond breaking exothermic or endothermic?

Answer 7

Endothermic (as it always absorbs energy)

Question 8

Is bond making exothermic or endothermic?

Answer 8

Exothermic (as it always releases energy)

Question 9

What is the enthalpy change equation when using bond dissociation enthalpies?

Answer 9

ΔH = Σ bonds broken – Σ bonds formed

therefore… energy in-energy out

Question 10

Define bond dissociation energy

Answer 10

The enthalpy change when 1 mole of covalent bonds is dissociated into gaseous atoms

Question 11

What is mean dissociation enthalpy?

Answer 11

The value of the bond dissociation enthalpy averaged across many compounds containing that bond

Question 12

Why is the enthalpy change calculated using bond dissociation enthalpies theoretical?

Answer 12

• Mean bond dissociation enthalpies are just an approximation
• The amount of energy required to break a bond is dependent on the rest of the molecule

Question 13

Give the 4 steps of calculating enthalpy changes using bond enthalpies

Answer 13

1. Draw out the molecules showing all the covalent bonds
2. Multiply the mean bond enthalpy by the number of each bond
3. Sum up all the bonds on each side of the reaction
4. Calculate ΔH: ΔH = Σ bonds broken – Σ bonds formed

Question 14

Give a limitation of calculating theoretical enthalpy changes using bond enthalpies

Answer 14

• Only works for covalent molecules
• Requires you to know all the bond enthalpies
• Uses average data