M3 C9 ENTHALPY

Question 1

define enthalpy change of formation

Answer 1

enthalpy change of formation is the enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions

Question 2

define enthalpy change of combustion

Answer 2

the enthalpy change when a mole of a substance is fully combusted in oxygen under standard conditions

Question 3

define enthalpy change of reaction

Answer 3

enthalpy change of reaction is the enthalpy change when a reaction occurs according to a balanced chemical equation

Question 4

define Hess’ law

Answer 4

Hess’ law states that if a chemical change can occur by more than one route the overall enthalpy change for each route must be the same

Question 5

what are the standard conditions to calculate enthalpy change (temp, pressure, conc.)

Answer 5

temperature = 298k
pressure = 101 kpa
conc. = 1 mol dm-3

Question 6

define standard bond enthalpy

Answer 6

the average amount of energy needed to break one mole of a particular bond by homolytic fission in gaseous state

Question 7

how do you calculate bond enthalpy changes

Answer 7

energy in - energy out

Question 8

what are the limitations of using bond enthalpies

Answer 8

• bond enthalpies are average values

- actual individual bond enthalpies depend on the chemical environment of the molecule

Question 9

what are the limitations and their effect on calculated enthalpy change when carrying out a calorimetry

Answer 9

• HEAT LOSS, temperature change calculated is too low :. calculated enthalpy change is not negative/exothermic enough
• WATER EVAPORATION, energy is used to break bonds rather than raise the temperature so change in temp calculated is too low :. enthalpy change calculated is not exothermic enough
• INCOMPLETE COMBUSTION OF FUEL, mass of fuel completely combusted is overestimated :. moles of fuel overestimated :. enthalpy change of combustion isn’t exothermic enough
• NON STANDARD CONDITIONS, Depends on the conditions whether the enthalpy change calculated is too negative or not negative enough

Question 10

comment on the experimental value of enthalpy change calculated compared to the listed value

Answer 10

experimental value is not exothermic enough due to heat loss (or side reactions)

Question 11

what is the formula to calculate the amount of energy released during a calorimetry

Answer 11

Q= m (g) c AT (k)