Energetics

Question 1

Define the term enthalpy change

Answer 1

the energy change of a reaction per mole at a constant pressure

Question 2

What are the standard conditions for an enthalpy change to be
measured under?

Answer 2

298 K, 100kPa

Question 3

Define the term standard enthalpy change of formation

Answer 3

The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states (under standard conditions)

Question 4

Define the term standard enthalpy change of combustion

Answer 4

the enthalpy change when one mole of a substance is completely burned in oxygen, all products and reactants in standard state

Question 5

Define the term mean bond (dissociation) enthalpy

Answer 5

Then enthalpy change when one mole of covalent bonds in the gaseous state are completely broken

Question 6

Why are calculations from mean bond enthalpies often different
from those calculated from Hess’ Law (theoretical values)

Answer 6

Bond enthalpies are a mean average taken over a range of compounds

Question 7

How is enthalpy change (H) calculated from enthalpy change of
formation data? (H f )

Answer 7

H = SumH f (products) - H f (reactants)

Question 8

How is enthalpy change (H) calculated from enthalpy change of
combustion data? (H c )

Answer 8

H = SumH c (reactants) - H c (products)

Question 9

How is enthalpy change (H) calculated from bond enthalpy data?

Answer 9

H =  bond enthalpy (reactants) -  bond enthalpy (products)

Question 10

equation for energy change

Answer 10

energy change = mass of solution x heat capacity x temperature change
Q (J) = m (g) x cp (J g-1K-1) x T ( K)

Question 11

Explain why calculations from calorimetry experiments are often
lower than theoretical values

Answer 11

1) heat lost to the surroundings
2) incomplete combustion (of fuels)
3) evaporation (of fuels)

Question 12

How to improve accuracy of calorimetry experiments due to heat
loss?

Answer 12

1) better insulation (polystyrene cup)
2) heat shield (for fuels)

Question 13

How should temperature change for a calorimetry experiment for
an aqueous reaction be calculated?

Answer 13

1) measure start temperature every minute for 4
minutes
2) mix reactants
3) record temperature every minute for 10 minutes
4) plot a graph and regress line of temperature
change to find T

Question 14

What is Hess’ law?

Answer 14

The enthalpy change of a reaction is the same, regardless of the route taken

Question 15

What is bond enthalpy data?

Answer 15

an averaged value representing the energy required to break one mole of the bond stated in a gaseous state under standard conditions

Question 16

In a calorimetry experiment, what do we do to counteract the effect of simultaneous cooling and temp. rise on our results

Answer 16

-take readings at regular time intervals
-extrapolate the temperature curve/line back to the time
the reactants were added together

Question 17

Calculate enthalpy change from a calorimetry experiment

Answer 17

H = q / moles reacted (smallest number of moles present)