Enthalpy (F6examonly)

Question 1

What is enthalpy change (ΔH) and units?

Answer 1

the heat energy transferred in a reaction at constant pressure
units: kJmol-1​

Question 2

What are exothermic reactions in terms of ΔH? How do the surroundings change?

Answer 2

• exothermic reactions give out energy
• the ΔH is negative
• temperature of surroundings increases

Question 3

What are endothermic reactions in terms of ΔH? How do the surroundings change?

Answer 3

• endothermic reactions take in energy
• the ΔH is positive
• temperature of surroundings decreases

Question 4

Construct enthalpy profile diagrams for exothermic and endothermic reactions:

Answer 4

look at notes and do ppqs/test yourself on paper

Question 5

What is activation energy (Ea)?​

Answer 5

the minimum amount of energy required to begin breaking reactant bonds and start a chemical reaction.​

Question 6

What are the standard conditions for enthalpy change?​

Answer 6

temperature: 298 K​
pressure: 100 kPa​
concentration: 1 moldm-3
standard states: the physical states of reactants under standard conditions (solid, liquid, or gas)

Question 7

What is standard enthalpy change of reaction (ΔrH)?​

Answer 7

the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

Question 8

What is standard enthalpy change of formation (ΔfH)?​

Answer 8

the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions

Question 9

What is standard enthalpy change of combustion (ΔcH)?​

Answer 9

the enthalpy change when one mole of a substance is completely burned in oxygen, under standard conditions

Question 10

What is standard enthalpy change of neutralisation (ΔneutH)?

Answer 10

the enthalpy change when an acid and an alkali react together, under standard conditions, to form one mole of water

Question 11

The equation for calculating enthalpy change using q:

Answer 11

ΔH = q/n
- ΔH: enthalpy change (kJmol-1)
- q: heat energy (kJ)
- n: amount of substance (moles)

Question 12

The equation for calculating heat energy, q (and important things to remember):

Answer 12

q = mcΔT
- q: heat energy (J) need to change to kJ if using in ΔH = q/n
- m: mass (g) of water/solution!
- c: specific heat capacity of water (4.18 Jg-1K-1)
- ΔT: change in temperature of the water or solution (K) same as the change in °C

Question 13

What is average bond enthalpy?​

Answer 13

the amount of energy required to break one mole of bonds in the gas phase, averaged over many different compounds (always endothermic, with a +ve enthalpy value)

Question 14

What are exothermic and endothermic reactions in terms of breaking and making chemical bonds?

Answer 14

exothermic = bond making (energy is released)
endothermic = bond breaking (energy is required/taken in)

Question 15

How do you calculate enthalpy change using average bond enthalpies?

Answer 15

break take away make
ΔH = ∑(bond enthalpies in reactants) - ∑(bond enthalpies in products)

Question 16

What is Hess’s law?

Answer 16

• the total enthalpy change is independent of the route taken (the total enthalpy change is always the same, no matter which route is taken)
• therefore, we can work out enthalpy changes indirectly

Question 17

How are Hess cycles used to find enthalpy changes of formation?

Answer 17

draw out the hess cycle on paper
(the elements should be at the bottom)
ΔrH = ΔfH of products - ΔfH of reactants

Question 18

How are Hess cycles used to find enthalpy changes of combustion?

Answer 18

draw out the hess cycle on paper
(the combustion products should be at the bottom)

Question 19

How do you use unfamiliar Hess cycles to find enthalpy changes?

Answer 19

• if going with the arrow, add
• if going against the arrow, subtract
• remember the definitions
• do exam ppqs

Question 20

Describe the practical involving determining enthalpy change of combustion:

Answer 20

1. using a clamp stand, clamp a beaker
2. using a measuring cylinder, measure 150 cm3 of water. pour the water into the beaker
3. record the initial temperature of the water
4. add methanol to the spirit burner and weigh the spirit burner containing the methanol
5. place the spirit burner directly under the clamped beaker. light the burner and burn the methanol whilst stirring the water with the thermometer
6. after about 3 minutes, extinguish the flame. record the maximum temperature reached by the water and calculate the temperature change of water
7. re-weigh the spirit burner containing the methanol to determine the mass of fuel burnt
8. calculate the energy change using q = mcΔT
9. calculate the ΔH using q

Question 21

Describe the practical involving determining enthalpy change of reaction:

Answer 21

1. pipette 25.0 cm3 of 1.00 moldm-3 CuSO4 into a polystyrene cup
2. weigh out an excess of zinc powder
3. start a stop-clock and take the temperature of the CuSO4 solution every minute for 3 min
4. on the 4th minute, add the zinc to the solution and stir the mixture. do not measure the temperature
5. continue measuring the temperature from the 5th minute every minute until the temperature stays constant
6. plot a graph of temperature against time
7. extrapolate the cooling curve back to the time when the zinc was added to determine the temperature of the solution at the moment the zinc was added. this corrects for cooling
8. calculate the temperature change and use that to calculate q
9. using q, calculate ΔH

Question 22

Sources of error: enthalpy change

Answer 22

• Common sources of error:
• Heat loss - Heat escaping to the surroundings means less is absorbed by the water, leading to a lower measured enthalpy change.
• Incomplete combustion - When fuel doesn’t burn completely, less heat is produced, resulting in a lower calculated enthalpy change.
• Fuel evaporation - If some fuel evaporates before burning, it reduces the amount of heat generated, thereby decreasing the measured enthalpy change.