Energetics, Kinetics, Thermodynamics and Redox

Question 1

State the meaning of the term “Standard enthalpy of combustion”

(2 marks)

Answer 1

• Enthalpy change when 1 mole of a substance burns completely in oxygen
• With all substances in their standard states (stated temp and 100kPa)

Question 2

The enthalpy of combustion determined experimentally is less than that calculated using enthalpies of formation

Give one possible reason for this, other than heat loss.

(1 mark)

Answer 2

• Incomplete combustion
• Evaporation of fuel
• Not completed under standard conditions

Question 3

Define the term enthalpy change

(1 mark)

Answer 3

• Heat energy change at constant pressure

Question 4

Explain why a value given for O=O bond enthalpy is not a mean value

(1 mark)

Answer 4

• Oxygen/ O2 is the only substance with an O=O bond

Question 5

When measuring the bond enthalpy of a reaction suggest 2 reasons why using a glass beaker on a tripod and gauze, instead of a clamped copper calorimeter

(2 marks)

Answer 5

Any 2 from:
- Glass is a poorer conducter than copper
- Tripod and gauze would reduce heat transfer
- Tripod and gauze would have a fixed height above the flame

Question 6

Suggest one way to improve the accuracy of the enthalpy value obtained

(1 mark)

Answer 6

• Use a wind sheild
• Use a lid
• Insulate sides of calorimeter

Question 7

A student determines the enthalpy change for the reaction between
calcium carbonate and hydrochloric acid.

The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be
improved to provide more accurate data.
Describe how this data from the improved method can be used to
determine an accurate value for the temperature change.

(6 marks)

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
* measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid
using a measuring cylinder and pour the acid into a 100 cm3 glass
beaker
* weigh out 2.50 g of solid calcium carbonate on a watch glass and tip
the solid into the acid
* stir the mixture with a thermometer
* record the maximum temperature reached.

Answer 7

Step 1: Apparatus:
- Use a burette/ pipette instead of measuring cylinder
- Use a polystyrene cup/ insulate beaker
- Reweigh the watch glass after adding the solid
- Use powdered solid

Step 2: Temperature measurements:
- Measure and record the initial temperature of the solution for a few minutes before addition
- Measure and record the temperature at regular intervals until a trend is observed

Step 3: Temperature determination
- Plot a graph temperture against time
- Extrapolate to the point of addition
- Determine temp change at point of addition

Question 8

In an enthalpy change experiment, how could the percentage uncertainty in temperature change can be decreased.

(1 mark)

Answer 8

Increase the concentrations of the solutions

Question 9

In a Maxwell Boltzmann curve, explain why the curve starts at the origin

(1 mark)

Answer 9

No molecules with no energy

Question 10

What does the highest point on a Maxwell-Boltzmann curve represent

(1 mark)

Answer 10

Most probable energy

Question 11

Explain, in general terms, how a catalyst increases rate of reaction

(2 marks)

Answer 11

• Provides a different pathway for a reaction
• With a lower activation energy

Question 12

The preperation of hydrogen on an industrial scale using the reversible reaction between methane and steam is done at a temperature of 800 degrees and a low pressure of 300 kPa in the prensence of a nickel catalyst.

Explain, in terms of equillibrium yield and cost, why these conditions are used.

(6 marks)

Answer 12

Stage 1: Temperature
- The reaction is endothermic
- Higher temperature increases yield
- High temperatures are costly

Stage 2: Pressure
- More moles of gas on the right hand side (equillibium shifts to Right)
- So lower pressure increases the yield
- Low pressure= low cost

Stage 3: Catalyst
- A catalyst has no effect on yield
- Adding a catalyst increases ROR so lower temp can be used
- Lowers the cost

Question 13

State the effect, if any, of the addition of a catalyst on the value of Kp for this equillibrium. Explain your answer

(2 marks)

Answer 13

• No effect
• Increases rate of forward and bakward reaction equally

Question 14

For a reaction at 1050K, Kp=7.62x10^5
For a reaction at 500K, Kp=3.94x10^4

Explain how this information can be used to deduce that the forward reaction is endothermic

(2 marks)

Answer 14

• Kp is higher at higher temperature
• At higher temperature equillibrium shifts to the right

Question 15

An enthalpy change for a reaction is -92kJmol-1
State the effect, if any, of an inrease in temperature on the value of Kp

(3 marks)

Answer 15

• Decrease
• Equillibrium shifts in the endothermic direction (left)

Question 16

CO2(g) + 3H2(g) <-> CH3OH(g) + H2O(g) ∆H = −49 kJ mol−1
Use Le Chateliers principle to justify why the reaction is carried out at a high pressure rather than at an atmospheric pressure

(3 marks)

Answer 16

• 4 mol of gas form 2 mol
• At high pressures equillibrium shifts to the right to oppose increase in pressure
• This increases the yield of methanol

Question 17

Suggest why a catalyst may be less efficient if the reagents contain impurities

(1 mark)

Answer 17

Impurities block the active sites

Question 18

Explain why H2O is not included in the Kp expression

(1 mark)

Answer 18

H2O conc is constant

Question 19

Explain why the value of Kw increases as temperature increases

(2 marks)

Answer 19

• Equillibrium is endothermic (in forward reaction)
• Equillibrium shifts to the right to oppose change in temperature