P- Energetics =)

Question 1

The enthalpy of combustion determined experimentally is less exothermic than that calculated using enthalpies of formation.
Give one possible reason for this, other than heat loss. (1)

Answer 1

Incomplete combustion

Question 2

Define the term enthalpy change. (1)

Answer 2

Heat change at constant pressure

Question 3

Explain why the value given for the O=O bond enthalpy in part (b) is not a mean value. (1)

Answer 3

O2 is the only substance that has O=O bond

Question 4

The student considered using a glass beaker on a tripod and gauze instead of the clamped copper calorimeter.
Suggest two disadvantages of using a glass beaker on a tripod and gauze. (2)

Answer 4

1. tripod & gauze- ↓ heat transfer
2. glass- poorer conductor than copper
3. tripod & gauze- fixed height above flame

Question 5

Copper calorimeter-
Suggest one addition to this apparatus that would improve the accuracy of the enthalpy value obtained. (1)

Answer 5

• use a windshield- ↓ heat loss
• // use a lid
• insulate sides of calorimeter

Question 6

State Hess’s Law. (1)

Answer 6

• enthalpy change of a reaction is
• independent of the route taken

Question 7

Adding water to anhydrous copper (II) sulfate to form copper (II) sulfate solution (exo reaction):
Suggest one reason why the calculated value of △H is less accurate than the data book value. (1)

Answer 7

• heat loss
• incomplete reaction
• x all dissolved

Question 8

Suggest one reason why the △H for forming hydrated copper(II) sulfate from anhydrous copper(II) sulfate cannot be directly measured. (1)

Answer 8

• imposs. to add exact amount of water
• difficult to measure temp rise of solid
• difficult to prevent solid dissolving to form solution

Question 9

Define mean bond enthalpy. (2)

Answer 9

• average enthalpy needed to break 1 mole of covalent bond in the gaseous state
• measured over a range of diff. compounds/ molecules
  //
• av. of several bond enthalpy values for a given bond type
• taken from a range of diff. compounds

Question 10

Define enthalpy change. (1)

Answer 10

• heat change under constant pressure

Question 11

State why the standard enthalpy of formation of oxygen is zero. (1)

Answer 11

• △Hϝ of an element is 0 by def.

Question 12

A bomb calorimeter is a container of fixed volume that withstands the change in pressure during the reaction.
The fuel is mixed with pure oxygen in the calorimeter, ignited and the temperature change is recorded.
State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change. (1)

Answer 12

• pressure x constant in bomb calorimeter
  (enthalpy change requires constant pressure)

Question 13

Burning fuel to measure heat change:
Suggest one change to this experiment that decreases the percentage uncertainty while using the same thermometer. (1)

Answer 13

• ↑ mass of fuel- (↑ temp. change)

Question 14

This experiment can be repeated by using a different volume of water that would result in a more accurate value for the enthalpy of combustion because there would be a reduction in the heat lost.
.
State a change in the volume of water that would cause a reduction in heat loss and explain your answer.(2)

Answer 14

• smaller ↑ in temp above room temp /
• ↑ contact between calorimeter & water /
• ↓ heat loss by evaporation

Question 15

A student carried out a laboratory experiment to determine the enthalpy change when a sample of ethanol was burned. The heat produced was used to warm some water in a copper calorimeter.
.
Deduce two reasons why the student’s value for the standard enthalpy of combustion of ethanol is higher than the Data Book value. (2)

Read the question.

Answer 15

• incomplete combustion
• heat loss
• x included specific heat capacity of Cu
• some ethanol evaporate- lost
• experiment x done under standard conditions

Question 16

Suggest one improvement that would reduce errors due to heat loss in the student’s experiment (combustion in a calorimeter). (1)

Answer 16

• use a lid (x insulate around)

//
- ↓ distance between flame & beaker
- use copper calorimeter instead of glass beaker

Question 17

Explain why the value of enthalpy of reaction obtained using mean bond enthalpies and △Hϝ are different. (1)

Answer 17

• mean bond enthlpies are not exact

Question 18

Explain why and how the value of enthalpy of combustion would differ if it referred to the formation of water in the liquid state than in the gaseous state. (2)

Answer 18

• ↑ -ve
• ∵ heat released when water vapour condenses

Question 19

Write an equation, including state symbols, for the bond enthalpy of I-I. (2)

Answer 19

I-I (g) –> 2I (g)

Question 20

As the chain length in the alkanes increases, the value of ΔHc becomes more negative.
Use your understanding of bond breaking and bond making to explain this trend. (2)

Answer 20

• ↑ bonds broken
• ↑ bonds formed per mole

Question 21

The enthalpy of combustion of an alkane was calculated using mean bond enthalpies.
Suggest one reason, other than the use of mean bond enthalpies, why a value for
the enthalpy of combustion of a liquid alkane is different from the value obtained. (1)

Answer 21

alkane x gaseous (takes energy to convert to gas)

Question 22

In a calorimetry experiment to calculate the enthalpy of combustion of almitic acid as it burns in air, state how the value calculated is likely to differ from data book values.
Give one reason, other than heat loss, for this difference. (2)

Answer 22

• ↓ exothermic/ ↓ -ve
• incomplete combustion