Energetics

Question 1

define standard enthalpy of combustion

Answer 1

enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions, all substances in their standard state
under standard conditions

Question 2

molar enthalpy change equations:

Answer 2

• q (heat energy / J) = mc^T
• ^H (kJ mol) = q (kJ) / n (limiting reactant)

Question 3

What is enthalpy change △H? (1)

Answer 3

heat change at constant pressure;

Question 4

reactivity and enthalpy of C-X (halogenoalkane) relationship

Answer 4

if less energy require to break C-X bond, molecule will react faster and more readily as activation energy will be lower

Question 5

how would the boiling points vary between an alkane and a halogenoalkane of the same Mr

Answer 5

• Halogenoalkanes have higher b.p.
• C- X bond is polar ➜ halogenoalkane forms PDD interactions and VdW forces between molecules the alkanes only form VdW forces between molecules

Question 6

How does the boiling point change of a halogenoalkane down the group

Answer 6

larger Mr -> Increased VdW forces between molecules
* Even though the polarity of the C-X bond decreases, so weaker PDD interaction,
* HOWEVER, higher Mr means stronger VdW forces, which outweighs the PDD interaction

Question 7

Suggest one reason, other than incomplete combustion or heat transfer to the atmosphere, why the student’s value for the enthalpy of combustion of methanol is different from that in a Data Book.

Answer 7

Experiment not done under standard conditions

Question 8

The student said correctly that using a thermometer with an overall uncertainty for the rise in temperature of ±0.5 °C was adequate for this experiment. Explain why this thermometer was adequate for this experiment.

Answer 8

Temperature rise is significantly bigger than uncertainty.

Question 9

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

Answer 9

Reduce the distance between the flame and the beaker

Question 10

what is bond enthalpy

Answer 10

energy required to break or form a bond

Question 11

Standard conditions are:

Answer 11

• 298 K (25°C)
• a pressure of 1 bar (100 kPa).
• where solutions are involved, a concentration of 1 mol dm-3

Question 12

When calculation mean bond enthalpies ALWAYS remember:

Answer 12

the C-C / C=C bonds

Question 13

suggest how the students method and analysis of results could be improved: (6)

Answer 13

• a polystyrene cup could be used with an insulating lid
• to reduce heat loss
• temp readings recorded every 30 sec for 3 minutes before addition of solid to measure accurate starting temp
• regular temp readings after adding every 30 sec
• graph of temp v time plotted, extrapolating two lines of best fit to gain max temp change

Question 14

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 using the expression for enthalpy of combustion: (1)

Answer 14

• (enthalpy of combustion) equation only relates to gaseous alkanes (by definition)

Question 15

Hess’ Law

Answer 15

enthalpy change is independent of the route taken

Question 16

Describe thermal cracking:

Answer 16

• high temperature (500C)
• high pressure (7000 kPa)
• produces mostly alkenes, (+some H2 for haber process)

Question 17

Describe catalytic cracking:

Answer 17

• moderate pressure
• high temp (450C)
• zeolite catalyst
• prod. branched and cyclic alkanes and aromatic HC’s
• used for making motor fuels

Question 18

A student planned and carried out an experiment to determine the enthalpy of reaction when magnesium metal displaces zinc from aqueous zinc sulfate.

The student used this method:
* A measuring cylinder was used to transfer 50 cm’ of a 1.00 mol dm aqueous solution of zinc sulfate into a glass beaker.
* A thermometer was placed in the beaker.
* 2.08 g of magnesium metal powder were added to the beaker.
* The mixture was stirred and the maximum temperature recorded.

The student recorded a starting temperature of 23.9 °C and a maximum temperature of
61.2 °C.

Suggest how the students method and method of analysis could be improved.

Don’t mention equipment precision (6)

Answer 18

Stage 1 Improved insulation
1a Insulate the beaker or use a polystyrene cup or a lid
1b To reduce heat loss

Stage 2 Improved temperature recording
2a Record the temperature for a suitable time before adding the metal
2b To establish an accurate initial temperature
OR
2c Record temperature values at regular time intervals
2d To plot the temperature results against time on a graph

Stage 3 Improved analysis of results
3a Extrapolate the cooling back to the point of addition
3b To establish a (theoretical) maximum temperature OR temperature change (e.g.
at the 4 th minute) OR adjust for the cooling /apply a cooling correction
3a and 3b could be seen on an extrapolated sketch graph