Energetics Flashcards
define standard enthalpy of combustion
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
molar enthalpy change equations:
- q (heat energy / J) = mc^T
- ^H (kJ mol) = q (kJ) / n (limiting reactant)
What is enthalpy change △H? (1)
heat change at constant pressure;
reactivity and enthalpy of C-X (halogenoalkane) relationship
if less energy require to break C-X bond, molecule will react faster and more readily as activation energy will be lower
how would the boiling points vary between an alkane and a halogenoalkane of the same Mr
- 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
How does the boiling point change of a halogenoalkane down the group
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
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.
Experiment not done under standard conditions
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.
Temperature rise is significantly bigger than uncertainty.
Suggest one improvement that would reduce errors due to heat loss in the student’s experiment.
(1 Mark)
Reduce the distance between the flame and the beaker
what is bond enthalpy
energy required to break or form a bond
Standard conditions are:
- 298 K (25°C)
- a pressure of 1 bar (100 kPa).
- where solutions are involved, a concentration of 1 mol dm-3
When calculation mean bond enthalpies ALWAYS remember:
the C-C / C=C bonds
suggest how the students method and analysis of results could be improved: (6)
- 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
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)
- (enthalpy of combustion) equation only relates to gaseous alkanes (by definition)
Hess’ Law
enthalpy change is independent of the route taken
Describe thermal cracking:
- high temperature (500C)
- high pressure (7000 kPa)
- produces mostly alkenes, (+some H2 for haber process)
Describe catalytic cracking:
- moderate pressure
- high temp (450C)
- zeolite catalyst
- prod. branched and cyclic alkanes and aromatic HC’s
- used for making motor fuels
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)
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
