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 molar enthalpy change △H?
The amount of heat energy absorbed or released for every mole of the substance which reacts
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
Definition: Enthalpy change
Enthalpy change is the amount of heat energy transferred out during any change in a system provided the pressure is constant.
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)
