Energetics questions Flashcards
(14 cards)
State 2 disadvantages of using a glass beaker on a tripod and gauze instead of a copper caliometer 2 marks
Glass is a poorer conductor than copper
Tripod and gauze would reduce heat transfer
Define the term enthalpy change 1 mark
The heat energy change at constant pressure
Why might the enthalpy of combustion data be less than the data book value of this experiment? 2 marks
- Incomplete combustion
- Heat loss to the surroundings/ copper caliometer
Describe an addition to the caliometry experiment which may improve the accuracy of the results obtained
1 mark
- Use a lid
- Use a wind sheild
-Insulate sides of the caliometer
Enthalpy change( basic method)
-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
Apparatus improvement:
- Use a biurette or a pipette instead of a measuring cylinder
-Use a polystyrene cup instead of a glass beaker
-Reweigh the watch glass after adding the solid, use powdered solid
Temp measurements:
-Measure and record the inital temp of the solution for a few minutes before addition
- Measure and record the temperature after the addition at regular intervals for 8 plus minutes until a trend is observed
Temp determination
-Plot a graph of temperature against time
- Extrapolate until the point of addition
-Determine temp change at point of addition
Suggest how, without a change in apparatus, how a change in experiment can reduce percentage unceirtanty of an experiment
1 mark
Inc concentrations of solutions
Which reaction has an enthalpy change equal to the enthalpy change of formation of lithium flouride?
- Li(s) + 1/2F2(g) =LiF(s)
Suggest why the heat change calculated from the bomb caliometer experiment is not an enthalpy change 1 mark
Bomb caliometer doesnt have a constant pressure
Data book value of the enthalpy of combustion was higher than the experimental data, why?
1 mark
Options
- Incomplete combustion
-Heat transferred to the caliometer
-Evaporation
-No lid- heat loss
- No insulation- heat loss
Write an equation using state symbols for the reaction with an enthalpy change equal to the enthalpy of formation of iron (III) oxide 1 mark
2Fe(s)+ 3/2 O2(g)= Fe2O3(s)
Suggest how students method and analysis of results can be improved in order to determine a more accurate value of the enthalpy of reaction
- Dont refer to presicion of measuring equipment
- Dont refer to amounts or conc of chemicals
6 marks
Improved insulation
-Insulate the beaker or use polystyrene cup or a lid
- To reduce heat loss
Improved temp recording
-Record temp for a suitable time before adding the metal
-To establish an accurate initial temperature
-Record temp values at regular time intervals
-To plot temperature results against time on a graph
Improved analysis of results
Extrapolate the cooling back to addition
To establish a theoretical maximum temperature or temp change
- Or apply a cooling correction
-
Write an equation to show the reaction taking place when standard enthalpy of combustion for ethanol is measured
1 mark
C2H5OH(l)+ 3O2(g)= 2CO2(g)+ 3H20(l)
Correct formula and balancing
Water is l
Oxygen is needed for combustion
Need standard states
Why is the enthalpy change of C(s) + 1/2O2(g)= CO2(g)
Difficult to determine directly 2 marks
Hard to prevent the C reacting with some O2 in order to form carbon dioxide
Why is mean bond enthaply referred to as mean?
The energy to break one mole of a specific bond avarged across a range of compounds
However not all bond enthalpies are averages though, if you are given the bond enthalpy for H-H or Cl-Cl e.g these cannot be averages as these bonds only exist in one molceule each
