energetics PMT Flashcards
Q3b 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
Glass is a poorer conductor than copper
Tripod and gauze would reduce heat transfer
Q4b A student determines the enthalpy change for the reaction between calcium carbonate and hydrochloric acid.
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
* measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid using a measuring cylinder and pour the acid into a 100 cm3 glass beaker
* weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into the acid
* stir the mixture with a thermometer
* record the maximum temperature reached.
The student uses the data to determine a value for the enthalpy change.
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.
Stage 1: Apparatus
1a. Use a burette/pipette (instead of a measuring cylinder)
1b. Use a polystyrene cup (instead of a beaker) / insulate beaker
1c. Reweigh the watchglass after adding the solid 1d: Use powdered solid
Stage 2: Temperature Measurements
2a. Measure and record the initial temperature of the solution for a few minutes before addition
2b. Measure and record the temperature after the addition at regular intervals (eg each minute) for 8+ minutes/until a trend is observed
Stage 3: Temperature Determination
3a. Plot a graph of temperature against time 3b. Extrapolate to the point of addition
3c. Determine ΔT at the point of addition
Q4c Suggest how, without changing the apparatus, the experiment in part (c) could be improved to reduce the percentage uncertainty in the temperature change.
Increase the concentration of the solutions
Q13b Give one reason why the bond enthalpy that you calculated in part (c) is different from the mean bond enthalpy quoted in a data book (388 kJ mol−1).
Data book value derived from (a number of) different compounds (not just different NH3 molecules)
Q20c State how your answer to part (b) is likely to differ from the value quoted in reference sources.
Give one reason for your answer.
Less negative than the reference
Heat loss
Q20d A 50.0 g sample of water was used in this experiment.
Explain how you could measure out this mass of water without using a balance.
Water has a known density (of 1.0 g cm–3)
Therefore, a volume of 50.0 cm3 could be measured out
