RP2- Calorimetry

Question 1

Give 2 sources of error in this practical?

Answer 1

• heat/energy lost to surroundings
• incomplete reaction/combustion (for flame calorimetry)
• approximation of specific heat capacities

Question 2

How could you improve accuracy of calorimetry?

Answer 2

use a bomb calorimeter

Question 3

How do you measure temperature changes during calorimetry?

Answer 3

• using a thermometer
• taking readings at regular intervals at eye level

Question 4

Why do you put the cup in a beaker?

Answer 4

insulation and support

Question 5

What should you do before mixing reactants together?

Answer 5

take initial temperatures (at regular intervals)

Question 6

Why do you stir the mixture?

Answer 6

ensures solution is at the same temperature

Question 7

Why might the correct temperatures be difficult to obtain for a slow reaction?

Answer 7

cooling occurs at the same time as the reaction

Question 8

How do we counteract any cooling that has happened simultaneously?

Answer 8

• regular readings
• extrapolate graph back to point when reactants mixed

Question 9

What needs to be measured in an investigation measuring enthalpies of combustion?

Answer 9

• mass of spirit burner before and after
• temperature change of water
• volume of water

Question 10

What does q stand for in q = mcΔT? Give units

Answer 10

energy (Joules, J)

Question 11

What is the value of c for water?

Answer 11

4.18 J/g/K

Question 12

What does m mean in q = mcΔT?

Answer 12

mass of water in g

Question 13

Explain how to calculate enthalpy change from an energy change and moles reacted

Answer 13

ΔH = q / moles reacted

Question 14

Describe the steps you would take to determine an accurate minimum temperature to calculate the enthalpy of solution of an ionic solid that is not influenced by heat from the surroundings

Answer 14

• measure the temperature of water every 30 seconds for 4 minutes
• measure the temperature of the solution every 30 seconds for 10 minutes after the addition of solid
• plot this data on a graph
• extrapolate the temperature change back to the start of the reaction and the average start temperature
• find the change in temperature

Question 15

Why is measuring accurate temperature change difficult?

Answer 15

• as soon as a reaction starts the temperature of the surroundings starts to have an effect almost immediately
• this can be heat lost to the surroundings or heat absorbed from the surroundings

Question 16

How could you change a method to reduce percentage uncertainty in temperature change?

Answer 16

• increase magnitude of temperature change
• by increasing concentration/amount of solid/alkali/acid

Question 17

Describe the method for a calorimetry practical

Answer 17

• measure 25cm3 water with measuring cylinder/volumetric pipette
• put in to insulated container/polystyrene cup
• add (mass of) solid
• weigh by difference

Question 18

Describe the measurements in a calorimetry practical

Answer 18

• record initial temperature at least twice
• record T at regular timed intervals (every 30 seconds) for 10 minutes
• plot a graph of T vs Time

Question 19

Describe how you would use results in a calorimetry practical

Answer 19

• draw line of best fit
• extrapolate line to point when solid added
• T final - T initial = change in Temp
• q = mcΔT
• ΔH = q / moles reacted

Question 20

How can heat loss be minimised in calorimetry?

Answer 20

use polystyrene cup.insulated beaker

Question 21

Why might calorimetry be less accurate when hydrating a salt?

Answer 21

no way to stop some of the salt dissolving/the salt is soluble

Question 22

Why might the enthalpy change of neutralisation be different from what was obtained in an experiment?

Answer 22

• weak acid/not fully dissociated
• more energy needed to complete dissociation

Question 23

State why the heat change calculated from the bomb calorimeter experiment is not an
enthalpy change

Answer 23

not under constant pressure

Question 24

What are disadvantages of using a glass beaker and tripod and gauze in this experiment?

Answer 24

• Glass is a poorer conductor than copper
• Tripod and gauze would reduce heat transfer (from combustion)