PAG 3- Measuring enthalpy change of a reaction (calirometry)

Question 1

Energy change equation

Answer 1

energy change (J)= mass (g) x shc (water) x temp change (K)

Question 2

Calirometric method

Answer 2

• Wash equipment (cup and pipettes etc) out with the solutions to be added to them, dry cup after.
• Place polystyrene cup in a 250cm^3 beaker, for insulation and support.
• Measure out desired volume of solutions using volumetric pipettes and transfer to cup.
• Place thermometer into cup and measure initial temperature of solution.
• Start stopwatch and continue to record the temperature of the solution at regular minute intervals for 2-3 minutes.
• At minute 3, transfer the second reagent to the cup after being measured out in a measuring cylinder (or using mass balance if solid- do ‘before, after’ weighing method to get exact mass).
• Record temperature every minute for 10 minutes, stir each time temperature recorded.

Question 3

Calculation of enthalpy change

Answer 3

• Plot a graph of temp (y axis) against time (x axis).
• Draw a line of best fit until addition of second reagent, then draw a second line of best fit through points after addition.
• Use the graph to calculate temperature change at final minute.
• Calculate energy change- q= mc x temp change
• Calculate moles of reactants used.
• Divide q/number of moles of reactant not in excess.
• Divide by 1000 to get kJmol^-1
• Add the right sign (- if temp increases, as reaction is exothermic, + if temp decreases, as reaction is endothermic)

Question 4

Possible errors in calorimetric method

Answer 4

• Energy loss to surroundings
• Method assumes that all solutions have the shc of water, may not be the case
• Reaction may not go to completion
• Conditions may not be exactly standard conditions

Question 5

Measuring enthalpy changed of combustion using flame calorimetry

Answer 5

• Fuel is burnt and flame is used to heat up water in a metal cup
• Need to measure mass of spirit burner before and after
• Need to measure temp change of water (using thermometer)
• Need to measure volume of water in cup

Question 6

Possible errors in calculating enthalpy change of combustion using flame calorimetry

Answer 6

• Heat loss from calorimeter
• Incomplete combustion of fuel
• Incomplete transfer of energy
• Evaporation of fuel from spirit burner after weighing
• Experiment may not be carried out under standard conditions