Measuring Enthalpy Changes (Calorimetry) Practical Working and Solutions

Main Menu Part 1: Determining the heat capacity of the calorimeter Analysis: 1. Draw a graph for your first run of results with three lines of best fit, one for the initial cool water, one for the initial hot water and one for the mixed water, extrapolating the initial lines forwards to 4 minutes and the mixed line back to 4 minutes. 2. Reading from your graph at 4 minutes, calculate  Temperature drop and heat lost by the hot water = ΔT = 7.9 q = mcΔT = 50 x 4.18 x 7.9 = J  Temperature rise and heat gained by the cool water = ΔT = 5.0 q = mcΔT = 50 x 4.18 x 5.0 = 1045J  The energy gained by the calorimeter: this is the difference between the energy lost by the hot water and gained by the cool water Difference in energy lost by hot water and gained by cold water = = 606.1J Energy gained by the calorimeter = 606.1J 3. Calculate the heat capacity of the calorimeter by dividing the amount of energy it gained by the temperature rise of the calorimeter (this is the same as the temperature rise of the cool water). Heat capacity of the calorimeter = q/ ΔT = 606.1/5 = Jg -1 K Repeat with the second set of results so that you can calculate the average heat capacity of the calorimeter.

Main Menu Part 2: Measuring the enthalpy change 1.Apply the ‘Total Heat Change’ equation to calculate the heat change for each set of results, and then average them. q = mcΔT 2Use your answer to 1 and the mass of magnesium used to determine the molar enthalpy change for the reaction. n(Mg) = m/M r = 0.5/24.31 = moles Mg Total Heat Change = (C (cal) *  T) + (C p(contents) *m contents *  T) Total Heat Change (per mole) = ((121.22*  T) + (4.18*100.5*  T)) / Research the literature value for this reaction, how does you measured value compare to the literature value and to the one calculated in the ‘Exothermic and Endothermic Reaction’ experiment. Literature value for this reaction = -426 kkJmol-1 4.Try to determine the % uncertainty in your experimental method and compare this to the percentage difference between your own and the literature values. This will help you determine any sources of systematic error.  Calculate relative uncertainties of measurements apparatus (beaker, thermometer, weighing balance) and each type of measurement you made (volume of water and acid, mass of Mg, each thermometer reading). 5. List all the assumptions you have made in the experiment and for each one state whether/why it is a valid assumption.  No heat loss from the system (not valid – experimental value lower than literature value, apparatus does not allow for a completely closed system)  All heat goes from reaction to the water (not valid – even though we took into account for the calorimeter gaining enthalpy, the experimental value lower than literature value, apparatus does not allow for a completely closed system)  The solution is dilute: V(HCl)= V(H 2 O) (valid – the volume and mass of the acid is dilute enough to be equal to the volume and mass of water)  Water has a density of 1.00gcm -3 (valid – this is the density of water)