A LEVEL CHEMISTRY
Name :
You will determine the enthalpy change for the reaction of magnesium ribbon, Mg, with dilute
hydrochloric acid.
Method
1.
Place the polystyrene cup provided into a glass beaker.
2.
Weigh the polystyrene cup in the glass beaker. Record the mass.
3. Add the piece of magnesium ribbon to the polystyrene cup in the glass beaker.
Record the mass.
4. Using a measuring cylinder, measure out 25 cm3 of HCl (aq), which is an excess for the
reaction.
5. Measure and record the temperature of the acid in the measuring cylinder, to the nearest
0.5 °C.
6.
Place the polystyrene cup in the glass beaker on a heat-proof mat.
The reaction that follows is vigorous.
7.
Add the HCl (aq) to the magnesium ribbon in the polystyrene cup.
8. Stir the mixture with the thermometer until the maximum temperature is reached.
Record this value to the nearest 0.5 °C. Calculate the temperature change (ΔT).
9.
From your mass readings, calculate the mass of magnesium used.
Evaluation
1. State whether the reaction is exothermic or endothermic.
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2. Write a balanced equation, including state symbols, for the reaction of solid magnesium with
aqueous hydrochloric acid.
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3. Other than the increase in temperature what other two observations can you identify for this
reaction? The observations must be consistent with the balanced equation that you have written.
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4. Suggest why a polystyrene cup was used to contain the reactants and make the measurements.
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Calculations
1. Assume that the mass (m) of solution is 25g and the specific heat capacity (c) of the solution
(mainly water) is 4.2J g-1K-1
Use the value you recorded for the change in temperature (ΔT) to calculate the energy
transferred (Q) during the course of this reaction:
Q = m x c x ΔT
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2. Using the value that you recorded for the mass of magnesium and a molar mass value of
24.3 g mol-1 calculate the number of moles of magnesium that you used in the reaction:
Number of mol = Mass of magnesium/Molar mass of magnesium
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3. Using your answers from (1) and (2), calculate the enthalpy of reaction in kJ mol-1 for 1 mol of
magnesium.
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