Use Hess’s Law to find ΔH for the following reactions: Express your answers in kJ/mol of the first reactant on the left in each equation.
Heat of Reaction for the Combustion of Magnesium
Although you will not be working with actual chemicals during this laboratory, since you will be doing it virtually, the procedure is given in order for you to be aware of the process that would be followed in a laboratory setting.
In this experiment, you will use Hess’s Law to determine the heat for a reaction that is difficult to measure directly. Magnesium metal burns rapidly, releasing light and heat, as observed in photo flashbulbs or by burning magnesium ribbon. The reaction is presented by the equation:
This equation can be obtained by combining equations (2), (3) and (4):
By combining these three equations, you can obtain equation (1). The heats of reaction for equations (2) and (3) can easily be determined by experiments. The heat of reaction for equation (4) can be obtained from a table of values for previously measured reactions.
magnesium, magnesium oxide, hydrochloric acid
Polystyrene calorimeter, graduated cylinder, electronic balance, filter paper, spatula, thermometer, glass stirring rod, stop watch,
· Safety goggles and a lab apron should always be worn when working with chemicals.
· This lab will be performed using small quantities of chemicals (micro-scale chemistry).
1. Pour 100 mL of 1.0 M hydrochloric acid into your polystyrene calorimeter.
2. Weigh out 1.00 g of magnesium oxide on a filter paper.
3. Place the thermometer in the hydrochloric acid to obtain the initial temperature, T1 (precise to 0.1oC). It may take several seconds for the temperature to stabilize.
4. Add the magnesium oxide powder to the solution.
5. Use a glass stirring rod to stir the cup contents until a maximum temperature has been reached and the temperature starts to drop.
6. Record the maximum temperature, T2
7. Pour 100 mL of 1.0 M hydrochloric acid into your polystyrene calorimeter.
8. Weigh out 0.50 g of magnesium ribbon (this is about 46 cm). Roll it up loosely around a pencil so that you end up with a coil. Make sure it is not too tightly coiled.
9. Place the thermometer in the hydrochloric acid to obtain the initial temperature, T1 (precise to 0.1oC). It may take several seconds for the temperature to stabilize.
10. Add the magnesium ribbon to the solution. n.
11. Use a glass stirring rod to stir the cup contents until a maximum temperature has been reached and the temperature starts to drop.
12. Record the maximum temperature, T2.
· Use the table of standard enthalpies of formation to calculate the theoretical value.
· Sample results are provided below.
Table #1: Volume, Temperature and Mass Values for Reactions 2 and 3
|Volume of HCl|
|Final temperature (T2)||32.1oC||48.2oC|
|Initial temperature (T1)||24oC||24oC|
|Change in temperature (ΔT)|
|Mass of solid||0.96 g||0.55 g|
1. Combine equations (2), (3) and (4) to obtain equation (1).
2. Calculate the change in temperature for reaction (2).
3. Calculate the energy released (ΔH) in (2) (in J), using Q=mcΔT. The mass to be used is the mass of HCl. Assume the density of hydrochloric acid to be 1 g/mL.
4. Convert your answer into kJ.
5. How many moles of MgO were used?
6. Convert your answer into kJ/mol MgO.
7. Calculate the change in temperature for reaction (3).
8. Calculate the energy released (ΔH) in (3) (in J), using Q=mcΔT. The mass to be used is the mass of HCl.
9. Convert your answer into kJ.
10. How many moles of Mg were used?
11. Convert your answer into kJ/mol Mg.
12. Calculate the heat of reaction (1) in kJ/mol of MgO by combining (2), (3) and (4) and their ΔH values that you obtained, in order to get equation (1) and its ΔH for the combustion of Mg.
13. Determine the theoretical value for the combustion of magnesium, from the table of standard enthalpies of formation.
14. Calculate the percentage error.
Briefly state the results for the laboratory, in general. This should be no more than two sentences.
|Please ensure that your lab report contains the following:|
· Title page
· Three sources of error