Thinking /24 Comm /19

Determining the Enthalpy of Combustion of Magnesium using Hess’ Law

The combustion of magnesium is very rapid and exothermic, and is represented by the equation:Mg(s) + ½ O2(g) MgO(s) + heat ΔHtarget = ? kJ

It is possible to observe and measure a series of reactions that enable us, with the use of Hess’ law, to determine the enthalpy for this reaction (i.e. the ΔHcombustion of magnesium)

(Reaction 1) Magnesium reacts in hydrochloric acid to form hydrogen gas and a salt: (provide balanced chemical equation with state symbols) (C: 2 marks)

ΔH1 = ? kJ

(Reaction 2) Magnesium oxide reacts in hydrochloric acid to form water and a salt: (Provide balanced chemical equation with state symbols) (C: 2 marks)

ΔH2 = ? kJ

* The values ΔH1 and ΔH2 will be determined empirically (i.e. via experiment) using a simple calorimeter.

(Reaction 3) Hydrogen and oxygen gases react to form liquid water (one mol): (Provide balanced chemical equation with state symbols) (C: 2 marks)

ΔH3 = -285.8 kJ

Purpose: The purpose of this investigation is to use _________________ law to determine the molar enthalpy of ________________________, ΔHcomb of magnesium, using _______________________. (C: 3 marks)

EXPERIMENTAL DESIGN: Measured masses of magnesium and magnesium oxide will be added to measured volumes of hydrochloric acid solution of known concentration. The temperature changes will be determined. Calculated enthalpies of reaction will be combined, using Hess’ law, to determine the enthalpy of combustion of magnesium.* Always include formulas, and answer in complete sentences.

PREDICTION:a. Show how the three known equations and their enthalpies of reaction may be combined, using

Hess’ law, to yield the target equation and its enthalpy of combustion. (TI: 3 marks)

MATERIALS:- eye protection - 100mL graduated cylinder- steel wool - scoopula- electronic balance - 10 cm strip of magnesium ribbon- thermometer - magnesium oxide powder- polystyrene calorimeter - 1.0 mol/L hydrochloric acid

PROCEDURE:1. Measure 80.0 mL of 1.0 mol/L hydrochloric acid into a graduated cylinder and pour into a

polystyrene cup. Measure the initial temperature of the acid solution with a thermometer to the nearest 0.1 C.

2. Polish a length of 5-10 cm magnesium ribbon with steel wool. Determine the mass, to the nearest 0.1 g, of approximately 0.3 g of magnesium metal. Add the solid to the solution and stir it, and record the maximum temperature that the solution attains.

3. Dispose of the products in the waste beaker located in the fume hood.4. Repeat the first three steps, using approximately 1g of magnesium oxide powder measured to the

nearest 0.1g.5. Clean all materials and workstation.

OBSERVATIONS: to be done as your ‘pre-lab’ and then filled in during the lab. See below.

ANALYSIS1. Were the changes exothermic or endothermic? Explain in detail. (TI: 2 marks)

2. For reaction 1, calculate the enthalpy change per mole of magnesium, ΔH1. (TI: 4 marks)

3. For reaction 1, write the thermochemical equation for the reaction of magnesium with acid, including your experimental value (TI: 2 marks)

4. For reaction 2, calculate the enthalpy change per mole of magnesium oxide, ΔH2. (TI: 4 marks)

5. For reaction 2, write the thermochemical equation for the reaction of magnesium oxide with acid, including your experimental value (TI: 2 marks)

6. Using Hess’ law, the values you have found experimentally, and the given value for the enthalpy change for the formation of water from its elements, determine the molar enthalpy of combustion of magnesium. *No need to repeat the work from previous questions, just show the formula used and the math. (TI: 2 marks)

EVALUATION:7. The accepted value for the molar enthalpy of combustion of magnesium is -601.6 kJ/mol.

Calculate the percentage difference by comparing your experimental value with the accepted value. (TI: 2 marks)

8. Suggest two possible sources of experimental error (OTHER THAN HUMAN ERROR) in this investigation. Explain. (TI: 3 marks)

9. Explain why (and how) your calculated enthalpies of reaction would be inaccurate if some heat were transferred to the air or the Styrofoam cup. Be specific! (TI: 3 marks)

Pre-lab (C: 10 marks)A number of quantitative measurements need to be taken in order to complete this lab. Construct a data table to organize this data. You may use more than one table if you wish.

*A good table:- includes a descriptive title (include brief descriptive caption- Table.1…) - includes columns with proper labels and the units of measurement are indicated - is neat and easy to read/interpret- contains measurements with appropriate decimal places- is accentuated with detailed quantitative observations in paragraph form after raw data tables