Name:

Applying Hess’s Law Lab

Purpose:

a)  Use Hess’ law to determine molar enthalpy of combustion

b)  Confirm the molar enthalpy of combustion for magnesium using Hess’ law

Introduction:

There are many reactions for which the heat of reaction cannot be easily measured experimentally. Some of these are dangerous to perform in the lab. Others generate so much heat that simple calorimeters cannot be used. In such cases, it is practical to examine a series of reactions whose net effect is the desired reaction, but whose heats of reaction are more easily measured. Once that has been done, the required heat of reaction can be calculated using Hess's Law.

The burning of magnesium can be represented by the equation:

Mg(s) + ½O2(g) ----> MgO(s) ΔcHo = ΔrHo = -601.6 kJ

The molar enthalpy of combustion of magnesium is the same as the molar enthalpy of formation of magnesium oxide because both processes have the same chemical reaction equation

The enthalpy change for the combustion of magnesium can be determined by applying Hess’ law to the following three chemical equations.

(1) MgO(s) + 2 HCl(aq) ----> MgCl2(aq) + H2O ΔrH =

(2) Mg(s) + 2 HCl(aq) -----> MgCl2(aq) + H2(g) ΔrH =

(3) H2(g) + ½O2(g) ------> H2O(l) ΔrHo = -285.8 kJ

Safety

Materials

Each group / Communal (teacher monitored)
·  lab aprons
·  eye protection
·  1 – 50mL graduated cylinder
·  1 – 400mL beaker
·  3 – polystyrene cups
·  Thermometer / ·  90cm of Magnesium Ribbon
·  10g of Magnesium oxide powder
·  (500mL) of 1.00M hydrochloric acid
·  Scoopula
·  Steel wool
·  Weigh paper
·  Balance
·  Ruler

Experimental Procedure

Build calorimeter that will be used for the lab

a.  Nestle 2 of the polystyrene cups together.

b.  Place these cups inside a 400 mL beaker.

c.  Remove the rim of a third cup to serve as a lid. Punch a hole in the top for your thermometer.

MgO heat of reaction

1.  Measure out 50 mL of 1.0 M Hydrochloric acid solution and pour into the styrofoam cup. Record the temperature of the solution.

2.  Accurately find and record the mass of about 1 g of solid magnesium oxide.

3.  Add the solid magnesium oxide to the acid solution in the cup. Stir the mixture and record the highest temperature possible.

4.  Discard the solution appropriately and rinse the cup thoroughly with water.

Mg heat of reaction

1.  Measure out 50 mL of 1.0 M HCl solution into the styrofoam cup. Record the temperature of the solution.

2.  Accurately find and record the mass of about 10cm of magnesium ribbon.

3.  Add the magnesium ribbon to the acid solution. Stir the mixture and record the highest temperature reached.

4.  Discard the solution appropriately and rinse the cup thoroughly with water. Dispose of all waste materials and clean up the lab bench area

Observations

Conc. Of HCl(aq)

Reacting 1.00g magnesium oxide with 50 mL of hydrochloric acid in calorimeter.

Mass of Magnesium Oxide (g) / Amount of HCl added (mL) / Initial Temperature in Celsius / Final Temperature in Celsius
Trial 1
Trial 2
(only complete
if necessary)

Reacting a 10cm magnesium ribbon with 50 mL of hydrochloric acid in calorimeter.

Mass of Magnesium Ribbon (g) / Amount of HCl added (mL) / Initial Temperature in Celsius / Final Temperature in Celsius
Trial 1
Trial 2
(only complete
if necessary)

Calculations
Part One: Magnesium Oxide

1.  From your data, calculate the following from the experiment.

a.  The temperature change of the solution.

b.  The heat absorbed by the solution. (assume c=4.184 J/goC)

c.  The number of moles of magnesium oxide

d.  Heat of reaction in kJmol-1of magnesium oxide reacting with hydrochloric acid.

Part Two: Solid Magnesium

1.  From your data, calculate the following from the experiment.

a.  The temperature change of the solution.

b.  The heat absorbed by the solution. (assume c=4.184 J/goC)

c.  The number of moles of magnesium

d.  The heat of reaction in kJmol-1of magnesium reacting with hydrochloric acid.

Calculations of Data for Magnesium Oxide and solid Magnesium

ΔT
(oC) / Energy
Produced, E
(kJ) / Moles used
(mol) / Hrxn
MgO(s)
Balanced Chemical Reaction
Mg(s)
Balanced Chemical Reaction

Data Analysis

1.  Using Hess’ Law, show how equations (1),(2) and (3) in the introduction can be combined to give the equation for the combustion of magnesium.

2.  Use your experimental values for the two reactions in the experiment, along with the value for the heat of reaction for equation to calculate the experimental heat of combustion for magnesium.

Error Analysis

1.  Compare your calculated value with that found in tables for the standard heat of formation of magnesium oxide. Find the experimental error.

2.  Give possible sources of error (from improper lab technique)

Conclusion

1.  Confirm purpose of the lab. Exaplain

a.  Include: enthalpy result, use of Hess’s law, % error

b.  What did you learn from the lab (related to our Chem 30 concepts)