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Name: Lab#_____

Hess’s Law Lab

In this experiment you will determine the (H values for the following reactions using both Hess’s Law and a calorimeter:

MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) (H = ? kJ/mol

Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) (H = ? kJ/mol

You will then use these reactions and Hess’s Law to determine a heat of reaction that would be difficult to obtain by direct measurement. The reaction is as follows:

Mg(s) + ½O2(g) ( MgO(s)

Hess’s Law states that the above equation can be obtained by combining the following equations labeled (1), (2), and (3):

(1) MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) (H = ? kJ/mol

(2) Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) (H = ? kJ/mol

(3) H2(g) + ½O2(g) ( H2O(l) (H = -285.8 kJ/mol

In this lab you will experimentally determine the (H values for equations (1) and (2). It would be difficult for you to calculate the (H value for equation (3), so it is given (–285.8kJ/mol). You will then use the values you obtain for equations (1) and (2) and the given one for equation (3) to calculate the (H value for the reaction: Mg(s) + ½O2(g) ( MgO(s).

Prelab Question:

1. Use ΔHf values given in the table below to solve the following problems.

Determine the ΔH values for each of the following reactions. Classify each reaction as either exothermic or endothermic.

|Substance |ΔHf (kJ/mol) |

|MgO(s) |-601.6 |

|HCl (aq) |-167.2 |

|MgCl2(aq) |-801.2 |

|H2O (l) |-285.8 |

a. MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) (H = ? kJ/mol

b. Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) (H = ? kJ/mol

2. Prove your answer for 1a is correct by combining the 4 given thermochemical equations.

Determine the (H value for the following reaction:

MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) (H = __________ kJ/mol

Solve using the following 4 thermochemical equations and (H values.

(1) Mg(s) + ½ O2(g) ( MgO(s) (Hf = -601.6 kJ/mol

(2) ½ H2(g) + ½ Cl2(g) ( HCl(aq) (Hf = -167.2 kJ/mol

(3) Mg(s) + Cl2(g) ( MgCl2(aq) (Hf = -801.2 kJ/mol

(4) H2(g) + ½ O2(g) ( H2O(l) (Hf = -285.8 kJ/mol

3. Prove your answer for 1b is correct by combining the 2 given thermochemical equations.

Determine the (H value for the following reaction:

Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) (H = ___________kJ/mol

Solve using the following 2 thermochemical equations and (H values.

(1) Mg(s) + Cl2(g) ( MgCl2(aq) (Hf = -801.2 kJ/mol

(2) ½ H2(g) + ½ Cl2(g) ( HCl(aq) (Hf = -167.2 kJ/mol

4. Calculate the ΔH value for the following reaction. Is this reaction exothermic or endothermic?

Mg(s) + ½O2(g) ( MgO(s) (H = ___________kJ/mol

Use the following equations and data:

(1) MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) (H = -150.9 kJ/mol

(2) Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) (H = -466.8 kJ/mol

(3) H2(g) + ½O2(g) ( H2O(l) (H = -285.8 kJ/mol

Safety: HCl is corrosive to skin and eyes and is highly toxic by ingestion or inhalation. Mg metal is extremely flammable. MgO is a white powder that is toxic by inhalation.

Procedure for:

MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l)

1. Obtain and wear goggles.

2. Obtain a labquest with a temperature probe attached.

3. Place the Styrofoam cup (calorimeter) into a beaker (there should also be a lid for the cup).

4. Measure 50.0mL of 2.0M HCl and pour it into the Styrofoam cup. Use the temperature probe to determine the temperature of the solution. Wait a few seconds and record the temperature under “Initial Temperature (T1)” in data table.

5. Obtain 1.00 gram of magnesium oxide. Add the white magnesium oxide powder to the solution. QUICKLY place the lid on the cup and place the temperature probe in the hole of the lid. Use the temperature probe to stir the cup contents until a maximum temperature, T2, has been reached and the temperature starts to drop. Record the maximum temperature reached under “Final or Maximum Temperature (T2)” on data table.

6. Discard the solution in the sink and clean out your cup with copious amounts of water.

Procedure for:

Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g)

7. Measure 50.0mL of 2.0M HCl and pour it into the Styrofoam cup. Use the temperature probe to determine the temperature of the solution. Wait a few seconds and record the temperature under “Initial Temperature (T1)” in data table.

8. Obtain 0.22 gram of magnesium. Add the magnesium to the solution. QUICKLY place the lid on the cup and place the temperature probe in the hole of the lid. Use the temperature probe to stir the cup contents until a maximum temperature, T2, has been reached and the temperature starts to drop. Record the maximum temperature reached under “Final or Maximum Temperature (T2)” on data table.

9. Discard the solution in the sink and clean out your cup with copious amounts of water.

Processing the Data – (Use the following information to complete your Data Table.) SHOW ALL WORK IN YOUR DATA TABLE!!!

1. Calculate the change in temperature, (T, for the reactions. (T = T2 - T1

2. Calculate the heat released by each reaction, q, using the formula:

q = (Cp)(m)((T)

where Cp = 4.18 J/g(C and m = 50.0g H2O (1mL H2O = 1g H2O)

Convert J to kJ in your final answer. (1kJ = 1000J)

3. Determine (H for each reaction (both were exothermic). (H = -q

4. Convert 1.00 gram MgO to moles. Convert 0.22 gram Mg to moles. (These were the amounts of MgO and Mg you used in this lab.)

5. Use your Step 3 and Step 4 results to calculate (H/mol for MgO and (H/mol for Mg.

6. Determine the percent error for the answers you obtained in Step 5 (Step 5 answers are the experimental values). Accepted values shown below.

MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) accepted value (H = -150.9 kJ/mol

Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) accepted value (H = -466.8 kJ/mol

Percent Error = (Accepted Value – Experimental Value( X 100

(Accepted Value(

|Data Table |  |  |

|  |MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) |Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) |

|Final or Maximum |__________°C |__________°C |

|Temperature (T2) | | |

|Initial Temperature |__________°C |__________°C |

|(T1) | | |

|1. Change in |__________°C |__________°C |

|Temperature (ρT) | | |

|2. q |__________kJ |__________kJ |

|3. ρH |__________kJ |__________kJ |

|4. Moles (Show Work) |__________mol MgO |__________mol Mg |

|5. ρH/mol (Show Work) |__________kJ/mol |__________kJ/mol |

| 6. Percent Error | __________% |__________% |

|(Show Work) | | |

Conclusion Questions

1. Using the data you obtained in this lab determine the (H value for the following reaction:

Mg(s) + ½O2(g) ( MgO(s) (H = __________ kJ/mol

Solve using the following 3 thermochemical equations, the (H values you calculated, and the given (H value for water.

(1) MgO(s) + 2HCl(aq) ( MgCl2(aq) + H2O(l) (H = box #5 in your data table

(2) Mg(s) + 2HCl(aq) ( MgCl2(aq) + H2(g) (H = box #5 in your data table

(3) H2(g) + ½ O2(g) ( H2O(l) (H = -285.8 kJ/mol

2. Reread the prelab questions and look over your data obtained in this lab. Write a summary of this lab. Use the terms Hess’s Law, calorimeter, and the equation

(H reaction = (sum of (Hf products) – (sum of (Hf reactants)

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