Hesss Law Lab, use Hesss law to find the enthalpy change of combustion of magnesium which is a highly exothermic reaction.

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Haroon Sahibi                                                                   1. februar 2011

                                Hess’s Law Lab

 

Hess's Law

Hess's Law states that that the value of ΔH for a reaction is the same whether it occurs directly or as a series of steps.

Purpose

To use the Hess’s law to find the enthalpy change of combustion of magnesium which is a highly exothermic reaction.

 Determining the enthalpy change for the reactions of Mg in HCl(aq) and MgO(s) HCl(aq) experimentally and alongside that use the enthalpy change for hydrogen gas (Table value) when it is combusted , to find the enthalpy change for the combustion of magnesium.

HYPOTHESIS

The heat of combustion for magnesium can be determined by determining the heat of reaction for the following reactions:

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

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

3) H2(g) + ½ O2(g) →  H2O(l)

Once these values have been obtained, I can add or subtract these values in order to achieve the heat of combustion for magnesium.

Apparatus

  • Goggles
  • Lab apron
  • Electric balance
  • 100 mL graduated cylinder
  • 2 plastic foam cup
  • Thermometer
  • Beaker ( transportation of HCl)

Procedure

I began this experiment by gathering all the required lab equipment and the necessary chemicals. I found a lab station and started my investigation. I decided to start with hydrochloric acid and magnesium first. I measured 50 cm3 1.00 mol/dm3 amount of HCl solution using a graduated cylinder and put it in the foam-cup and took the initial temperature reading. I cleaned  approx. 10 cm of the magnesium ribbon using emery paper and weigh off approx. 0.25 g (cut into small pieces) using a weighing boat and a scale.  I measured the temperature of the of the HCl solution every 30 seconds for 3 minutes. After exactly 3 minutes, I added the magnesium from the weighing boat. I Stirred using the thermometer and read the temperature every 10 seconds for 2 minutes and every 30 seconds for another 3 minutes.I recorded all the changes I observed during the course of the reaction.

Part 2 I measure out 50 cm3 of 1.00 mol/dm3 of the HCl solution using a graduated cylinder. Then I transferred it into the Styrofoam calorimeter. I weighed off approx. 0.40 g of magnesium oxide using a weighing boat and a scale. I measured the temperature of the of the HCl solution every 30 seconds for 3 minutes. After exactly 3 minutes, I added the magnesium oxide from the weighing boat.  I stirred it using the thermometer and read the temperature every 10 seconds for 2 minutes and every 30 seconds for another 3 minutes.

Once again, I recorded all changes  observed during the course of the reaction.

 

VARIABLES

Independent:     • Type of chemicals used

                        • Amount of chemicals used

Dependant:       • Change in enthalpy

Controlled:       • Elevation

                        • Air pressure

                        • Room temperature

                        • Equipment

                        • Humidity

 

OBSERVATIONS

 

Qualitative

 

Hydrochloric acid solution

It was a transparent water like solution. It had same texture as water.

 Magnesium metal strip

Its color was dull silver and it was a grey strip before cleaning but was shiny after I rubbed it with a sand paper. It was very ductile. 

Magnesium oxide

It was a fine white powder and it looked like floor.

 

Reaction: 1 (hydrochloric acid (HCl) solution and magnesium (Mg) metal strip)

It made a soft fizzing noise and the cup was a bit warm after the reaction started. At the end of the reaction we were left with a clear solution. The magnesium strip had completely dissolved in the hydrochloric acid solution. I could notice a pungent smell during and after the reaction. Bubbles had been formed at the side of the cup.

 Reaction: 2 (hydrochloric acid solution and magnesium oxide)

The cup was slightly warm and there was no apparent noise made by the reaction. The magnesium oxide powder had completely dissolved in the hydrochloric acid solution. I could notice a pungent smell during and after the reaction.

Quantitative

 Table 1: Data observed for Reaction #1 between hydrochloric acid solution and magnesium metal strip

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Table 2: Data observed for Reaction #2 between hydrochloric acid solution and magnesium oxide

 

 

 

CALCULATIONS (Processed Data)

Graph with extrapolated lines for experiment 1 A

Graph with extrapolated lines for experiment 1 B

Graph with extrapolated lines for experiment 1 C

 Graph with extrapolated lines for experiment 2 A

Graph with extrapolated lines for ...

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