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# Hess

Background: There are many reactions for which direct measurement of the enthalpy reaction is difficult, the hydration of magnesium sulfate being such a reaction. In these cases it is often possible to determine the enthalpy of reaction by direct measurements on more accessible related systems and then applying Hess' Law.
Aim: to determine the change in enthalpy for the hydration of magnesium sulfate using Hess' Law.
Hypothesis: It looks like there are being more bonds formed than broken in this reaction, therefore, I think the change in enthalpy will be exothermic.
Variables: .
- Temperature of the surroundings (this should not effect our experiment because it will be carried out in a polystyrene cup).
- Air pressure (will have no effect on our experiment as it should not change during the short time our experiment will take.).
- Temperature of the water (there will be an error with the thermometer).
- Mass of MgSO4 and MgSO4•7H2O (error on the electronic balance).
- Specific Heat Capacity (we are using the specific heat capacity of water).
- Volume of Water - this is the controlled variable, as we will use the same amount of water in each experiment.
- Water may have impurities (but we will use distilled water to decrease the error here).
Equipment:.
.
Distilled Water.
Measuring Cylinder.
Polystyrene cup with lid.
Thermometer.
MgSO4 .
MgSO4•7H2O.
Stopwatch.
Spatula.
Electronic Balance.
Weighing Container.
.
Method: .
1. Measure out, using a measuring cylinder, 100mLs of distilled water into a polystyrene cup, and record the temperature.
2. Accurately weigh out about 7.5 grams of anhydrous magnesium sulfate.
3. Add this to the water and record the temperature of the water every 20 seconds during the reaction.
4. Record the maximum temperature reached.
5. Repeat steps 1 -   4, but taking approximately 15.5 grams of hydrated magnesium sulfate.
Explaining the method:.
We want to find out the change in enthalpy for the following equation: .

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