Circuit Experiment 10: Hess’s Law Aim of the Experiment (Essay Sample)

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Experiment 10: Hess’s Law
Aim of the Experiment
The main objective of this experiment is to gain a better understanding of Hess’s law. First, we shall determine the calorimeter constant. Using the calorimeter constant and the preceding experiments, we will calculate the heat of reaction of magnesium oxide.
Introduction
In chemistry, there are a number of experiments that cannot be done in the laboratory as they may lead to explosions. Some cannot be done as they are highly poisonous. As such, it becomes quite difficult to figure out their heat of reaction. However, Rudolf Walter Hess (Foulkes 175) discovered that carrying out successive reactions and making the reaction in question an intermediate reaction, it is possible to come up with its heat of reaction. He came up with a law that would enable us to find the heat reaction of such.
Hess’s law states that regardless of the path taken by a reaction, the overall heat of reaction will be the same. In this experiment, we shall consider the following reactions:
MgO +2HCl------ MgCl2 + H2O
Mg +2HCl------ MgCl2 + H2
H2 + ½ O2 ---- H2O
From the three equations, we can formulate the equation which is required which is:
Mg + ½ O2 ------ MgO
The theoretical heat of formation of water is -285.83 kj/mol (Ebbing, Darrell, 195).
In the experiment, we shall calculate the heat of reaction of when magnesium oxide reacts with hydrochloric acid and the heat of reaction when magnesium reacts with hydrochloric acid. Using the three experiments and their heat of formation, we shall then calculate the heat of reaction of magnesium oxide.
Apparatus and Chemicals
Calorimeter
Graduated beaker, 100ml
1.0M HCl
Magnesium ribbon
Cold water
Warm water
Magnesium oxide
Procedure
Part A: Determination of Calorimeter Constant
The weight of the empty calorimeter was measured and recorded. 25ml of water was carefully measured using a graduated beaker and poured into the calorimeter. The weight of the calorimeter and its new contents was measured and recorded. The cover of the calorimeter was then replaced and a thermometer inserted so as to measure the temperature. Tis temperature was recorded as the initial temperature. Water was boiled and its temperature was measured and recorded. 50ml of the boiled water was carefully measured using a graduated beaker and poured into the calorimeter. The calorimeter and its new contents was measured and recorded. Using the thermometer, the water was stirred until the hot and cold water completely mixed. The final temperature was then measured and recorded.
Part B: Enthalpy Changes for Reactions of Mg Ribbon with HCl
The weight of an empty calorimeter was measured and recorded. 50ml of 1.0M HCl was carefully measured using a graduated beaker and carefully poured into the calorimeter. The weight of the calorimeter and its contents was measured and recorded. A thermometer was inserted in the calorimeter and the reading taken and recorded. A magnesium ribbon was cut and its weight measure and recorded. The ribbon was then dipped in the calorimeter and allowed to react with the hydrochloric acid. As soon as the reaction was complete, the temperature was measured and recorded. The temperature change was then calculated.
Part C: Enthalpy Changes for reactions of MgO with HCl
The weight of an empty calorimeter was measured and recorded. 50ml of 1.0M HCl was carefully measured using a graduated beaker and carefully poured into the calorimeter. The weight of the calorimeter and its contents was measured and recorded. A thermometer was inserted in the calorimeter and the reading taken and recorded. Magnesium oxide of known weight was then put into the calorimeter and allowed to react with the hydrochloric acid. As soon as the reaction was complete, the temperature was measured and recorded. The temperature change was then calculated.
Results:
Part A: Determination of Calorimeter Constant
Atmospheric pressure

100.43 kPa

Boiling point of water at this pressure

101.1˚C

Mass of empty calorimeter

9.2289g

Mass of calorimeter plus 40ml of water

47.4319g

Mass of calorimeter after adding 50ml of hot water

79.3115g

Temperature of the cold water just before addition of the hot water

22.8˚C

Time at which the cold and hot water were mixed together(s)

195 seconds

Final temperature

50.2 ˚C

Part B: Enthalpy Changes for Reactions of Mg Ribbon with HCl
Mass of empty calorimeter

9.2041g

Mass of calorimeter plus 50ml of 1.0M HCl

50.9177g

Mass of the magnesium ribbon

0.2673g

Initial temperature of the HCl in the calorimeter

22.7˚C

Maximum temperature of the reaction

49.0˚C

ΔTc

26.3˚C

Part C: Enthalpy Changes for reactions of MgO with HCl
Mass of empty calorimeter

9.2193g

Mass of calorimeter plus 50 ml of 1.0M HCl

52.9036g

Mass of MgO

0.400g

Initial temperature of the HCl in the calorimeter

22.4 ˚C

Maximum temperature of the reaction

28.2 ˚C

ΔTc

5.8 ˚C

Sources of errors
During the reaction, some heat was lost to the environment while some heat was absorbed from the environment. However, it is not possible to determine exactly the amount of heat that was either lost or gained.
Some of the apparatus may not be functioning properly and as such it would not have been possible to obtain accurate readings.
The chemicals used may have been contaminated or have a slightly different concentration from the indicated. This will make the reaction precede either faster than expected or slower than expected.
Calculations
Determining the heat constant of the calorimeter:
ΔT of hot water = 101.1 ˚C – 50.2 ˚C = 50.9 ˚C
Mass of hot water = 79.3115g – 47.4319g = 31.8796g
ΔQhot water = mass of hot water * temperature change * 4.18J/g ˚C
ΔQhot water = 31.8796g * 50.9 ˚C * 4.18J/g ˚C = 6782.7674J
ΔT of cold water = 22.8 ˚C – 50.2 ˚C = -27.4 ˚C
Mass of hot water = 47.4319g - 9.2289g = 38.2030g
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