EXPERIMENT 1: CALORIMETRY : HESS’S LAW
OBJECTIVES The purpose of this experiment is to determine the standard enthalpy of formation of magnesium oxide, ∆Hof.
INTRODUCTION The heat evolved or absorbed when a process occurs at constant pressure is equal to the change in enthalpy. Since H is defined by the equation:
H=U+pV then ΔH = ΔU + pΔV at constant pressure; where ΔH represents the change in enthalpy. Reactions which occur in unsealed containers in the laboratory, occur essentially at constant pressure (= atmospheric pressure). Chemical processes which occur in plants and animals also occur at constant pressure. This is why enthalpy is such an important thermochemical parameter for physical and chemical processes. It can be related directly to the heat evolved or absorbed when the processes occur under "natural" conditions. When processes occur in a pressure-tight, sealed container, such as a bomb calorimeter, the heat evolved or absorbed is equal to the change in internal energy, ΔU, since the process occurs at constant volume. Enthalpy is a state function, and so if one wants to define uniquely the enthalpy change in a physical or chemical process, one needs to define only the initial and final states of the system when the process occurs. For a physical process such as the melting of ice, once the pure substance is identified and the pressure is specified, the enthalpy change is uniquely defined. The value which is now most often quoted for the enthalpy change in this process, is the molar enthalpy of melting (or "latent heat" of melting) when the process occurs at a pressure of 1 bar. (1 bar = 10x5 Pa)
For chemical reactions, one can define a "standard" enthalpy of reaction by specifying "standard" initial and final states of the reacting system. The standard enthaly of formation of a chemical compound, ΔHf, is the heat evolved or absorbed when the compound is formed in its standard state from its constituent elements in their standard states. The standard state of a substance is defined as the stable form of that substance at a pressure of 1 bar and a specified temperature. The standard molar enthalpies of formation of elements are zero at all temperatures - by definition. The standard molar enthalpy of formation of a compound is therefore a uniquely defined quantity, ΔHf(T), and values given in thermodynamic tables are usually at 298.15 K. These quantities are useful because they can be used to obtain enthalpy of any reactions in which the individual compounds are involved. The heat, Q required to change the temperature of a substance from Ti to Tf is given by:
Q = mC(Tf - Ti) where m is the mass whose temperature is changed from Ti to Tf and C is the heat capacity of the substance. When m is in kg, C is in J K-1 kg-1, and T is in °C or K, the heat is in Joule. Note that the heat capacity, C, quoted here, bears no indication of conditions, that is, whether it is Cp or Cv. This is because only solids and liquids are usually involved in calorimetry at this level, and Cp and Cv are very nearly the same value for matter in these "condensed" phases.
PROCEDURE A burette was used to transfer 50cm3 tap water into the calorimeter and was covered. Thermometer was put into the calorimeter. The temperature of water was recorded for four
minutes at one minute intervals. 50cm3 of 44oC hot water was measured using graduated cylinder and was poured into a beaker. Another thermometer was used to record the temperature of hot water and the hot water was quickly transferred into the calorimeter at the fifth minute. The lid was replaced and the water was stirred carefully using thermometer. The temperature was recorded every 15 seconds for the next 3 minutes. The experiment was repeated using the copper calorimeter.
Then 1.1 g of magnesium powder was weighted and was recorded. 50cm 3 of 2 M HCl was drain from a burette into the calorimeter and was covered and thermometer was replaced. The temperature was recorded every minutes for four minutes. At the fifth minute, the magnesium powder was quickly poured into the HCl. The lid was carefully covered and the content in the calorimeter was stirred. The temperature of room was recorded every 15 seconds for 3 minutes interval. This second reaction was repeated again using 1.7037 g of magnesium oxide. All the data was recorded in the table.
RESULTS Temp. of hot water (oC) Mass of Mg (g) Mass of MgO (g)
44 1.0113 1.7037
Data sheet for Experiment 1
Coffee cup Calorimeter Time
DISCUSSION
T (oC)
Copper Calorimeter
Mg and HCl
MgO and HCl
Time
T (oC)
Time
T (oC)
Time
T (oC)
1 minute
25
1 minute
22
1 minute
22
2 minutes
25
2 minutes
22
2 minutes
22
3 minutes
25
3 minutes
22
3 minutes
22
4 minutes
25
4 minutes
22
4 minutes
22
15 s
30
15 s
67
15 s
34
30 s
30
30 s
67
30 s
36
45 s
30
45 s
66
45 s
36
60 s
30
60 s
65
60 s
36
75 s
30
75 s
64
75 s
36
90 s
29
90 s
64
90 s
37
105 s
29
105 s
64
105 s
37
120 s
29
120 s
63.5
120 s
37
135 s
29
135 s
62
135 s
38
150 s
29
150 s
61
150 s
38
165 s
29
165 s
60
165 s
38
180 s
29
180 s
60
180 s
38
In this experiment, the standard enthalpy of formation of magnesium oxide is determined by conduct some reaction which is involved in using calorimeter. One of the reaction that form is between the Magnesium metal and hydrochloric acid. Another reaction that involve is between Magnesium oxide and hydrochloric acid. The last reaction requires the standard heat formation for the formation of water from hydrogen and oxygen. The reaction can be define as equations, Mg (s) + 2HCL (aq) → MgCl2 (aq) + H2
∆Hrxn (1)
MgO (s) + 2HCl (aq) → MgCl2 (aq) + H2O
∆Hrxn (2)
H2 + 1/2O2 → H2O
∆Hf (H2O) (3)
The results obtained from the experiment by calculation of the heat of reaction for the reaction of magnesium metal and magnesium oxide. With these heat of reactions combined with the heat of reaction of water which were given, the value of the heat of reaction of the combustion of magnesium metal was calculated using Hess's law, which came out to be -189.10 kJ/mol. This value can be expected because the combustion of magnesium metal is an exothermic process, so the heat of reaction should be negative. Also, at the molecular level, the addition of magnesium oxide or magnesium metal to hydrochloric acid increases the temperature of the reaction meaning that as the reaction proceeds, the reaction is giving off heat. Energy must be supplied to break chemical bonds and energy must be released when chemical bonds are formed.
CONCLUSION
From this experiment, the heat capacity for copper calorimeter was determined that is 238.86 J/oC and the standard enthalpy of formation of magnesium oxide is -395.28kJ mol -1. Exothermic reaction occurs that is energy release in the form of heat.
REFERENCE Weiner, S. A. And Harrison, B. (2010). Introduction to Chemical principles: A Laboratory Approach. 7th Edition, Brooks/Cole Cengage Learning, USA Morris Hein, Leo R Best, Robert L. Miner (2006), Foundations of Chemistry in the Laboratory. 12th Edition, John Wiley.
