Objective
Able to determine the heat of formation of magnesium oxide using Hess’s law.
Introduction
Hess’s Law states that the enthalpy change accompany with a chemical change is independent of the route by which the change of chemical occurs. It can be explained that Hess’s Law if the reactants A convert into products B, the overall enthalpy change will be the same no matter how many steps taken.
Enthalpy in Chemistry and Physics means a thermodynamic property of a system. It is the addition of internal energy plus with the volume of the system. The units used to express enthalpy are the joule, calorie of BTU (British Thermal Unit). The importance of enthalpy are it can allow us to determine whether the reaction was endothermic or exothermic, it is used to calculate the heat of reaction of chemical process and to measure heat flow in calorimetry.
The enthalpy formulas are H = E + PV (where H is enthalpy, E is internal energy of the system, P is pressure, and V is volume) Second Law of Thermodynamics states that the entropy of the universe in a spontaneous process and remains unchanged in an equilibrium process. Entropy means the number of ways of a system can be arranged.
Apparatus
• Calorimeter (polystyrene cup or plastic with a lid with a hole to insert the thermometer) • Thermometer (110oC) • Stirrer • Burette • Retort stand • Beaker • Electronic balance
Chemicals
• 2.0 M Hydrochloric acid (HCl) • Magnesium powder • Magnesium oxide powder
Procedure
1. The burette was filled with 2.0 M hydrochloric acid. 2. 30 mL 2.0 M of hydrochloric acid was measured out and was poured into the calorimeter. The thermometer was inserted.
The temperature was recorded after a few minutes. 3. 0.60 g of magnesium powder (limiting reactant) was weighed. The magnesium powder was added into the calorimeter containing hydrochloric acid. 4. The mixture was stirred gently and the highest temperature reached was recorded.
5. Step (1) until Step (4) was repeated using 1.00 g magnesium oxide instead of magnesium. 6. All readings were recorded in table. ? RESULT: Mass and Temperature of Substance Magnesium (Mg) Magnesium oxide (MgO) Mass, m (g) 0.6030 g 1.0170 g Initial temperature ( oC) (HCl) 25.0 oC 24.0 oC Final temperature ( oC) (HCl + Mg or MgO) 75.0 oC 28.0 oC Temperature change (?T) 50.0 oC 4.0 oC Volume of acid is used = 30.0 mL Mass of acid, ma = 30.0 g (assume density of acid is 1 g mL -1 )
Questions
1. Calculate ?H2 and ?H3.. 2. State whether the reaction is endothermic or exothermic. 3. Calculate ?H1, the standard enthalpy of formation of magnesium oxide. Given that the enthalpy of formation of water (value of ?H4) is -286 kJ/mol) ANSWERS: 1. ?Q = (ma x C x ?T) ÷ 1000 kJ = (30 x 4.2 x 62) ÷ 1000 kJ = 7.812 kJ ?H2 (kJ mol -1) = ?Q/n = 7.812/0.025 = – 312.48 kJmol -1 ?Q = (ma x C x ?T) ÷ 1000 Kj = (30 x 4.2 x 4) ÷ 1000 kJ = 0.504 kJ ?H3 (kJ mol -1) = ?Q/n = 0.504/0.025 = – 20.16 kJmol -1 2. It is an exothermic reaction.
3. ?H1 = ?H2 + (-?H3) + H4 = (-312.48) + (-20.16) + (-286.00) = – 618.64 kJmol -1 ?
DISCUSSION: The reaction between hydrochloric acid and magnesium powder gaves off exothermic reaction and it was the extreme one than the reaction between hydrochloric acid and magnesium oxide. It is a reaction where a gas begins to form and the solution becomes hotter. In periodic table, magnesium is an active element as it stays in Group 2 in the periodic table. Thus, it will react with hydrochloric acid actively.
Mg + 2HCl = MgCl2 +H2 The weaknesses in this experiment were the mistake of taking the amount of magnesium powder and magnesium oxide accurately. Therefore, the result of the experiment was slightly affected in taking of accurate heat given off by both compounds. Other than that, the experiment between hydrochloric acid and magnesium oxide was conducted twice due to the confusion between the first experiment and the second experiment in result of temperature change.
Conclusion
To summarise, in this experiment Hess’s Law is applied. For the first experiment, which is between hydrochloric acid and magnesium powder, the initial heat and the final heat is largely different and thus can calculate the internal energy in the compound.
For the second experiment, by using the same procedure as the first experiment, which is between hydrochloric acid and magnesium oxide, the initial heat and the final heat is slightly different but still can calculate the internal energy of the compound. These two experiments gave the same products at the end. Thus, Hess’s Law is applied.
