Hess Law Lab
Problem: What is the molar enthalpy of formation of magnesium oxide?
Controlled: Isolation of calorimeter, concentrations of substances involved.
Assumptions: Specific heat capacity of water, we assume that the acid has the
same qualities as water including heat capacity, and we assume the enthalpy of formation for magnesium oxide from the data booklet for theoretical value.
Hess’s Law: Hess' law states that the energy change for any chemical or physical process is independent of the pathway or number of steps required to complete the process provided that the final and initial reaction conditions are the same. In other words, an energy change is path independent, only the initial and final states being of importance.
Waste disposal and safety: Corrosive substance which causes severe but delayed burns. DO NOT INGEST. Avoid Inhalation. Dispose as instructed by the lab technician or the teacher.
Apparatus Diagram: Figure from Inquiry into chemistry textbook pg 352
1.00 mol/L HCl(aq)
100 mL graduated cylinder
Mg(s)+ O2(g) MgO(s)
H2(g)+ O2(g) H2O(l) + 285.8kJ
Part 1: Determining ΔH of Reaction (2)
Set up the simple calorimeter refer to figure 9.16 above. Using a graduated cylinder, add 100 mL of 1.00mol/L HCl(aq) to the calorimeter. 2.
Record the initial temperature of the HCl(aq) to the nearest tenth of a degree. 3.
Find the mass of no more than 0.80 grams of MgO(s) powder. Record exact mass. 4.
Add the MgO(s) powder to the calorimeter containing the HCl(aq) swirl the solution gently, recording the highest temperature reached. 5.
Dispose as directed by teacher.
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