# 11.3 Hess's Law on Combustion of Magnesium

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Design
Objective: Find the standard molar enthalpy of combustion for magnesium.

Manipulated Variables- volume of HCl, length of magnesium strip, mass of magnesium oxide,
Controlled Variable- type of calorimeter (Styrofoam cup)
Responding Variable- change in temperature
Procedure:
1. Cut out 3 pieces of magnesium strips each with lengths of maximum 5 centimeters. 2. Measure and record the lengths of the magnesium. 3. Scrub the magnesium strips with steel wool to clean out impurities. 4. Pour out approximately 310 mL of hydrochloric acid from the stock container. 5. Use a 50mL or 100mL graduated cylinder to measure approximately 50mL of hydrochloric acid. 6. Record the volume of hydrochloric acid. 7.

*Similarly, points from 90s to 140s show no or minimal change in temperature.
Calculations for the Enthalpy of Reaction for Magnesium for 3 Trials
Mg(s) + 2 HCl(aq) MgCl2(aq) + H2(g)
ΔrHm = mcΔt/n
Trial 1:
ΔrHm = (0.04g + 50.1g*)(4.19 J/(g °C))(4.5°C)0.04g24.31g/mol = -574560.5902J/mol**
*Assuming that the density of HCl(aq) solution is 1g/mL
** Value is negative because the reaction is exothermic

Trial 2:
ΔrHm = (0.04g + 50.2g)(4.19 J/(g °C))(4.2°C)0.04g24.31g/mol = -537326.0293J/mol

Trial 3:
ΔrHm = (0.04g + 50.1g)(4.19Jg °C)(4.2°C)0.04g24.31g/mol = -536256.5508J/mol

**The discrepancy between ΔrH of first trial and the other trials is approx. 7%, therefore the trials can still be counted as precise.

Average ΔrHm= (-574560.5902J/mol + -537326.0293J/mol + -536256.5508J/mol)/3 = -549381.0568J/mol
ΔrH= (n of Mg) ×(Average ΔrHm) = (1mol ) × (549381.0568J/mol) = -549381.0568J

Calculations for the Molar Enthalpy of Reaction for Magnesium Oxide
MgO(s) + 2 HCl(aq) MgCl2(aq) + H2O(l)
ΔrHm = mcΔt/n
Trial 1:
ΔrHm = (0.90g + 50.1g)(4.19 J/(g °C))(10.6°C)0.90g40.31g/mol = -105.4519393kJ/mol

Trial 2:
ΔrHm = (0.82g + 51.0g)(4.19Jg °C)(10.5°C)0.82g40.31g/mol
= -112.0726591kJ/mol

Trial 3:
ΔrHm = (0.82g + 50.5g)(4.19Jg