Find the enthalpy of combustion of magnesium using Hess' law given the following data (assume that exactly 100.0 mL of HCl was used in each trial and that HCl is in excess)
Magnesium/HCl reaction:
Grams of Mg= 0.244g
Initial temp= 32.2 degrees celsius
Final temp= 21.3 degrees celsius
Magnesium Oxide/HCl reaction:
Grams of MgO= 0.996g
Initial temp= 28.8 degrees celsius
Final temp= 21.4 degrees celsius
What's the answer though
To find the enthalpy of combustion of magnesium using Hess' law, we need to manipulate and combine the given reactions to cancel out the common reactants and obtain the desired equation. Here's how you can proceed:
1. Write the balanced equations for the given reactions:
Reaction 1: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Reaction 2: MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
2. Calculate the change in enthalpy for each reaction using the given temperature data and the formula:
ΔH = q / n
where ΔH is the enthalpy change, q is the heat absorbed or released, and n is the number of moles of the limiting reactant.
For Reaction 1:
ΔH1 = q1 / n1
where q1 is the heat absorbed or released in Reaction 1 and n1 is the number of moles of Mg.
For Reaction 2:
ΔH2 = q2 / n2
where q2 is the heat absorbed or released in Reaction 2 and n2 is the number of moles of MgO.
3. Convert the grams of Mg and MgO used to moles using their molar masses:
Molar mass of Mg = 24.31 g/mol
Molar mass of MgO = 40.31 g/mol
n1 = mass of Mg / molar mass of Mg
n2 = mass of MgO / molar mass of MgO
4. Calculate the heat absorbed or released, q1 and q2, using the formulas:
q = m*c*ΔT
where q is the heat absorbed or released, m is the mass of the substance, c is the specific heat capacity of the substance, and ΔT is the change in temperature.
For Reaction 1:
q1 = (100.0 mL) * (1.00 g/mL) * (4.18 J/g°C) * (21.3°C - 32.2°C)
For Reaction 2:
q2 = (100.0 mL) * (1.00 g/mL) * (4.18 J/g°C) * (21.4°C - 28.8°C)
Note: The specific heat capacity used here is for water.
5. Substitute the values of q1, q2, n1, and n2 into the equations for ΔH1 and ΔH2 to calculate their respective values.
ΔH1 = q1 / n1
ΔH2 = q2 / n2
6. Apply Hess' law to calculate the enthalpy change for the desired reaction, Reaction 3:
Reaction 3: Mg(s) + 1/2O2(g) → MgO(s)
ΔHcombustion = ΔH2 - ΔH1
Finally, substitute the calculated values of ΔH1 and ΔH2 into the equation to find the enthalpy of combustion of magnesium.