Need help determining the molar enthelpy of Magnesium with HCl

• Sythe777
In summary, the student attempted to solve a Hess law problem and came up with a value of -563.7 kJ/mol.
Sythe777

1. Homework Statement

What is the molar enthalpy of combustion ΔH(c) of magnesium?

Mg(s) + 2HC(aq) → H2(g) + MgCl2(aq)

MgO(s) + 2HCl(aq) → H2O(l) + MgCl2(aq)

m(Mg) = 0.5 g
m(MgO) = 1.0 g
V(HCl) = 100.0 mL
C(HCl) = 1.0 mol/L
T(HCl) = 19° C
T(rxn1) = 38° C
T(rxn2) = 25° C

rxn1 relates to Mg(s) + 2HCl(aq)

rxn2 relates to MgO(s) + 2HCl(aq)2. Homework Equations

n(Mg) = m/M
q = mcΔT
ΔH(C) = -q/n

3. The Attempt at a Solution

n(Mg) = m/M
n(Mg) = (0.5 g)/(24.31 g/mol)
n(Mg) = 0.02056766762649115590292060880296 mol
n(Mg) = 0.021 mol

q = mcΔT
q = (3.6 g)(4.18 J/g °C)(38° C – 19° C)
q = 285.912 J
q = 285.9 J

ΔHC = -q/n
ΔHC = -285.9 J/0.021 mol
ΔHC = 13614.285714285714285714285714286
ΔHC = 13.6 kJ/mol

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If anyone can help me with this it would be greatly appreciated!

Wouldn't hurt if you will explain what you are doing and what the data is about.

Do you know the Hess law?

My class touched on Hess's Law for a bit but not enough to make much of an impact on me. I would edited the post above, but it shows no edit button to use. Basically, I am finding the moles of magnesium for molar enthalpy and the heat quality of the HCl for the other part of molar enthalpy and plug values into the molar enthalpy equation.

Forgot to mention, the data above is experimental

Last edited:
This is a Hess law problem, so you can't solve it without understanding the Hess law.

all right, I think I've got it after checking with Hess's Law and doing a full solution.

this is what I have come up with: http://imgur.com/a/Cs95f (sorry, I am lazy)

overall, I got a value of -563.7 kJ/mol.

1. What is molar enthalpy?

Molar enthalpy, also known as molar heat of reaction, is the amount of heat released or absorbed during a chemical reaction per mole of a substance.

2. How is molar enthalpy determined?

Molar enthalpy is determined through calorimetry, which involves measuring the change in temperature of a reaction mixture and using the heat capacity of the system to calculate the molar enthalpy.

3. What is the equation for calculating molar enthalpy?

The equation for calculating molar enthalpy is: ΔH = q/moles, where ΔH is the change in enthalpy, q is the heat transferred, and moles is the number of moles of the substance.

4. How do I determine the molar enthalpy of Magnesium with HCl?

To determine the molar enthalpy of Magnesium with HCl, you will need to perform a reaction between a known amount of Magnesium and HCl in a calorimeter. The change in temperature of the reaction mixture can then be used to calculate the molar enthalpy.

5. Why is it important to determine the molar enthalpy of a reaction?

Determining the molar enthalpy of a reaction is important because it gives insight into the energy changes that occur during a chemical reaction. It also allows for the prediction and optimization of reaction conditions, which can be useful in industrial processes and research.

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