I'm trying to calculate the enthalpy of the reaction

Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2 (g)

however I cannot find the enthalpy of the compound MgCl2 (aq) and I cannot calculate the total enthalpy without it

My text gives -641.8 kJ/mol MgCl2(s)as delta Hformation.

To calculate the enthalpy change of the reaction, you can use Hess's Law, which states that the total enthalpy change of a reaction is the sum of the enthalpy changes of the steps that make up the overall reaction.

Since you cannot find the enthalpy of formation for MgCl2(aq), you can use the enthalpy changes of other reactions to indirectly determine it. Here's how you can proceed:

1. Look for known reactions involving MgCl2. For example, you can consider the following reaction:
Mg(s) + Cl2(g) -> MgCl2(s)
The enthalpy change of this reaction is -641 kJ/mol.

2. Consider the reaction between HCl(aq) and Cl2(g):
HCl(aq) + Cl2(g) -> Cl2(aq) + HCl(g)
The enthalpy change of this reaction is -57 kJ/mol.

3. Consider the reaction between H2(g) and HCl(g):
H2(g) + Cl2(g) -> 2HCl(g)
The enthalpy change of this reaction is -185 kJ/mol.

4. Combine all the reactions to obtain the desired reaction:
Multiply the second reaction by 2 and reverse the third reaction to obtain:
2HCl(aq) -> 2HCl(g)
2H2(g) + Cl2(aq) -> 2HCl(g)

Subtract the sum of the enthalpies of the reversed reactions from the enthalpy of the desired reaction:
ΔH = (-641 kJ/mol) - 2*(-57 kJ/mol) - (-185 kJ/mol)
ΔH = -342 kJ/mol

By doing this, you have indirectly calculated the enthalpy change for the formation of MgCl2(aq) as -342 kJ/mol.

To calculate the enthalpy of the reaction, you need to know the enthalpy values of the individual compounds involved in the reaction. In this case, you are specifically looking for the enthalpy of formation of MgCl2(aq).

Unfortunately, the enthalpy of formation of MgCl2(aq) is not readily available in most standard references or databases. One possible approach to estimating its value is to use Hess's Law.

Hess's Law states that the total enthalpy change of a reaction does not depend on the path taken, but only on the initial and final states. In this case, you can make use of the following reactions:

Equation 1: Mg(s) + HCl(aq) -> MgCl2(aq) + 1/2H2(g)

Equation 2: 1/2H2(g) -> H2(g)

Equation 3: Mg(s) + Cl2(g) -> MgCl2(aq)

The enthalpy change for Equation 1 can be determined using available data sources. The enthalpy change for Equation 3 can also be determined using standard enthalpy of formation values since it involves the elements in their standard states.

By manipulating and combining these three equations, you can cancel out the MgCl2(aq) term, leaving you with the enthalpy change for the desired reaction:

Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

Once you have determined the enthalpy change for this combined reaction, you can then calculate the enthalpy of formation for MgCl2(aq) by subtracting the enthalpy changes of the other two equations.

It's important to note that this approach involves some approximations and assumptions, as the enthalpy of formation of MgCl2(aq) is not directly measured. Nonetheless, this method can provide a reasonable estimation of the enthalpy of the reaction you are interested in.