[Kimixi]

This is the 4th question of this set, but i cant get it, i got the previous 3 answers.

A student, using an ice calorimeter, studies the enthalpy of reaction of the following exothermic chemical change:


Mg(s) + 2H+(aq) ---> Mg2+(aq) + H2(aq)


From data collected, it was determined that a magnesium sample having a mass of 0.0676 grams would react with excess HCl and melt the ice in the calorimenter that resulted in a volume change of 0.308 ml.

1. Calculate the number of grams of ice melted during this experiment.
= 3.3958g

2. Calculate the number of moles of ice melted during this experiment.
= 0.1886g

3. Calculate the heat released during this reaction in Kcal/mole.
= 0.2709

4. Calculate the enthalpy of formation of magnesium, DHf(Mg2+), in Kcal/mole for this reaction.

(I dont know how to do this one)

thanks for your help

[Alpha-Omega]

Start by writing the RXN EQNS for what is going on in the system.

HINT:  You need the DHf for water from the literature.

[Kimixi]

Isn't the reaction equation just

Mg(s) + 2H+(aq) ---> Mg2+(aq) + H2(aq)

I dont know what numbers i would need to use to figure out the enthalpy of the formation of Mg2+

[Kimixi]

It says, the heat of fusion of ice is 1.436 K Calories per mole, (6.008  KJ/mole) is this what you mean by dHF. Or is dHF 4.184 J/g-K (found this online, i think this is specific heat)

Once i get this number do I have to Subtract it from the heat of the reaction, so that remained is Mg2+'s Enthalpy. (this is prob all wrong)

Im really confused by this question.

[Alpha-Omega]

Yes, you are correct - if you are seeking the enthalpy of formation of Mg2+...then the eqn

Mg(s) + 2H+(aq) ---> Mg2+(aq) + H2(aq)  is correct

In your system you have ice melting.  So is there anything else you have to take into account?

Check the procedure that is given here:

The accepted heat of formation of Mg2+(aq) is -462.0 kJ/mol (25 °C, 1 M).

The heats of formation of Mg(c), 2H+(aq), and H2(g) are defined to be 0 kJ/mol at standard conditions. Therefore,

ΔH reaction = ΔH products - ΔH reactants
= [(ΔHf Mg²⁺(aq)) + (ΔHf H₂(g))] - [ (ΔHf Mg(c)) + 2 x (ΔHf H+(aq))]
= [(ΔHf Mg²⁺(aq)) + 0]-[0 + 0]
= ΔHf Mg²⁺(aq)