# Enthalpy changes

1. Jun 9, 2009

### mouk92

1. The problem statement, all variables and given/known data

Magnesium will also displace copper from copper (II) sulphate solution. If an excess of magnesium is added to 100 cm3 of 1.0 moldm-3 copper(II) sulphate, the temperature increases by 46.3 oC.

a) Calculate the molar enthalpy change for the reaction

b) Calculate the minimum quantity of magnesium required to ensure it is in excess.

c) Calculate the temperature change if only 0.8 g of magnesium is added.

2. Relevant equations

Q=m*c *DT
Moles=concentration*volume
DH=q in KJ/moles

3. The attempt at a solution

a) Calculate the molar enthalpy change for the reaction. ( i got the answer to be -193.534 KJmol -1 )

However im stuck on the rest of this question.......

b) Calculate the minimum quantity of magnesium required to ensure it is in excess.
c) Calculate the temperature change if only 0.8 g of magnesium is added.

thanks.

2. Jun 9, 2009

### vertciel

Hello there,

I will start by writing the chemical equation:

Mg + Cu(SO4)2 -> Cu + Mg(SO4)2

b) For this part, you know that Mg is in excess. Therefore, you simply need to find the amount of Cu(SO4)2 in mols. When you have this, then you know that the excess amount of Mg will be anything > the amount of Cu(SO4)2 because Cu(SO4)2 is the limiting reagent.

c) You know that Q = mc∆T.

You know the specific heat capacity, mass of magnesium, and you can calculate Q. Since you know the heat enthalpy, you can multiply this value by the mols of magnesium used to find the heat energy evolved from adding this magnesium.

Now, you just need to rearrange the above equation for ∆T and solve.

I hope that this helps!