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Enthalpy help

I'm stuck on a few questions, please help me out if you can!

2. 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.

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

I worked out the enthalpy change in the reaction with magnesium in excess and then divided by the number of moles in the reaction (0.1) to give me193534 J/mol. Then I worked out the number of moles in 0.8g of magnesium (which is 0.03). I used this value to work out the mass of the coppersulphate which will react (assuming that the density of CuSO4=water as required by the question). This is roughly 30. I also used the number of moles to work out the enthalpy change (5806.02J). I subbed the values in to E=m(4.18)t to give me roughly 4 degrees.
The answer is 15.2 degrees. Where did I go wrong?

2) When 2.3 g of magnesium chloride dissolves in 200 cm3 of water, the temperature rose by 3.4 oC. Calculate the enthalpy change for the reaction
Number of moles in mgcl2= (2.3/71+24.3)
Mass of water= moles x 1000. its roughly 24. I multiplied 24,4.18 and 3.2 together and divided it by the number of moles. I got roughly 10000. Did I miss something?
Answer: -118KJ/mol

3) I dont understand how to work out when there is a minus sign in front of the enthalpy ( I know it is a negative enthalpy when the reaction is exothermic). How do I work out if it is a negative or positive enthalpy mathematically?

4) In this question, I have been given the average bond enthalpies of some bonds.

Work out he enthalpy of formation of 1,2-dibromopropane.
I drew out 1,2-dibromopropane, counted the number of C-C bonds (4) and the number of C-H bonds (12) and multiplied each with the corresponding number on the table. I got 6608J
Answer:-95KJ/mol

I must be missing something major. Please help!!
Original post by thevegetarian
I'm stuck on a few questions, please help me out if you can!

2. 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.

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

I worked out the enthalpy change in the reaction with magnesium in excess and then divided by the number of moles in the reaction (0.1) to give me193534 J/mol. Then I worked out the number of moles in 0.8g of magnesium (which is 0.03). I used this value to work out the mass of the coppersulphate which will react (assuming that the density of CuSO4=water as required by the question). This is roughly 30. I also used the number of moles to work out the enthalpy change (5806.02J). I subbed the values in to E=m(4.18)t to give me roughly 4 degrees.
The answer is 15.2 degrees. Where did I go wrong?

With magnesium in excess all of the copper(II) reacts = 0.1 mol

The energy release = mcΔT = 0.1 x 4.2 x 46.3 = 19.446 kJ

So the reaction releases 19.446/0.1 = 194.5 kJ per mol

Hence if 0.03 mol of Mg completely reacts you will get 0.033 * 194.5 = 6.48 kJ released.

6.48 = 0.1 * 4.2 * ΔT

ΔT = 6.48/(0.1 * 4.2) = 15.4ºC


2) When 2.3 g of magnesium chloride dissolves in 200 cm3 of water, the temperature rose by 3.4 oC. Calculate the enthalpy change for the reaction
Number of moles in mgcl2= (2.3/71+24.3)
Mass of water= moles x 1000. its roughly 24. I multiplied 24,4.18 and 3.2 together and divided it by the number of moles. I got roughly 10000. Did I miss something?
Answer: -118KJ/mol

3) I dont understand how to work out when there is a minus sign in front of the enthalpy ( I know it is a negative enthalpy when the reaction is exothermic). How do I work out if it is a negative or positive enthalpy mathematically?

4) In this question, I have been given the average bond enthalpies of some bonds.

Work out he enthalpy of formation of 1,2-dibromopropane.
I drew out 1,2-dibromopropane, counted the number of C-C bonds (4) and the number of C-H bonds (12) and multiplied each with the corresponding number on the table. I got 6608J
Answer:-95KJ/mol

I must be missing something major. Please help!!

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