Chemistry Unit 3 help needed !

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this is a question on edexcel chemistry unit 3B paper for June 2009. the answer is from the edexcel mark scheme

Question
The H for the reaction between zinc and copper(II)sulphate was determined to be -216.8kJ/mol while that for the reaction between zinc and lead (II) nitrate was found to be -154.0 kJ/mol.

Place the three metals (Copper, lead and zinc) in DECREASING reactivity and justify your answer using these H values.

Answer
- Zn>Pb>Cu
Zinc displaces both so is mostreactive (1)
The more exothermic / negative(accept ‘the larger’) the
H thegreater the difference in reactivity(so lead more reactive than copper)(1)

what do u nt understand in this question?

I want to know why they have this order

Zinc+ copper sulfate= zinc sulfate + copper
zinc + Lead (II) Nitrate= Zinc (II) Nitrate+ lead

As you can see from the reactions above. zinc displaces both lead and copper. from that u can tell that zinc is more reactive than the two metals. so first comes zinc.

Now to figure out which is the most reactive between Lead and copper. Energy required for zinc to displace copper from copper sulfate is -216.8 kj per mol and for zinc to displace lead from lead (II) Nitrate, the energy required is -154.0 kj per mol. less amount of energy is required to displace lead compared to copper. this tells us that lead is more reactive than copper. As you have learnt in your chemistry class the more reactive a substance the more easily it loses/gains electrons, thus requiring less amount of energy to break the bonds.

therefore you get 1)zinc 2) lead (as it is the second most reactive) 3)copper (as it is the least reactive)

Really hope u understood all that! if not let me know!

Yea...... You are very helpful...Thanks..
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