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OCR Chemistry Redox help

Hay everyone, Iv'e been going through all the pat papers for the exam today and there is one type of question that I become stuck on. Is there anyone that can show me HOW to solve it.

Question:
The student dissolved another small piece of the broze relic in dilute (8 mole dm-3) nitric acid. A blue solution containing Cu2^+ ions was again formed but this time a colourless gas was produced with the molecular formula NO.

Equation 4.2 reprsents the unbalanced equation for this second reaction.
Cu + H^++ NO3- ------> Cu^2+ + NO + H2O

By considering oxidaton numbers, balance equation 4.2

Answer:
3Cu + 8H^+ + 2NO3^- ----> 3Cu^2+ + 2NO + 4H2O

How did they get that equation? :s
Reply 1
yeah i was doing that paper last night and have no clue how they got that answer??
since the oxidation state of Cu increases by two and N decreases by three you need two find a common factor to balance the oxidation states. so times Cu by 3 which gives a change of +6 and times N by 2 which gives a change of -6 so they balance.
Reply 3
First balance the number of each element on each side (like normal) then balance the charges...

Cu + H+ + NO3- ----> Cu2+ + NO + H2O
Cu + 4H+ + NO3- ----> Cu2+ + NO + 2H2O
double... 2Cu + 8H+ + 2NO3- ----> 2Cu2+ + 2NO + 4H2O
+6 on left, +4 on right SO... we need another Cu2+
3Cu + 8H+ +2NO3- ----> 3Cu2+ + 2NO + 4H2O ok?
you might not get full marks for that tho cos you were asked to balance it by considering the oxidation numbers and two of the three marks were for stating the changes in oxidation states
Reply 5
thanks
Reply 6
can u guys help me with a question from jan 04 paper if u have it. this type of questions comes up often but i dont see how they get the answers. its the rates question where they give u a table with the initial amounts of reactants and tell u to work out the equilibrium amounts or reactant and product. thanks
your told that 1.0 moles of CH3COOH remain so therefore 6.0-1.0= 5.0 moles of CH3COOC2H5 have been formed. This requires 5.0 moles from the C2H5OH aswell therefore that leaves 7.5 moles of that and 5 moles of H2O are made just to balance everything.