Question

Which equation represents a redox reaction?

Answer 1

 Transfer of electrons from one species to another species is called a redox reaction. it consists of one species that will give out electrons called an oxidation reaction and the other species take in electrons called reduction reactions. both these half equations together make up a redox reaction.
In oxidation, the oxidation state increases of the species that is oxidised.
In reduction, oxidation state of the species decreases of the species that is reduced.
1)
2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2Ag(s)
single displacement reaction
oxidation number of Ag decreases from +1 to 0 - reduction reaction
oxidation number of Cu increases from 0 to +2 - oxidation 
since both oxidation and reduction reactions take place its a redox reaction.

2)
BaCl₂(aq) + Na₂CrO₄(aq) → BaCrO₄(s) + 2NaCl(aq)
its a double displacement reaction and the ions have been exchanged.
Oxidation numbers of the elements do not change therefore this is not a redox reaction.

3)
H₃PO₄(aq) + 3KOH(aq) → 3H₂O(l) + K₃PO₄(aq)
acid base reaction where the oxidation state of the elements remain the same so its not a redox reaction.
 4)
CaCO₃(s) → CaO(s) + CO₂(g)?
decomposition reaction where the oxidation states remain the same hence not a redox reaction 

Answer 2

Answer:

2agno3(aq) + cu(s) → cu(no3)2(aq) + 2ag(s)

Explanation:

Now let us take a good look at the reaction presented in the answer.

If we look at silver on the lefthand side of the reaction equation, we will realize that silver is in the +2 oxidation state but on the right hand side of the reaction equation, silver is in the zero oxidation state. This implies that silver has been reduced. Reduction refers to a decrease in oxidation number.

Now let us move to copper. Copper is in the zero oxidation state on the lefthand side of the reaction equation but on the right hand side of the reaction equation, it is in the +2 oxidation state. Hence, there is an increase in the oxidation number of copper- oxidation of cooler has occurred. Oxidation rs to a positive increase in oxidation number.

Hence the reaction involves oxidation and reduction, thus it s a redox reaction.