I need help understanding this question

Question

How can you combine the half-reactions below to give the overall reaction for the electrolysis of water?

 

Reaction A: 2H⁺ + 2e⁻ → H₂

 

Reaction B: ½ O₂ + 2H⁺ + 2e⁻ → H₂O

How can you combine the half-reactions below to give the overall reaction for the electrolysis of water?

 

Reaction A: 2H⁺ + 2e⁻ → H₂

 

Reaction B: ½ O₂ + 2H⁺ + 2e⁻ → H₂O

A.2A + 2B

B.2A – 2B

C.2B – 2A

D.2A – B

Answer 1

Answer is ... (B) 2A – 2B

The equation for the electrolysis of H₂O is ...

2H₂O ==> 2H₂ + O₂

How can you combine reaction A and B to get this result?

We need 2H₂O and 2H₂, so right off the bat, we can see we will need 2A and 2B. That eliminates D.

We need 2H₂O on the left side and H₂ and O₂ on the right side. We need to subtract 2B or add the reverse of 2B

That leaves us with option B (2A - 2B). Let's take a look at that.

4H⁺ + 4e- ==> 2H₂ (2 x A)

2H₂O ==> O₂ + 4H⁺ + 4e- (reverse of 2 x B)

--------------------------------------------------------------

4H⁺ + 4e- + 2H₂O ==> 2H₂ + O₂ + 4H⁺ + 4e-

2H₂O ==> 2H₂ + O₂ = desired reaction