You are Here: Home >< A-levels

# Half equations watch

1. I have some half equation questions however I checked the mark scheme and according the the mark scheme i have been adding the electrons in the wrong place?
Would someone be able to explain to me why the electrons are added in the way shown in the mark scheme?

question:
a) PbO2 -> Pb2+ , Cl- -> Cl2

PbO2 + 4H+ + 2e -> Pb2+ + 2H2O . (how come the electrons are not added on the right side of the equation here because would that not make it equal on both sides?)
2Cl- ->Cl2 + 2e
PbO2 + 4H+ + 2Cl- -> Pb2+ + 2Cl- + 2H2O

sorry if this doesn't make sense i'm just very confused!
2. Because you need to add the hydrogens on the left which makes the left hand side 4+ and the right 2+, so 2 electrons go on the left

So:
PbO2 -> Pb2+
1. You require oxygen so you add water to the right hand side

PbO2 -> Pb2+ + 2H2O

2. Now you require hydrogen, so you add hydrogen to the left

PbO2 + 4H+ -> Pb2+ + 2H2O

3. Now balance with electrons

PbO2 + 4H+ + 2e -> Pb2+ + 2H2O

Are you fine with the chlorine?
3. (Original post by AzureCeleste)
Because you need to add the hydrogens on the left which makes the left hand side 4+ and the right 2+, so 2 electrons go on the left
thank you so much!
4. (Original post by AzureCeleste)
Because you need to add the hydrogens on the left which makes the left hand side 4+ and the right 2+, so 2 electrons go on the left

So:
PbO2 -> Pb2+
1. You require oxygen so you add water to the right hand side

PbO2 -> Pb2+ + 2H2O

2. Now you require hydrogen, so you add hydrogen to the left

PbO2 + 4H+ -> Pb2+ + 2H2O

3. Now balance with electrons

PbO2 + 4H+ + 2e -> Pb2+ + 2H2O

Are you fine with the chlorine?

I think I am fine with the chlorine one.

I am working my way through the list of questions and I am struggling with d and e!

d) ClO- -> ClO3-, ClO- -> Cl-

e) H2SO4 -> SO2, Br- ->Br2
5. (Original post by Chiaralia)
I think I am fine with the chlorine one.

I am working my way through the list of questions and I am struggling with d and e!

d) ClO- -> ClO3-, ClO- -> Cl-

e) H2SO4 -> SO2, Br- ->Br2
Take it in steps, add the water first to balance the oxygen. Then add any additional hydrogen ions to balance the hydrogen and finally add the electrons where they're required.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 25, 2018
Today on TSR

### He lied about his age

Thought he was 19... really he's 14

### University open days

Wed, 25 Jul '18
2. University of Buckingham
Wed, 25 Jul '18
3. Bournemouth University
Wed, 1 Aug '18
Poll

## All the essentials

### Student life: what to expect

What it's really like going to uni

### Essay expert

Learn to write like a pro with our ultimate essay guide.

### Create a study plan

Get your head around what you need to do and when with the study planner tool.

### Resources by subject

Everything from mind maps to class notes.

### Study tips from A* students

Students who got top grades in their A-levels share their secrets