Writing half equations.

[Potter]

Hey guys, I've been going through exams and 3/4 of my marks lost are from not being able to write half equations properly.

Would just like some tips on how to write for example..

The oxidation and reduction half equations for the fermentation of glucose

Or, the oxidation and reduction half equations occurring in a car battery (Pb/SO4)

[scocliffe09]

Assuming acidic conditions (be careful with this)
1. Balance significant molecules and ions (e.g. make sure you have the same number of Pb or C or whatever is the most "important" atom on both sides)
2. add water to balance the number of O atoms
3. add H+ to balance number of H atoms
4. add electrons to make sure charge is equal on both sides.

[mankay]

and if its a alkaline solution?

[drdr]

With the car battery half equations, do we need to include the SO4 2- ions? Or would something like Pb --> Pb2+ + 2e be ok?

[toshibaj]

With the car battery half equations, do we need to include the SO4 2- ions? Or would something like Pb --> Pb2+ + 2e be ok?

You need to include the SO4 2- as it forms a solid product, PbSO4 not Pb2+ using your example.

[drdr]

With the car battery half equations, do we need to include the SO4 2- ions? Or would something like Pb --> Pb2+ + 2e be ok?

You need to include the SO4 2- as it forms a solid product, PbSO4 not Pb2+ using your example.

Ah right, cheers. Do we need to memorise these equations, and others such as Downs Cell/Hall Helroit Aluminium etc?

[toshibaj]

With the car battery half equations, do we need to include the SO4 2- ions? Or would something like Pb --> Pb2+ + 2e be ok?

You need to include the SO4 2- as it forms a solid product, PbSO4 not Pb2+ using your example.

Ah right, cheers. Do we need to memorise these equations, and others such as Downs Cell/Hall Helroit Aluminium etc?

Don't think so

[jasonn93]

They'll always give you enough information to derive those equations, but it's always helpful to memorise the common ones

[Potter]

Thanks for the feedback guys. If anyone is interested, just a quick google search came up with;
http://www.av8n.com/physics/lead-acid.htm

It's not a bad explanation of what happens.

[scocliffe09]

No you don't need to memorise stuff, just understand the principles...

and if its a alkaline solution?

You can sometimes work out the half equations (e.g. if they give you overall and one half equation) just by balancing what's leftover or using other information they give you, but in balancing from scratch in alkaline solution is not on the year 12 course.