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OCR Gateway Chemistry C4C5C6 - 13/06/13

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Original post by ryanb97
You only need an A to get your A* ... this time round :biggrin:


Oh really?? I hope I do. I've only done c4 though. Need to start ASAP
Ok my teacher and revision guide are telling me opposite things, is anybody able to explain this:
'Explain the changes that take place at each electrode in a hydrogen-oxygen fuel cell: 1.) Construct the equations for the electrode reactions, given the formulae of the ions present and the products, and 2.) Redox reactions at each electrode.'
Original post by L'Evil Fish
Well, it won't bond ionically :wink:


urgh i find chemistry hard -_- it just depends on the exam i guess!
Original post by L'Evil Fish
Okay:

Positive = Anode
Negative = Electrode

Electrolysis is breaking down through electricity.

I'll start with the easiest to understand.

Molten Lead Bromide = PbBr(2)

It's an ionic bond made up of ions:
Pb2+ and Br-

So these are now molten and flowing freely.

The Pb2+ is positive and so is attracted to the cathode. It gains 2 electrons there and becomes lead (no charge). Lead is reduced.

The Br- ions are negative and are attracted to the anode. It gives an electron to the anode and becomes Br (no charge). It is oxidized.

Yeah?


Thanks this makes it so much easier. :smile:
Original post by AsianNoodles
Thanks this makes it so much easier. :smile:


It's the same with other solutions, but there are also OH- and H+ ions as well then from water.
Reply 245
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Knowing
1
Just got back from Geography. Time to do chemistry revision!


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Reply 246
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dazzer19
Can anyone help me with Equilibrium?

I know for the Haver Process that pressure moves it right and temperature left and, like always, the catalyst doesn't move it, but what do the different conditions do for theContact Process?

thanks in advance :smile:
Original post by dazzer19
Can anyone help me with Equilibrium?

I know for the Haver Process that pressure moves it right and temperature left and, like always, the catalyst doesn't move it, but what do the different conditions do for theContact Process?

thanks in advance :smile:


High temperatures move equilibrium to the left, but increase the rate of reaction. So a compromise is used.
The catalyst doesn't move the position but increases rate of reaction.
Even at low pressure, the Contact Process has the position of equilibrium on the right side, so expensive high pressure isn't used. Atmospheric pressure is used :biggrin:
Reply 248
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Elcor
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Original post by Knowing
Just got back from Geography. Time to do chemistry revision!


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How did i go? :biggrin:
Reply 249
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dazzer19
Original post by BrokenS0ulz
High temperatures move equilibrium to the left, but increase the rate of reaction. So a compromise is used.
The catalyst doesn't move the position but increases rate of reaction.
Even at low pressure, the Contact Process has the position of equilibrium on the right side, so expensive high pressure isn't used. Atmospheric pressure is used :biggrin:


Thanks haha, I still get a bit confused over the whole equilibrium topic tbh...

If the equilibrium is on the left, does that mean that there are MORE reactants in relation to products? Or just more reactants in general?
Reply 250
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Elcor
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Original post by dazzer19
Thanks haha, I still get a bit confused over the whole equilibrium topic tbh...

If the equilibrium is on the left, does that mean that there are MORE reactants in relation to products? Or just more reactants in general?


It means that the rate of reaction from products into reactants is higher than the rate of reaction from reactants to products.
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dazzer19
Original post by andersson
It means that the rate of reaction from products into reactants is higher than the rate of reaction from reactants to products.



Ah I see,

one last thing,

is it a recurring factors that:
temperature always moves the equilibrium lleft and pressure always moves it right? Or does it change?

like, we only neeed to know about the contact ad Haber processes, so in each one of these does the temp always move it left and pressure right? Thanks
Reply 252
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Elcor
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Original post by dazzer19
Ah I see,

one last thing,

is it a recurring factors that:
temperature always moves the equilibrium lleft and pressure always moves it right? Or does it change?

like, we only neeed to know about the contact ad Haber processes, so in each one of these does the temp always move it left and pressure right? Thanks


Nope it's possible they could give you a random reversible reaction and ask you predict what effect increasing temperature, pressure etc will have on it, so you need to know what actually happens:

Increasing the temperature makes the equilibrium move in the direction that is an endothermic and vice versa. This won't always be the backward reaction.

Changing the pressure only has an effect on gases. Increasing it makes the equilibrium move in the direction with fewer moles i.e. whichever side has the smallest amount of molecules made.

Hope that helps.
So In the electrolysis of sodium hydroxide, hydrogen is made rather than sodium. Why is that? :cool:
Reply 254
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dazzer19
Original post by andersson
Nope it's possible they could give you a random reversible reaction and ask you predict what effect increasing temperature, pressure etc will have on it, so you need to know what actually happens:

Increasing the temperature makes the equilibrium move in the direction that is an endothermic and vice versa. This won't always be the backward reaction.

Changing the pressure only has an effect on gases. Increasing it makes the equilibrium move in the direction with fewer moles i.e. whichever side has the smallest amount of molecules made.

Hope that helps.


Ahh ok, thank you!
Reply 255
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1
Original post by andersson
How did i go? :biggrin:


It was OK. Not too bad.

Unfortunately I don't think I did so well in Paper 1 so I don't know whether I'll get A* or not. We'll have to see :wink:


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Original post by Turtlefushsia
So In the electrolysis of sodium hydroxide, hydrogen is made rather than sodium. Why is that? :cool:


it is easier for the hydrogen ions to be given off rather than sodium.
Original post by andersson
Nope it's possible they could give you a random reversible reaction and ask you predict what effect increasing temperature, pressure etc will have on it, so you need to know what actually happens:

Increasing the temperature makes the equilibrium move in the direction that is an endothermic and vice versa. This won't always be the backward reaction.

Changing the pressure only has an effect on gases. Increasing it makes the equilibrium move in the direction with fewer moles i.e. whichever side has the smallest amount of molecules made.

Hope that helps.


is there any way of figuring out which is exo and which is endo?
Original post by benwalters1996
it is easier for the hydrogen ions to be given off rather than sodium.

But why is that? Do we need to know
Reply 259
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Red Fox
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Got 69/85 on the specimen paper with a consistent 21 marks on each module. Feeling very confident of an A* now.

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