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So stuck on a topic for chem :(

Im having a hard time understanding a topic. When I sit to revise it its okay but the past paper questions for the topic are something else.

Its equilibrium btw and also kinetics (order of reaction) Idk what to do I feel like giving up -__-
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RMNDK
3
Original post by aladdin818
Im having a hard time understanding a topic. When I sit to revise it its okay but the past paper questions for the topic are something else.

Its equilibrium btw and also kinetics (order of reaction) Idk what to do I feel like giving up -__-


Really? Equilibrium and Kinetics?

There must be a lack of core understanding when it comes to the past paper questions, there's really nothing more to it.

What exactly do you struggle with? Could you post an example question and see how you approach the question? Or perhaps the notes you have revised from aren't helping you well
Original post by aladdin818
Im having a hard time understanding a topic. When I sit to revise it its okay but the past paper questions for the topic are something else.

Its equilibrium btw and also kinetics (order of reaction) Idk what to do I feel like giving up -__-


What do you find so hard about kinetics? I think they are the easiest things on the paper! Partly because I do maths so it helps a lot.
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Avatar for aladdin818
aladdin818
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Original post by SirRaza97
What do you find so hard about kinetics? I think they are the easiest things on the paper! Partly because I do maths so it helps a lot.


Original post by RMNDK
Really? Equilibrium and Kinetics?

There must be a lack of core understanding when it comes to the past paper questions, there's really nothing more to it.

What exactly do you struggle with? Could you post an example question and see how you approach the question? Or perhaps the notes you have revised from aren't helping you well



I find it the most difficult because those topics involve a lot of graphs. I hate graphs. A lot of application needed:/ I don't do maths lol. Ill try find some questions, I'm doing wjec but I get so confused because on some questions Kw apparently has no units and Kc and on others it does have units. I haven't fully covered kinetics topic because 1. its so off-putting to me the whole graphs and table thing and 2. I tried to understand it once and completely failed :/
Original post by aladdin818
I find it the most difficult because those topics involve a lot of graphs. I hate graphs. A lot of application needed:/ I don't do maths lol. Ill try find some questions, I'm doing wjec but I get so confused because on some questions Kw apparently has no units and Kc and on others it does have units. I haven't fully covered kinetics topic because 1. its so off-putting to me the whole graphs and table thing and 2. I tried to understand it once and completely failed :/


I don't do WJEC so I don't know what you need to know exactly. I can help you with units. This is how I do it so bare with my fairly long explanation.

I am going to make up a reaction with letters.

2A + B ----> C + 3D

Kc = [C] [D]³ / [A]²

Okay so the units for this will be mol dm-3. I worked this out in my head and this is how.

You should know that [C] mean concentration of C which has the units mol dm-3.
I use the rules of powers to work out the units this way. There are no other units in this equation. Only concentration so our units for Kc have to be mol dm-3. But to what power? In this case it's to the power of one by working it out.

Look at the total number of powers on each side.

LHS = 1 + 3 = 4 RHS = 2 + 1 = 3.

Using rule of powers, when we divide powers we take them away. Kc is a division right? There are 4 powers - 3 powers which make 1 power.

So are units are (mol dm-3)1 = mol dm-3

•

If there were 5 powers on the LHS and 2 powers on the RHS then our final amount is 5-2 = 3 powers. So Kc in this case is (mol dm-3)3 = mol3 dm-9

•

If there were 2 powers on the LHS and 4 powers on the RHS then our final amount is 2-4 = -2 powers. So Kc in this case is (mol dm-3)-2 = mol-2 dm6

Use this method the same way for rate constant accept just add s-1 at the end your units you have worked out. It's always LHS - RHS (or TOP - BOTTOM depending how you write out the equation). Hopefully this helps.

Ka and Kw usually have the same units for all cases. I have never come across a situation where they have different units.


EDIT: You might find looking up the rules of powers to help you even though you don't do maths.
(edited 8 years ago)

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