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OCR AS - Chemistry Unit F322 - Chains, energy and resource - REVISION! watch

1. (Original post by SmithytheDrummer)
The way I see that, after like half an hour of looking through the chapter, is that if you have the enthalpy changes for formation, you use the total enthalpy of the products-total enthalpy of the reactants. In this case it is (6(-394)+6(-286))-(-1250)=2830. So it doesn't matter if they are asking for the enthalpy change of combustion, you do it with enthalpy change of formation values. This is the same for the reverse situation.
oh yhh u r right.. thank you soooooooooooooo much... the questions are actually easy and are easy marks in the exam but confuse me all the time
2. (Original post by SmithytheDrummer)
Reaction Conditions

Alkene to Halogenoalkane- RTP
s
+rep but dont what RTP is!!
3. (Original post by Killjoy-)
You are using enthalpy changes of formation to calculate enthalpy changes of combustion, I don't know how you got CO2 and H2O at the bottom.

Crap I just saw 'combustion' and assumed that was it.

Omg I doing do so bad for this compared to unit 1! :/
4. (Original post by sixthformer)
YOU LEGEND

one more question
thank you

enthalphy change = bonds broken - bonds made
enthalphy of cobustion = bonds broken - bonds made
enthalphy of formation = bonds made - bonds broken

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation.
5. (Original post by sixthformer)
YOU LEGEND

one more question
thank you

enthalphy change = bonds broken - bonds made
enthalphy of cobustion = bonds broken - bonds made
enthalphy of formation = bonds made - bonds broken

Why don't you just use the the cycle? It's 1000x easier and you just follow the arrows?
6. (Original post by lolo-x)
C6H12O6 + 602 -----------------------> 6CO2 + 6H20

.........................6C + 9O2 + 6H2

and the arrows from the bottom line need to go up to the top equation..
then you just use the formation value of C6H12O6 on the left arrow going up
and the formation value for 6CO2s and 6H2Os on the right arrow going up

and subtract the left arrow from the right arrow
thanks
7. (Original post by lolo-x)
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation.
Define average bond enthalpy
8. (Original post by J DOT A)
Define average bond enthalpy
the average enthalpy change when one mole of a given type of bond is broken in the molecules of a gaseous species
9. Definition of enthalpy!?!?
10. (Original post by arvin_infinity)
Definition of enthalpy!?!?
is just the heat change? it can be endothermic or exothermic
11. Enthalpy change is the change in energy.

RTP = room temperature and pressure, so standard conditions [I believe it's 298K and 100kpa, and 1mol?]
12. (Original post by arvin_infinity)
+rep but dont what RTP is!!
Room temperature and pressure And thankyou
13. (Original post by medusa cascade)
Enthalpy change is the change in energy.

RTP = room temperature and pressure, so standard conditions [I believe it's 298K and 100kpa, and 1mol?]
yeah you don't have to say 1 mole for standard conditions

just say 298K and 100kPa OR you can just say 25C and 1atmosphere pressure

i find the second one easier to remember
14. (Original post by J DOT A)
Why don't you just use the the cycle? It's 1000x easier and you just follow the arrows?
Yes! I remember how to do hess cycles again now thanks for that!
15. hey can someone plz explain to me the equilibrium question when they ask you stuff like
predict and explain the condition tht would giv the max equilibrium yield of ethanol from ethene n steam q 5 b iii) from jan 2010 ... the answer is its an exothermic reaction so low temperature for high yield I DONT GET IT HOW DOES THIS WORK?

and what is the stuff about compromising
16. (Original post by arvin_infinity)
Definition of enthalpy!?!?
The heat content that is stored in a chemical system.
17. (Original post by godknowsprerna)
hey can someone plz explain to me the equilibrium question when they ask you stuff like
predict and explain the condition tht would giv the max equilibrium yield of ethanol from ethene n steam q 5 b iii) from jan 2010 ... the answer is its an exothermic reaction so low temperature for high yield I DONT GET IT HOW DOES THIS WORK?
It's Le Chatelier's Principle. If a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise this change.

Basically, it means if a reaction is exothermic, and you decrease the temperature of the surroundings, more of the products will be produced, because the system will 'try' to balance the equilibrium.

Same for pressure. If there are more moles on the reactants side, and you decrease the pressure, more products will be produced, again 'trying' to minimize change.

Also, the compromise bit just means that a low temperature and low pressure will make the reaction ridiculously slow, so it can't be used in industry. The manufacturers try to a balance between speed and yield.
18. (Original post by lolo-x)
yeah you don't have to say 1 mole for standard conditions

just say 298K and 100kPa OR you can just say 25C and 1atmosphere pressure

i find the second one easier to remember
Don't put 25C I've seen that disallowed wrote because the definition specifically states 298K best to play it safe!
19. (Original post by Ttawwab)
It's Le Chatelier's Principle. If a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise this change.

Basically, it means if a reaction is exothermic, and you decrease the temperature of the surroundings, more of the products will be produced, because the system will 'try' to balance the equilibrium.

Same for pressure. If there are more moles on the reactants side, and you decrease the pressure, more products will be produced, again 'trying' to minimize change.

Also, the compromise bit just means that a low temperature and low pressure will make the reaction ridiculously slow, so it can't be used in industry. The manufacturers try to a balance between speed and yield.
oooh i get it.. thankkk youuuuuuuuu could u also plz explain to me when they say if its a compromise :s
20. I have genuinely learnt more in the few hours that I've been here than my chemistry teacher has taught me all year. Thank you so much everyone.

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