Trial exams discussion

[kendraaaaa]

Question 7 MC VCAA 2008



Nobody never taught me how to do these bloody questions. Could someone, please? I general instruction with these types of questions would also be very appreciated. Thank youuuuuu

(answer is A btw)

[StringFever]

Question 7 MC VCAA 2008

(Image removed from quote.)

Nobody never taught me how to do these bloody questions. Could someone, please? I general instruction with these types of questions would also be very appreciated. Thank youuuuuu

(answer is A btw)

This is called Hess' Law of Summation

And pretty much what you do is re-jig your given equations so you end up with the equation you want. You may need to reverse equations, or multiply by factors to get what you need.

For your example, reverse equation 1 and multiply it by a factor of 2 (remember to apply those changes to your enthalpy value)
In equation 2, just multiply it by a factor of 2

Once done, add the equation together (remembering to cancel "like" terms - as is done in Methods) - and remember to add your enthalpies.

In doing that, you should get A!

Hope that helped!

[kendraaaaa]

How are people going in their studies? What are you averaging in trials?

[arthurk]

not too bad
trial exams arent too scary
although i have the kilbaha one at school tomorrow so that might change my mind:P

[jimmy999]

My concern is with part c. I know you should calculate [CH3CH(OH)COO-] to be 0.0033, but wouldn't the concentration of [CH3CH(OH)COOH] = 0.0900 - 0.0033?

[ryley]

I think because lactic acid is a weak acid, you have to assume the concentration is constant.

[jimmy999]

I know that normally you would do that, however they tell us what the concentration of the conjugate base is, which is how much of the original concentration has been ionised. So if you take my approach, from memory you get which is a slight difference but it's still a difference

[khalil]

Why is electrolyzing a dilute aqeous of sodium chloride a more efficient way to produce hydrogen than electrolyzing water?
From  Lisachem 08

[jimmy999]

Why is electrolyzing a dilute aqeous of sodium chloride a more efficient way to produce hydrogen than electrolyzing water?
From  Lisachem 08

An aqueous solution of sodium chloride contains sodium and chloride ions. As it contains a number of charged particles, it will conduct electricity better than water does. Therefore as it is a better electrolyte than water, it will be able to produce hydrogen at a much faster rate.

Unrelated to electrolysis, but consider what happens when you boil salt water. As there are ions in the solution, heat is able to distribute itself more easily throughout the solution hence it will boil quicker. Now put that into the context of electrolysis/galvanic cells and you might understand the question better

[khalil]

Thanks Jims

[khalil]

I don't understand this

[jimmy999]

As this is a weak acid, will only partially ionise meaning there'll be less and less than . Hence will exist in the highest concentration. That's the amount of information you need to solve the question. If you wish to take it further, you'll find that will also partially ionise. Therefore the order of concentrations will be

[kendraaaaa]

Just got 90% on Lisachem 08 ^_^

[khalil]

As this is a weak acid, will only partially ionise meaning there'll be less and less than . Hence will exist in the highest concentration. That's the amount of information you need to solve the question. If you wish to take it further, you'll find that will also partially ionise. Therefore the order of concentrations will be

Thanks again