Opinions on paper 1 and paper 2 chemistry 2010? TZ2

[narathiel]

i also put br.

were there anymore tricky multi choice- I think no or?

[tdubthebassist]

i also put br.

were there anymore tricky multi choice- I think no or?

well, I won't say that it was confusing tbh...

other confusing one was 'which was exothermic?' (which was everything apart from melting of ice)

and other one was when K_w was blabla... pH stays in 7.00 due to the Le Chetalier's. (acidity does not change)

[narathiel]

^^pH does not stay 7 it changes but OH- and H+ concentration remains constant

for multi choice what did you put with the qs where it says what is Kw if ka and kw are so and so

i put the fraction.

[IBer_101]

^^pH does not stay 7 it changes but OH- and H+ concentration remains constant

for multi choice what did you put with the qs where it says what is Kw if ka and kw are so and so

i put the fraction.

Yea I answered the pH that is like 7.27 which is half of 14.54 the pKw

I had no idea at all how to answer that question. Our teacher barely taught us the calculations for buffer solutions. As soon as I saw Acid and Bases in Section B, I didn't even read the question, I automatically skipped it. But yea, I also answered the fraction lol. Wasn't there more than one fraction though?

[TNT]

Did anyone do the question 9 in section B, what did you put as the two step mechanism at the last part?

[TNT]

for the question on exothermic neutralisation and combustion, is ice melting exothermic as well this was in the multiple choice?

[lolocakes]

for the question on exothermic neutralisation and combustion, is ice melting exothermic as well this was in the multiple choice?

Ice melting is a heat-absorbing process -- therefore, it's endothermic.

II and III were correct

[TNT]

Did anyone do the question 9 in section B, what did you put as the two step mechanism at the last part?

[tdubthebassist]

Did anyone do the question 9 in section B, what did you put as the two step mechanism at the last part?

1st one: from 1-bromo to 1-amine, Sn2 reaction with Cynide and reaction with H2 with the presence of Ni/Pt

I don't know if you are going to get penalized for not saying (warm) cynide.

2nd one: from from something to ketone? again, Sn2 with hydroxide and then Oxidation with the presence of K2Cr2O7

Edited May 14, 2010 by tdubthebassist

[tdubthebassist]

^^pH does not stay 7 it changes but OH- and H+ concentration remains constant

for multi choice what did you put with the qs where it says what is Kw if ka and kw are so and so

i put the fraction.

pH = -log(conc(H+)).

SO if conc(H+) remains constant, pH should remain constant.

[TNT]

What factor affects the arrangement in the periodic table?

For those that did question 6, what is the answer to the last bit where it was asking for the reasons behind why Fe(something) was coloured. What was the economic reason for using catalyst in the haber process?

Edited May 14, 2010 by TNT

[TNT]

Did anyone get a roll of letters in the paper 1?

[IBer_101]

^^pH does not stay 7 it changes but OH- and H+ concentration remains constant

for multi choice what did you put with the qs where it says what is Kw if ka and kw are so and so

i put the fraction.

pH = -log(conc(H+)).

SO if conc(H+) remains constant, pH should remain constant.

When the temperature of the water changes, the equilibrium H2O <--> H+ + OH- also changes. (Kw changes)

So, both the concentrations of H+ and OH- increase or decrease by the same value.

If Kw is given to us, (10^-14.54 I think), then, since [OH-] = [H+], then pOH = pH

pKw = pOH + pH

-log(10^-14.54) = 2 * pH (since pH = pOH)

so, pH = 14.54/2 = 7.27

Edited May 14, 2010 by IBer_101

[tdubthebassist]

What factor affects the arrangement in the periodic table?

For those that did question 6, what is the answer to the last bit where it was asking for the reasons behind why Fe(something) was coloured. What was the economic reason for using catalyst in the haber process?

In Haber process, catalyst decreases the activation energy, increasing the rate of reaction faster. So with lower temperature, they can produce more NH3. Moreover, decrease in temperature can also lead more forward reaction by Le Chetalier's Principle (it said it is exothermic). Hence, activation energy leads to the decrease of optimum temperature, saving energy and producing more NH3, which are economically beneficial.

But I think any sensible answers would be valid.

Fe is coloured because existence of ligands around Fe (central ion) causes two different energy levels of d-block. So when electron is excited and comes back down to the original place, you will get E=hf whose f is collinear to the frequency of visible light. Hence it is coloured.

[tdubthebassist]

^^pH does not stay 7 it changes but OH- and H+ concentration remains constant

for multi choice what did you put with the qs where it says what is Kw if ka and kw are so and so

i put the fraction.

pH = -log(conc(H+)).

SO if conc(H+) remains constant, pH should remain constant.

When the temperature of the water changes, the equilibrium H2O <--> H+ + OH- also changes. (Kw changes)

So, both the concentrations of H+ and OH- increase or decrease by the same value.

If Kw is given to us, (10^-14.54 I think), then, since [OH-] = [H+], then pOH = pH

pKw = pOH + pH

-log(10^-14.54) = 2 * pH (since pH = pOH)

so, pH = 14.54/2 = 7.27

ah. true that. it is not acidic NOT because the concentration is not changing but just because the balance between H and OH is the same.

[Tilia]

Did anyone get a roll of letters in the paper 1?

Yes, there were 4 Ds in a row, like question 20-24 or something like that.