Buffer and Energy Questions

[tcg93]

Hi

Can someone explain these MCQ answers for me. I got them wrong, but there is no explanation in the solutions.

6   Which of the following reagents could not be added together to make a buffer solution?

A   NaOH (aq) and CH3COOH(aq)
B   CH3COONa(aq) and CH3COOH(aq)
C   NaOH (aq) and CH3COONa(aq)
D   NH4Cl(aq)  and NH3 (aq)

(Answer is A)

_________________________________________________
   The following information refers to Questions 14,15 and 16.

   The heat energy produced during the combustion of 2 mol of carbon monoxide according to the    equation
         2CO(g) + O2(g)   <->   2CO2(g)       ∆H = -564 kJ mol-1



15   The heat energy produced during the combustion of 2 mol of carbon monoxide according to the    equation
         2CO(g) + O2(g)   <->   2CO2(s)
   would be

A     less than 564 kJ.
B     564 kJ.
C     greater than 564 kJ.
D     unable to be deduced as there is insufficient information.

(Answer is C)

[Shark 774]

First question: NaOH is a strong base so would completely neutralise the ethanoic acid, and hence no buffer (I assume... Maybe there is a better explanation?).

Second question: CO2 formed in the first equation is a gas. CO2 formed in the second equation is a solid. A solid is more stable than a gas and hence has less chemical energy, so more energy is released to make the solid form of carbon monoxide than there is released to make the gaseous form.

[Mao]

For the first question I'm fairly sure c won't work either. For a buffer solution you need both an acid and its conjugate, and it has to be a weak acid. In tgif case the equilibrium contains some acid and conjugate, any addition of acid/base would be absorbed by three conjugate pair.

[toshibaj]

For the first question I'm fairly sure c won't work either. For a buffer solution you need both an acid and its conjugate, and it has to be a weak acid. In tgif case the equilibrium contains some acid and conjugate, any addition of acid/base would be absorbed by three conjugate pair.

That's what I thought too. Is Shark's reasoning for A correct btw?

[Mao]

For the first question I'm fairly sure c won't work either. For a buffer solution you need both an acid and its conjugate, and it has to be a weak acid. In tgif case the equilibrium contains some acid and conjugate, any addition of acid/base would be absorbed by three conjugate pair.

That's what I thought too. Is Shark's reasoning for A correct btw?

For the first question I'm fairly sure c won't work either. For a buffer solution you need both an acid and its conjugate, and it has to be a weak acid. In tgif case the equilibrium contains some acid and conjugate, any addition of acid/base would be absorbed by three conjugate pair.

That's what I thought too. Is Shark's reasoning for A correct btw?

yes, he is correct

[kenhung123]

Can I ask, what H2SO4 is acting as in this equation:
Mg(s)+H2SO4(aq)=>H2(g) + MgSO4(aq)?

I wrote its acting as oxidant as ON of H decreases from +1 to 0. Solutions say its acting as reductant.

[toshibaj]

solutions are wrong

[tcg93]

Can I ask, what H2SO4 is acting as in this equation:
Mg(s)+H2SO4(aq)=>H2(g) + MgSO4(aq)?

I wrote its acting as oxidant as ON of H decreases from +1 to 0. Solutions say its acting as reductant.

Definitely an oxidant. (2H+ + 2e -> H2 on elec series) btw what exam is that from for reference?

[kenhung123]

2000 VCAA

[jackchan1993]

Question 6.

If the NaOH is a limiting reagent then wouldn't there be some CH3COOH remaining, and with the CH3COONa produced couldn't it also make a buffer solution?

[kenhung123]

I'm not particularly sure why this reaction isn't one that describes carbon monoxide displacement of oxygen:

8CO(g) + Hb4(O2)4=>Hb4 + 8CO2(g)

Isn't the carbon monoxide causing the oxygen to remove from the haemoglobin?

The answer says its only this:
4CO(g) + Hb4(O2)4 =>Hb4(CO)4 + 4O2

[Mao]

you'll find it very hard for CO to react with O2 in the conditions of a red blood cell. That reaction is quite energetic, considering you're going to have to break the O2 double bond, and a single bond in CO.