Unit 4 Questions MEGATHREAD!

[mickeymouse]

really confused,
is this correct? and would this apply to all acid-base reactions?

diluting a weak acid --> pH will decrease
diluting a base --> pH will increase

[tony3272]

Diluting a weak acid ( or any acid for that matter ) should increase the pH, at least according to my understanding.

When you dilute a weak acid, the [H⁺] will decrease, resulting in an increase in pH. The thing about weak acids though is that they do not fully ionise, and so the reaction does not go to completion. This is where our equilibrium concept comes in. The pH will initially increase due to the dilution, but then to partially oppose this change, the reaction will proceed to reduce the pH, but we know it will never go back to its original value as it is only partially opposing the change.

[HarveyD]

are both A and B correct for this question?

[LachieEvans]

The answer is B yeh? Because the ammonium chloride is added as a solid (presumably) it does not affect the equilibrium as a solution of gases or liquids.

[HarveyD]

ahh okay, yeah it is. I was just a bit confused about A
thanks

[nbhindi]

Hey guys, need some assistance with the following question:

A motorist blows into a fuel cell for 5 seconds and provides 1.4X10^-4 g of ethanol at a rate of 2.8X10^-5 g/sec. Calculate the max current that the cell would produce.

I know the steps involved to work it out but i'm not too sure about what I should use to calc the moles of ethanol.....the mass given in the rate or the final mass produced.
Thanks

[HarveyD]

mass given in the rate gives you the time (that you need for the formula)
other mass is what you need for mol of ethanol

( i think)

[Milkshake]

Hey guys, need some assistance with the following question:

A motorist blows into a fuel cell for 5 seconds and provides 1.4X10^-4 g of ethanol at a rate of 2.8X10^-5 g/sec. Calculate the max current that the cell would produce.

I know the steps involved to work it out but i'm not too sure about what I should use to calc the moles of ethanol.....the mass given in the rate or the final mass produced.
Thanks

You can use either one of those rates for that question, just later on when you're calculating the current, the time(s) you use needs to be the correct one according to whichever rate you use - one second or 5 seconds.

Is there more information to that question by any chance? Or are we expected to know the reaction for the oxidation of ethanol?

[nbhindi]

Is there more information to that question by any chance? Or are we expected to know the reaction for the oxidation of ethanol?

The first part is writing the half equation for oxidation of ethanol. Ok cheers guys

[HarveyD]

just wondering
when we're given a question we have to compare different fuel cells, i.e. adv and disadvantages
would it be viable to use the data booklet values to say that one releases more energy, hence is more efficient?
for example, with a methanol and hydrogen one

[sros]

Sorry for the quick question; but when calculating Equilibrium Constant equations (Kc); do we include [H2O] if it in liquid/gas state? I've heard so many different things to this it has got quite confusing!

[Hamdog17]

If water is reacting then yeah include H2O for a Kc expression. But you do not include H2O for a Ka expression.

[Hamdog17]

Does increasing the temperature of a reaction increase the rate of reaction by:
a) Increasing the proportion of particles with sufficient energy to react, so that when they do collide there are more successful collisions due to the higher average kinetic energies of the colliding particles

b) Increasing the frequency of collisions hence the likelihood of an individual successful collision remains the same but the greater number of collisions over a period increases the rate

I thought a) but STAV says b) ?

[Mao]

Does increasing the temperature of a reaction increase the rate of reaction by:
a) Increasing the proportion of particles with sufficient energy to react, so that when they do collide there are more successful collisions due to the higher average kinetic energies of the colliding particles

b) Increasing the frequency of collisions hence the likelihood of an individual successful collision remains the same but the greater number of collisions over a period increases the rate

I thought a) but STAV says b) ?

Increasing temperature does both of those things. The answers must be wrong..

[golden]

How does adding NaOH(s) decrease the equilibrium concentration of O2 in the reaction:
2CO2 (g) <-> 2CO (g) + O2 (g), dH = +556 kJ mol-1