VCE Chemistry Question Thread

[warya]

Yep that did scare me but I sorta get it I think thanks 

One more q, does a decrease in OH- mean an increase in H+ and therfore an increase in pH? Does this sort of relationship exist? I know it does at 298K but what about at other temps?

[Sine]

Q: What mass of NaOH must be dissolved in water to produce 750mL of solution with pH=13. I get the answer of 3g.

EDIT: My reasoning is that since pH=13, [H3O=]=10-13 thus [OH-]=10-1
          C(OH-)=10-1=n/0.750, n=0.075mol
          n(NaOH)=m/M n(NaOH)=n(OH-)=0.075=m/40  n(OH-)=3g     

Can someone check my working?

[keltingmeith]

Yep that did scare me but I sorta get it I think thanks 

One more q, does a decrease in OH- mean an increase in H+ and therfore an increase in pH? Does this sort of relationship exist? I know it does at 298K but what about at other temps?

Yup - this is actually something that temperature does not play too much of a role in (that is, it doesn't change what happens, just other things we need not worry about atm)

Consider the following general self-ionization of an acid and base:



Where A is our acid, and B is our base. Now, this will exist in equilibria, so if you increase the acidity (i.e, decrease the pH), the system will oppose this change, meaning that less base is produce, and more of the salt, meaning there is a decrease in the concentration of the base. If you were to increase the basicity (i.e., increase the pH), you'd see the same effect, but decrease in acid instead, hence lowering the concentration of the acid.

Same thing holds for your specific scenario - except, OH is the base, and H is the acid.

[BakedDwarf]

Can someone explain to me why the answer is D and not C?

Doesn't bond formation require energy in order to absorb and store the energy in the chemical bonds?

[lzxnl]

Can someone explain to me why the answer is D and not C?

Doesn't bond formation require energy in order to absorb and store the energy in the chemical bonds?

Do you do biology? If so, you have been confused by VCE bio's concept of breaking high energy bonds to release energy.
Chemical bonds ALWAYS release energy when formed because if they cost energy, why would they form? There is a physical law called the second law of thermodynamics which essentially says that molecules try to spread energy out as much as possible. This implies that atoms will not want to be bound to each other unless the forming of said bond releases enough energy.

Fyi, in biology you get energy from hydrolysing ATP because hydrolysis gets rid of a weak P-O-P bond and replaces that with two P-O bonds which are stronger. The energy comes from forming the strong bond, not just by breaking a weak bond.

[@#035;3]

Hey guys quick q
Explain why sulfuric acid of the same concentration as HCl (0.1M) has a pH less than 1.
Thanks in advance!

[izzywantsa97]

Hey guys quick q
Explain why sulfuric acid of the same concentration as HCl (0.1M) has a pH less than 1.
Thanks in advance!

If [H2SO4] is 0.1M, [H+] is 0.2M as there are twice as many hydrogen ions as sulfuric acid molecules. This means that the pH will be lower than if the acid were monoprotic like HCl, as [H+]=[HCl]

[lzxnl]

If [H2SO4] is 0.1M, [H+] is 0.2M as there are twice as many hydrogen ions as sulfuric acid molecules. This means that the pH will be lower than if the acid were monoprotic like HCl, as [H+]=[HCl]

Not quite true. If [H2SO4] = 0.1 M, [H+] is actually still pretty close to 0.1 M. Probably like 0.11 M. The reason is that H2SO4, while being diprotic, is only a strong acid for the first dissociation. HSO4- is a weak acid, albeit a much stronger weak acid than many of the other weak acids (its Ka is about 0.01). But the idea is the same.

[paper-back]

Is it always non-spontaneous reactions that occur in electrolytic cells?

I understand in Galvanic cells that the strongest reductant will react with the strongest oxidant, in a negative gradient (on the electrochemical series) 'manner', but I'm having trouble understanding what reacts with what in an electrolytic cell when multiple reactants are present

[lzxnl]

Is it always non-spontaneous reactions that occur in electrolytic cells?

I understand in Galvanic cells that the strongest reductant will react with the strongest oxidant, in a negative gradient (on the electrochemical series) 'manner', but I'm having trouble understanding what reacts with what in an electrolytic cell when multiple reactants are present

Exactly the same thing. Strongest reductant reacts with the strongest oxidant. It just so happens that in an electrolytic cell, this reaction is non-spontaneous (and for the reaction to be of practical use, the products should be separated to avoid reverse reactions taking place).

[paper-back]

Exactly the same thing. Strongest reductant reacts with the strongest oxidant. It just so happens that in an electrolytic cell, this reaction is non-spontaneous (and for the reaction to be of practical use, the products should be separated to avoid reverse reactions taking place).

So in electrolytic cells you use a positive gradient for the electrochemical series?

[tashhhaaa]

hey guys,

is stuff like facts about oil, coal etc examinable? The study design only talks about
"comparison of the renewability of energy sources including coal, petroleum, natural gas, nuclear fuels and biochemical fuels"

(I'm talking about Ch 23 of Heinemann)
this chapter just seems like a waste of my time, more "background info" than essential content?

[jyodesh.com]

hey guys,

is stuff like facts about oil, coal etc examinable? The study design only talks about
"comparison of the renewability of energy sources including coal, petroleum, natural gas, nuclear fuels and biochemical fuels"

(I'm talking about Ch 23 of Heinemann)
this chapter just seems like a waste of my time, more "background info" than essential content?

You'll definitely be asked at least one question thats something along the lines of "which of these fuels are renewable energy sources/which are not/what is a biofuel/define what makes a fuel renewable"
Whether you get asked a question like "compare the pros and cons of using each source of energy" is less likely but certainly possible.
so what you should know is:
 - what does renewable mean?
 - what is a biofuel
 - know what each of these fuels are:
       - fossil fuels: coal, petroleum/crude oil, natural gas
       - uranium
       - diesel/biodiesal
       - biogas
       - bioethanol
- know whether each of the fuels above are renewable or non-renewable e.g. methane? - depends on source nat. gas or biogas
- know how each of the biofuels are made (from unit 3)

as an extension you should know the pros and cons of using each source of energy.
(this is based on what has appeared in exams, your SACs may have more emphasis an pros and cons)

[tashhhaaa]

You'll definitely be asked at least one question thats something along the lines of "which of these fuels are renewable energy sources/which are not/what is a biofuel/define what makes a fuel renewable"
Whether you get asked a question like "compare the pros and cons of using each source of energy" is less likely but certainly possible.
so what you should know is:
 - what does renewable mean?
 - what is a biofuel
 - know what each of these fuels are:
       - fossil fuels: coal, petroleum/crude oil, natural gas
       - uranium
       - diesel/biodiesal
       - biogas
       - bioethanol
- know whether each of the fuels above are renewable or non-renewable e.g. methane? - depends on source nat. gas or biogas
- know how each of the biofuels are made (from unit 3)

as an extension you should know the pros and cons of using each source of energy.
(this is based on what has appeared in exams, your SACs may have more emphasis an pros and cons)

thank you so much!

[paper-back]

If two monoprotic acids of equal volume, but different Ka values, are reacted with a 0.1M solution of OH- ions, why is it that they both require the same amount of OH- for the solution to become neutralised?
Wouldn't one have a greater amount of H+ ions at equilibrium than the other, hence requiring more OH- ions to neutralise?