SQA Advanced Higher Chemistry Prelim HELP

Hi Prelim is in a few day, wondering if any of your geniuses on here can help us with a question?

From 2006 AH, For which of the following reactions does the (change in enthalpy value ΔH ) correspond to both the enthalpy of combustion of an element and the enthalpy of formation of a compound?

A) C₍₂₎ + 1/2 O₍₂₎ -> CO_(g)
B) H_2(g) + O_2(g) -> H₂O₂ (l)
C) 2Na_(s) + 1/2 O_2(g) -> Na₂O_(s)
D) Mg_(S)+ 1/2 O_2(g) -> MgO_(s)
The answer is "D"

Also, an analysis of a salt shows the formula is Pt(NH₃)_xCl.
0.02 moles of the salt were dissolved in nitric acid and excess silver (I) nitrate solution was added. The precipitate formed was filtered, washed and dried. It weighed 5.74g.
The number of moles of chloride ions per mole of the salt is
A) 1
B) 2
C) 3
D) 4

Answer is "B"

any help would be major appreciated! The reason i can't ask a teacher is i am self teaching! It's a rocky road at times haha

(edited 10 years ago)

Reply 1

TheFOMaster

Original post by Pliskin

For the first one, you have to learn your definitions of enthalpy of combustion and enthalpy of formation.

Enthalpy of combustion is the energy released when one mole of a reactant reacts completely with oxygen oxygen.
Enthalpy of formation is the energy required to form one mole of your product.

From those definitions, you can eliminate A (not burned completely), B (Too much oxygen, you should know from knowledge that water is formed) and C (Combustion is ONE mole of product)

The second questions harder, but basically you have to look up your precipitation table in your data book. I've not done the question (I'll give it a look over tomorrow for you :smile:

) but it looks like the number NH3 groups in the salt is irrelevant. So just use your table, create a balanced equation, and just follow it through. I'll do the question tomorrow when I have time so I can give you better advice, I'll edit this post.

Reply 2

Pliskin

You're awesome! Remember you helped me before, cheers mate its really appreciated, completely makes sense, though for number 2, Silver Chloride is formed (AgCl) and by finding the moles you get 0.04 moles, which is 2 chloride ions per mole, though struggling with the balanced equation lol

Reply 3

TheFOMaster

Original post by Pliskin

Is your answer correct? AgCl is produced, I'm sure of that. But 0.02 moles of salt would only give 0.02 moles of Cl ions in the product. So the answer should be A. Or maybe even the question, because if the salt had two Cl- ligands then this would work as well.

Reply 4

Pliskin

Yeah, that Question came up on my prelim, wrote out the whole balanced equation and the answer was D. Like you said, yeah, I am not too sure on the complete technicality of "if the salt had two Cl- ligands" , quite literally just plugged in the numbers and made them match haha, prelim went good though, thanks for help!

Reply 5

aliceruth

Hey, I hope this is the right place to post this :P Can anyone help me with these questions on pH?

500cm^3 of 0.022 mol l-1 HCl and 500cm^3 of 0.02 mol l-1 NaOH are mixed to produce a pH of...? Answer is 3 but I don't understand why. :tongue:

I mean I get that it's a 1 mol : 1 mol reaction and we have 0.011 mol monoprotic acid reacting with 0.01 mol base so the base is limiting.. pH is -log[H+] but -log(0.01) gives 2. -log(0.001) gives 3 but where does the 0.001 come from?

Also:

Which of the following 0.1 mol l-1 aqueous sol.ns has the highest pH value?
Sodium Fluoride
Sodium Benzoate
Sodium Propanoate
Sodium Methanoate.
Answer is Propanoate but why?

Reply 6

Asklepios

Original post by aliceruth

I suspect this is a "careless error" 0.011 - 0.01 = 0.001

Which of the following 0.1 mol l-1 aqueous sol.ns has the highest pH value?
Sodium Fluoride
Sodium Benzoate
Sodium Propanoate
Sodium Methanoate.
Answer is Propanoate but why?

You should know from Higher that a salt formed from a strong base and weak acid has a pH in solution of > 7 and that a salt formed from a strong acid and weak base has a pH in solution of < 7.

In this case we have various sodium salts - all formed from a strong base (NaOH). The correct answer would be the salt that derives from the weakest acid i.e. dissociates the least in solution.

Hydrofluoric acid is a strong acid - eliminate NaF.

So we are left with deciding the weakest acid from benzoic acid, propanoic acid and methanoic acid. How do we know which one dissociates the least (hint: you might want to use your data book).

Reply 7

aliceruth

Original post by Asklepios

I suspect this is a "careless error" 0.011 - 0.01 = 0.001

You should know from Higher that a salt formed from a strong base and weak acid has a pH in solution of > 7 and that a salt formed from a strong acid and weak base has a pH in solution of < 7.

In this case we have various sodium salts - all formed from a strong base (NaOH). The correct answer would be the salt that derives from the weakest acid i.e. dissociates the least in solution.

Hydrofluoric acid is a strong acid - eliminate NaF.

So we are left with deciding the weakest acid from benzoic acid, propanoic acid and methanoic acid. How do we know which one dissociates the least (hint: you might want to use your data book).