[scientific222]

My last chemistry exam is coming up, and I have no clue how to answer this question. Can anyone please help me with this, I have checked the mark scheme and I have no clue what they are looking for, how would I show that one is strong and the other is weak?

[blackjenny85]

what exam is this?
is this the unit 2 of the new spec?

[QuantumSuicide]

Do an acid-based reaction and plot the results against a pH chart...what else?

[Reemaa]

that question seems a bit hard -
Is that unit 3? ive got mine next week too...

what did the mark scheme say though?

[scientific222]

(Original post by blackjenny85)
what exam is this?
is this the unit 2 of the new spec?

OCR 21st Century Science C7 Further Chemistry

[scientific222]

Yes this is Unit 3, the mark scheme says that I have to compare a hydrochloric acid reaction to a reaction with a methanoic acid reaction but somehow I am meant to show that the hydrochloric is strong and methanoic is weak JUST through the balanced equation.

[blackjenny85]

(Original post by scientific222)
OCR 21st Century Science C7 Further Chemistry

oh okay, thanks.

[scientific222]

(Original post by QuantumSuicide)
Do an acid-based reaction and plot the results against a pH chart...what else?

You don't use charts or anything , you just use a balanced equation which is why I do not understand this.

[izzyd16]

I'm not sure but it may be talking about the way stong acids and weak acids are different because they ionise differently in terms of their hydrogen ions- ionisation of weak acids is reversible so ionise only very slightly and stong acids ionise almost completely and release most of their hydrogen ions. Maybe you have to show the reaction of each acid reacting with water (ionisation)? I may be completely wrong though sorry.

[scientific222]

(Original post by izzyd16)
I'm not sure but it may be talking about the way stong acids and weak acids are different because they ionise differently in terms of their hydrogen ions- ionisation of weak acids is reversible so ionise only very slightly and stong acids ionise almost completely and release most of their hydrogen ions. Maybe you have to show the reaction of each acid reacting with water (ionisation)? I may be completely wrong though sorry.

Thats been quite helpful actually, you may be correct. I could show a reversible reaction sign between a weak acid splitting up into hydrogen ions and a normal reaction sign for a strong acid splitting up into hydrogen ions perhaps?

[izzyd16]

Yes that would actually work. Hydrochloric acid is HCL--> H+ + Cl- or something like that.

[Teabo]

Okay, heres my view. Coming from a Masters student in Chemistry. Note <---> represents an equilibrium in these equations.

Firstly, show that methanoic acid is acidic. React it with a weak base.

CH3O2H(aq) + H2O(aq) <----> CH3O-(aq) + H3O+(aq)

So that's methanoate ions and hydronium ions being produced. H3O+ is the same as H+ at GCSE level, that is what is responsible for acidic solutions.

So now, do a similar reaction with HCl:

HCl + H2O <---> Cl- + H3O+

Now you can use both of these to demonstrate why one is a weak acid and why one is a strong acid.

Methanoic acid reacts with water, in an acid-base reaction to give you the products, but it only does so to a very, very small degree. I.e. the equilibrium of the reaction is far left of the forward reaction. A weak acid is one which only partially ionises in aqueous solution.

Hydrochloric acid reacts with water in the same fashion. But it does so to a large degree, nearly all of the HCl will ionise in aqueous solution. I.e. the equilbirium of the reaction is driven far right of the forward reaction, resulting in the production of more H+ ions that with methanoic acid. HCL causes the formation of more H+ ions in aqueous solution than an equally concentrated (1 molar) solution of methanoic acid.

TL;DR - CH3O2H is a weak acid because it doesn't fully ionise in aqueous solution. HCl is a strong acid because it nearly fully ionises in aqueous solution.

The factors that determine whether an acid is strong or weak is dependant on 2 factors:
1 - The stability of the ions that you form upon ionisation. Since they all product H+ we can leave that out because it's a constant. But, what matters with the negative species you created. If that can't stabilise the negative charge effectively, the acid will most likely be weak (relatively).
2 - The stability and strength of the bond that breaks to release H+ that protonates the substrate (water in this case). Generally, stronger the bond the weaker the acid. Weaker the bond the stronger the acid. As it requires less to ionise into the conjugate base + H+ ions.

Hope that's thorough enough. Might be too thorough.

[Konflict]

Would this be a valid answer? (Although, I probably did not receive all the marks...):

Methanoic Acid reacts with Sodium to produce Sodium Methanoate and a hydrogen ion. Sodium methanoate is a salt: it contains negative acid ions, and positive metal ions. When a carboxylic acid dissolves in water, the hydrogen ion leaves its functional group (COOH). Only about one out of one hundred methanoic acid molecules are ionised - it is, therefore, a weak acid. It's dynamic equilibrium lies towards the left.
Hydrochloric acid is formed when hydrogen chloride dissolves in water. HCl -> H+ and Cl- ions.
Almost all the hydrochloric acid molecules are ionised - the equilibrium lies towards the right.


After writing it out, I noticed that I did not know what the formula was for sodium methonoate, so, in hindsight, I should have done the reaction with water.

[Teabo]

(Original post by Konflict)
Would this be a valid answer? (Although, I probably did not receive all the marks...):

Methanoic Acid reacts with Sodium to produce Sodium Methanoate and a hydrogen ion. Sodium methanoate is a salt: it contains negative acid ions, and positive metal ions. When a carboxylic acid dissolves in water, the hydrogen ion leaves its functional group (COOH). Only about one out of one hundred methanoic acid molecules are ionised - it is, therefore, a weak acid. It's dynamic equilibrium lies towards the left.
Hydrochloric acid is formed when hydrogen chloride dissolves in water. HCl -> H+ and Cl- ions.
Almost all the hydrochloric acid molecules are ionised - the equilibrium lies towards the right.


After writing it out, I noticed that I did not know what the formula was for sodium methonoate, so, in hindsight, I should have done the reaction with water.

Bold points are wrong:
Negative acid ions? They're not acid ions once they have lost a proton (hydrogen ion). They are the conjugate base.
Avoid specifying any figure of ionisation; it's highly dependant on concentration of solution and external factors.

Avoid:
Describing the composition of the salt. It's not required.
Also you've said about methanoic acid plus sodium and then compared that to hydrochloric acid plus water. The question states compare similar reactions. EDIT: Ah, read that you where going to use water instead.

You've got 6 marks to collect:

1. Equation (CH3O2H + H2O <---> CH3OO- + H3O+) to show that methanoic acid IS an acid.
2. Briefly describe that H3O+ or H+ ions are responsible for acidic properties.
3. Equation (CH3O2H + H2O <---> CH3OO- + H3O+)
4. Briefly describe that equilibrium is on left hand side, thus a small proportion of methanoic acid is ionised, thus less H+ in solution, thus weak acid.
5. Equation (HCl + H2O <---> Cl- + H3O+)
6. Briefly describe that equilibrium is on right hand side, thus a high proportion of hydrochloric acid is ionised, thus greater H+ in solution than methanoic acid, thus a strong acid.

[Konflict]

Well thanks. Luckily, this question is not on my specification anyway.
After you broke it down, it seems a relatively easy question now.

[scientific222]

(Original post by Teabo)
Bold points are wrong:
Negative acid ions? They're not acid ions once they have lost a proton (hydrogen ion). They are the conjugate base.
Avoid specifying any figure of ionisation; it's highly dependant on concentration of solution and external factors.

Avoid:
Describing the composition of the salt. It's not required.
Also you've said about methanoic acid plus sodium and then compared that to hydrochloric acid plus water. The question states compare similar reactions. EDIT: Ah, read that you where going to use water instead.

You've got 6 marks to collect:

1. Equation (CH3O2H + H2O <---> CH3OO- + H3O+) to show that methanoic acid IS an acid.
2. Briefly describe that H3O+ or H+ ions are responsible for acidic properties.
3. Equation (CH3O2H + H2O <---> CH3OO- + H3O+)
4. Briefly describe that equilibrium is on left hand side, thus a small proportion of methanoic acid is ionised, thus less H+ in solution, thus weak acid.
5. Equation (HCl + H2O <---> Cl- + H3O+)
6. Briefly describe that equilibrium is on right hand side, thus a high proportion of hydrochloric acid is ionised, thus greater H+ in solution than methanoic acid, thus a strong acid.

Thank you for your help

[hairubina]

How did everyone find the unit 3 exam today? (thursday 24th May)
I found it okay however the one about whether all the glucose would be fermeneted put be off. The ideas section went rather well!
I hope the grades boundaries are similar to what they have been for an A*

[scientific222]

(Original post by hairubina)
How did everyone find the unit 3 exam today? (thursday 24th May)
I found it okay however the one about whether all the glucose would be fermeneted put be off. The ideas section went rather well!
I hope the grades boundaries are similar to what they have been for an A*

This exam was easier according to a lot of people so the grade boundary might be higher.