HSC Chemistry Question Thread

[bsdfjnlkasn]

Still going on The Acidic Environment 

Question 1

Spoiler

Why are more dilute acids stronger?

Question 2

Spoiler

Do we need to know how to interpret pH graphs of titration neutralisations?

Question 3

Spoiler

Do the product lines in an equilibrium graph always travel inverse to the reactants?

[bsdfjnlkasn]

Another way to think about this is that volume is inversely proportional to the pressure. And pressure is directly proportional the amount of gaseous moles. Thus, by increasing the volume, the pressure is decreased. So to increase the pressure, the equilibrium shifts to the side with the most GAS moles, and this is the backwards reaction. This explanation might be really confusing, so I hope someone else is able to clearly explain this

2a. Recall that LCP is not applied to pure solids and liquids, ie they do not affect the equilibrium at all. So increasing/decreasing the concentration of water has no effect on the equilibrium. So the rate of forward reaction will only increase if the concentration of CO2 is increased.

3. Basically, its not necessary for all the hydrogen in a molecule to be 'removable'. eg in ethnaoic, the H's bonded to the carbon won't ionise, only the one that is bonded with the oxygen. Similarly, If you look at the structure of citric acid, only the 3 hydrogens sticking out of the carboxylic acid groups are able to ionise, hence its tri-protic.

Hey ellipse,

First of all, thank you so much for taking the time to send me such detailed answers  !!

I just have a few things to clarify with your answers as I'm still a bit unclear on the theory behind some points.

1. What formula are you quoting for the second half of your answer to my 2nd question?
(I just can't remember if/when I learnt that volume is inversely proportional to the pressure etc.) But the rest of it makes perfect sense!

2. Why won't the H's bonds to the C ionise? Further, what allows the H in the O-H ionise?


Thanks again!!

[Shadowxo]

Hey ellipse,

First of all, thank you so much for taking the time to send me such detailed answers  !!

I just have a few things to clarify with your answers as I'm still a bit unclear on the theory behind some points.

1. What formula are you quoting for the second half of your answer to my 2nd question?
(I just can't remember if/when I learnt that volume is inversely proportional to the pressure etc.) But the rest of it makes perfect sense!

2. Why won't the H's bonds to the C ionise? Further, what allows the H in the O-H ionise?


Thanks again!!

For the formula, P1V1 = P2V2. As you increase the volume of a container, the molecules hit the sides of it less and therefore the pressure is less. Also pressure is proportional to concentration, so an increase in volume means c=n/v and concentration and therefore pressure decreases.

[jakesilove]

Still going on The Acidic Environment 

Question 1

Spoiler

Why are more dilute acids stronger?

Question 2

Spoiler

Do we need to know how to interpret pH graphs of titration neutralisations?

Question 3

Spoiler

Do the product lines in an equilibrium graph always travel inverse to the reactants?

Hey!

The question 'Why are more dilute acids stronger' doesn't make sense. Potentially, what you mean to ask is why more dilute acids ionise more? This is because the reaction will take place, a certain percentage of the time, between the acid and water. More water = more reactions. The more you dilute an acid, the more ionisation that occurs, but the higher the pH.

Yes, you need to know how to interpret pH graphs. You need to know whether the titration used a strong/weak acid and a strong/weak base, where the neutralisation point is, and what indicator to use.

I really don't understand your last question. Product lines? As reactants are used up (ie. the graph shows reactants decreasing), the products will increase.

[bsdfjnlkasn]

The question 'Why are more dilute acids stronger' doesn't make sense. Potentially, what you mean to ask is why more dilute acids ionise more? This is because the reaction will take place, a certain percentage of the time, between the acid and water. More water = more reactions. The more you dilute an acid, the more ionisation that occurs, but the higher the pH.

Hey thanks for clarifying my questions,

I have a few more if that's ok 

1. Why do only strong acids fully ionise?

2. For "why more dilute acids ionise more":
When the reaction is concerning the acid and water, does the rate of reaction increase?
- Would this be to increase concentration which decreases with increased volume?
Since dilute acids have higher pH readings, does that mean dilute acids are more acidic?
Or is it sort of by definition that a high concentration of H+ ions means high pH - not really related to the acidity of the dilute acid? (because how can a dilute acid be really acidic? Isn't this inaccurate?)

3. Do you have any general tips for interpreting equilibrium graphs? Are there many tricks to it? Why are they useful?

Also, sorry for being unclear - I'm still new to this topic and am still trying to understand what I don't know if that makes sense 

Thanks again 

[Shadowxo]

Hey thanks for clarifying my questions,

I have a few more if that's ok 

1. Why do only strong acids fully ionise?

2. For "why more dilute acids ionise more":
When the reaction is concerning the acid and water, does the rate of reaction increase?
- Would this be to increase concentration which decreases with increased volume?
Since dilute acids have higher pH readings, does that mean dilute acids are more acidic?
Or is it sort of by definition that a high concentration of H+ ions means high pH - not really related to the acidity of the dilute acid? (because how can a dilute acid be really acidic? Isn't this inaccurate?)

3. Do you have any general tips for interpreting equilibrium graphs? Are there many tricks to it? Why are they useful?

Also, sorry for being unclear - I'm still new to this topic and am still trying to understand what I don't know if that makes sense 

Thanks again 

Hi,
1. Strong acids want to give their H+ ions more, which is why they're strong acids, and that means they ionise more (create more H₃0⁺ ions)
2. A lower pH (eg 1 or 2) means more acidic, a higher pH means less acidic / more basic. So if a dilute acid has a higher pH reading it is less acidic, as the concentration of H₃0⁺ ions is lower.
3. Practice helps. See how many substances are going up/down, the ones going the same direction are on the same side of the equation, and see how fast it's changing, that should show you which one it is if it's eg 2a+b, the one being consumed faster is a.

About your earlier question about products/reactant lines, yes, as the reactants are consumed products are produced, so concentration of reactants decreases (goes down) and concentration of products increases (goes up) and vice versa for backwards reactions.

[jakesilove]

Hi,
1. Strong acids want to give their H+ ions more, which is why they're strong acids, and that means they ionise more (create more H₃0⁺ ions)
2. A lower pH (eg 1 or 2) means more acidic, a higher pH means less acidic / more basic. So if a dilute acid has a higher pH reading it is less acidic, as the concentration of H₃0⁺ ions is lower as it ionises less
3. Practice helps. See how many substances are going up/down, the ones going the same direction are on the same side of the equation, and see how fast it's changing, that should show you which one it is if it's eg 2a+b, the one being consumed faster is a.

About your earlier question about products/reactant lines, yes, as the reactants are consumed products are produced, so concentration of reactants decreases (goes down) and concentration of products increases (goes up) and vice versa for backwards reactions.

Shadow, you're a legend!

[Shadowxo]

Shadow, you're a legend!

[ellipse]

Hey thanks for clarifying my questions,

I have a few more if that's ok 

1. Why do only strong acids fully ionise?

2. For "why more dilute acids ionise more":
When the reaction is concerning the acid and water, does the rate of reaction increase?
- Would this be to increase concentration which decreases with increased volume?
Since dilute acids have higher pH readings, does that mean dilute acids are more acidic?
Or is it sort of by definition that a high concentration of H+ ions means high pH - not really related to the acidity of the dilute acid? (because how can a dilute acid be really acidic? Isn't this inaccurate?)

3. Do you have any general tips for interpreting equilibrium graphs? Are there many tricks to it? Why are they useful?

Also, sorry for being unclear - I'm still new to this topic and am still trying to understand what I don't know if that makes sensA e 

Thanks again 

A great answer by Shadow.
I would like to add some extra tips for q3  if you don't know already (to easily identity concentration/pressure/temperature changes);
a sharp verticle increase/decrease in one substance implies its change in concentration
If every substance sharply increases, then volume is decreased / pressure is increased
if every substance sharply decreases, then volume is increased / pressure is decreased
A gradual change of all substance imples a change in temperature

[ellipse]

Hey ellipse,

First of all, thank you so much for taking the time to send me such detailed answers  !!

I just have a few things to clarify with your answers as I'm still a bit unclear on the theory behind some points.

1. What formula are you quoting for the second half of your answer to my 2nd question?
(I just can't remember if/when I learnt that volume is inversely proportional to the pressure etc.) But the rest of it makes perfect sense!

2. Why won't the H's bonds to the C ionise? Further, what allows the H in the O-H ionise?


Thanks again!!

Sorry, but Im not really sure for question 2

[bsdfjnlkasn]

Hey thanks for the team effort everyone!

So I have managed to write a few notes on my question "why do more dilute acids ionise more?"with a few questions remaining. Can someone please check over to see if they're correct and possible answer some of the questions?

More dilute acids have higher pH readings because:
-The reaction, when occurring between the acid and water will increase the rate of reaction (idk if this is relevant)
-The more you dilute an acid, the more ionisation that occurs (what ions are created here?) resulting in a higher pH reading
->   Additional H2O causes ionisation equilibrium to favour the forward reaction, in order to counteract the decrease in concentration (a result of the dilution)
->    More ionisation occurs and concentration of [H+] increases making the solution more basic


Also, I have some more questions regarding the most recent replies 

1. What is it about strong acids that makes them "want to give their H+ ions more"?
(has it got something to do with concentration?)

2. Why won't the H's bonds to the C ionise in ethnoic acid? Further, what allows the H in the O-H ionise?

3. Are there any tricks to pH graphs of titration and neutralisations? 

4. Do more reactions occur with more water because the concentration of the product ions decrease?

Thanks again 

[Shadowxo]

Hey thanks for the team effort everyone!

So I have managed to write a few notes on my question "why do more dilute acids ionise more?"with a few questions remaining. Can someone please check over to see if they're correct and possible answer some of the questions?

More dilute acids have higher pH readings because:
-The reaction, when occurring between the acid and water will increase the rate of reaction (idk if this is relevant)
-The more you dilute an acid, the more ionisation that occurs (what ions are created here?) resulting in a higher pH reading
->   Additional H2O causes ionisation equilibrium to favour the forward reaction, in order to counteract the decrease in concentration (a result of the dilution)
->    More ionisation occurs and concentration of [H+] increases making the solution more basic


Also, I have some more questions regarding the most recent replies 

1. What is it about strong acids that makes them "want to give their H+ ions more"?
(has it got something to do with concentration?)

2. Why won't the H's bonds to the C ionise in ethnoic acid? Further, what allows the H in the O-H ionise?

3. Are there any tricks to pH graphs of titration and neutralisations? 

4. Do more reactions occur with more water because the concentration of the product ions decrease?

Thanks again 

- don't think rate of reaction is relevant here
- the more you dilute an acid, the more ionisation that occurs creating more H₃O⁺ ions (only a greater number, not concentration), meaning it has a higher % ionisation
but the more you dilute it, the less the concentration of H₃O⁺ ions resulting in the higher pH (less acidic, concentration of H₃O⁺ ions is less)
-Kind of like Le Chatlier's principle, if you decrease the concentration then the system partially opposes it by partially increasing (but still less than original value)
since the concentration of everything but the water (fairly constant) decreases and Ka = [H₃O⁺]²/[CH₃COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H₃O⁺ increases partially and concentration of CH₃COOH decreases.
- More ionisation occurs but concentration of [H+] decreases making the solution more basic

1. The definition of a strong acid is one that "completely ionizes (dissociates) in a solution" (according to google) so basically strong acids want to give their H+ ions. You don't really need to know details, but the definition of a strong acid is basically one that wants to donate their H+ ions strongly. Concentration is irrelevant, different acidic molecules are either strong or weak, irrespective of their concentrations
2. Don't really need to know this, I think it's just about the structure. Just make sure you know the main acids/bases and which elements they donate/accept.
3. There are different graphs for different titrations. There should be some examples in your textbook, you should just know what each looks like. If it starts above 7 you begin with a base and add an acid. If it starts below 7 you begin with an acid and add a base. Closer to 7 the weaker acid/base it is. Steep long incline at equivalence point usually means strong acid and base, short and not steep usually means weak either one or both, I'm sure there are more rules but don't remember them all atm
4. Yes, as I explained above, as all concentrations decrease, with 2 of the product concentrations decreasing and 1 reactant concentration decreasing (water is pretty much constant) more H₃O⁺ is created and CH3COOH consumed.

Hope I helped, if you need any clarification or if I made any mistakes let me know

[bsdfjnlkasn]

- don't think rate of reaction is relevant here
- the more you dilute an acid, the more ionisation that occurs creating more H₃O⁺ ions (only a greater number, not concentration), meaning it has a higher % ionisation
but the more you dilute it, the less the concentration of H₃O⁺ ions resulting in the higher pH (less acidic, concentration of H₃O⁺ ions is less)
-Kind of like Le Chatlier's principle, if you decrease the concentration then the system partially opposes it by partially increasing (but still less than original value)
since the concentration of everything but the water (fairly constant) decreases and Ka = [H₃O⁺]²/[CH₃COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H₃O⁺ increases partially and concentration of CH₃COOH decreases.
- More ionisation occurs but concentration of [H+] decreases making the solution more basic

1. The definition of a strong acid is one that "completely ionizes (dissociates) in a solution" (according to google) so basically strong acids want to give their H+ ions. You don't really need to know details, but the definition of a strong acid is basically one that wants to donate their H+ ions strongly. Concentration is irrelevant, different acidic molecules are either strong or weak, irrespective of their concentrations
2. Don't really need to know this, I think it's just about the structure. Just make sure you know the main acids/bases and which elements they donate/accept.
3. There are different graphs for different titrations. There should be some examples in your textbook, you should just know what each looks like. If it starts above 7 you begin with a base and add an acid. If it starts below 7 you begin with an acid and add a base. Closer to 7 the weaker acid/base it is. Steep long incline at equivalence point usually means strong acid and base, short and not steep usually means weak either one or both, I'm sure there are more rules but don't remember them all atm
4. Yes, as I explained above, as all concentrations decrease, with 2 of the product concentrations decreasing and 1 reactant concentration decreasing (water is pretty much constant) more H₃O⁺ is created and CH3COOH consumed.

Hope I helped, if you need any clarification or if I made any mistakes let me know

Hey thanks shadowxo!

I'm not sure if this is taught in the HSC syllabus, but if you could please explain what you meant with

"Ka = [H₃O⁺]²/[CH₃COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H₃O⁺ increases partially and concentration of CH₃COOH decreases."

it would probably help teach me another way of looking at the concept (as you were very clear with everything else  )

Also, when you highlighted that "[H+] decreases making the solution more basic" does the H+ reference the hydrogens in H3O+? I'm just a bit confused because I thought high H+ concentrations meant a substance was very basic and high H3O+ concentrations meant a substance was highly acidic.

 


Thanks for everything 

[Shadowxo]

Hey thanks shadowxo!

I'm not sure if this is taught in the HSC syllabus, but if you could please explain what you meant with

"Ka = [H₃O⁺]²/[CH₃COOH], if all the concentrations decrease then the value of this expression is less than Ka so the concentration of H₃O⁺ increases partially and concentration of CH₃COOH decreases."

it would probably help teach me another way of looking at the concept (as you were very clear with everything else  )

Also, when you highlighted that "[H+] decreases making the solution more basic" does the H+ reference the hydrogens in H3O+? I'm just a bit confused because I thought high H+ concentrations meant a substance was very basic and high H3O+ concentrations meant a substance was highly acidic.

 


Thanks for everything 

Hi, I'm actually from Victoria so might be slightly different content
First of all [H3O+] = [H+] ( the H+ ions react with water to form [H3O+], they're used interchangeably)
So high [H3O+] means highly acidic, and high [H+] also means highly acidic, as they're basically the same thing
If something's basic, it has a low [H+] (or high [OH-])
And with the K_a, not sure if it's in HSC course but likely is. Do you know K? If so, Ka is just the acidity constant as [H2O] is constant, so it's like K*[H2O]
Basically for each weak acid, there's an equilibrium. We use the term Ka to represent the acidity constant, there is a defined Ka for each weak acid. If the acid is HA and it reacts with H2O to form H3O+ and A- ions, the Ka = [H3O+][A-]/[HA], and the [H3O+] = [A-] as they react 1:1, so Ka = [H3O+]²/[HA]
So the Ka for ethanoic acid is Ka = [H3O+]²/[CH3COOH]. This Ka is always constant at equilibrium, regardless of the concentrations so if all the concentrations decrease then this expression (sometimes called Q) isn't equal to the Ka, so in order for it to be in equilibrium again, the concentrations need to change so it's equal to the Ka. If all concentrations decrease, since it's [H3O+]²/[CH3COOH] then [H3O+] must increase and [CH3COOH] must decrease to return to equilibrium

Hope this helps

[RuiAce]

All the following questions relate to The Acidic Environment topic:

How important is it that we know how to name acids?

Do we need to know the following indicators: Alizarine yellow and bromoscresol green?

Regarding this one you only need to know the 4 common off by heart:

Methyl orange
Bromothymol blue
Litmus
Phenolphthalein

Rest, nope you'll be given it on the exam