HSC Chemistry Question Thread

[Shadowxo]

Would anyone have  advice on how to do these questions?

Exothermic means heat is produced (feels hot) and the products have less energy than the reactants. Endothermic means heat is absorbed (feels cold) and the products have more energy than the reactants.
Burning kerosene -> produces heat -> exothermic
Melting ice -> heat absorbed & water has more energy than ice -> endothermic
See if you can do the rest and post any you can't do

[bsdfjnlkasn]

Could someone please check if the following notes I have written are correct?
I'm pretty sure my last line is incorrect as doesn't the presence of H3O+ determine a substance to be acidic not basic as I have concluded? I just tried to get to the fact that dilute acids have higher pH readings so can be seen as more basic than before but I don't get the link ... Hopefully that made some sense. Any help would be greatly appreciated!!

o   The more you dilute an acid, the more ionisation that occurs
•   LCP aims to maintain same ion concentration, which with an increase in volume naturally decreases
•   Maintains same ion concentration by increasing % ionisation
•   But comparatively, there are more H3O+ ions than before which results in a higher pH reading, so essentially it becomes more basic

Thank you!!

[RuiAce]

Could someone please check if the following notes I have written are correct?
I'm pretty sure my last line is incorrect as doesn't the presence of H3O+ determine a substance to be acidic not basic as I have concluded? I just tried to get to the fact that dilute acids have higher pH readings so can be seen as more basic than before but I don't get the link ... Hopefully that made some sense. Any help would be greatly appreciated!!

o   The more you dilute an acid, the more ionisation that occurs
•   LCP aims to maintain same ion concentration, which with an increase in volume naturally decreases
•   Maintains same ion concentration by increasing % ionisation
•   But comparatively, there are more H3O+ ions than before which results in a higher pH reading, so essentially it becomes more basic

Thank you!!

More H3O+ ions results in a lower pH reading.

The scale is a negative logarithmic scale

[Shadowxo]

Could someone please check if the following notes I have written are correct?
I'm pretty sure my last line is incorrect as doesn't the presence of H3O+ determine a substance to be acidic not basic as I have concluded? I just tried to get to the fact that dilute acids have higher pH readings so can be seen as more basic than before but I don't get the link ... Hopefully that made some sense. Any help would be greatly appreciated!!

o   The more you dilute an acid, the more ionisation that occurs
•   LCP aims to maintain same ion concentration, which with an increase in volume naturally decreases
•   Maintains same ion concentration by increasing % ionisation
•   But comparatively, there are more H3O+ ions than before which results in a higher pH reading, so essentially it becomes more basic

Thank you!!

I'm not familiar with LCP, but I'll see if I can help a little.
First of all, just some info about H3O+ and OH- ions in water, and whether something is an acid or a base:
In pure water, which is neutral, the pH is 7. Some water molecules react with each other to form H3O+ and OH- ions but these concentrations and amounts are equal, so water is neutral. So water, being completely neutral with a pH of 7 (at 25ēC) has both H3O+ and OH- ions.
Formula is H₂O + H₂O ⇌ H₃O⁺ + OH⁻
And as [H₃O⁺] decreases, the pH increases due to the negative logarithmic scale. 1 - very acidic, 7- neutral, 14 - very basic. If the [H₃O⁺] decreases, the pH increases as it becomes more basic, getting closer to the basic end.

pH measures acidity by using the concentration of H₃O⁺ ions in the formula pH = -log₁₀[H₃O⁺]. The number of H₃O⁺ ions increases during dilution, but the [H₃O⁺] decreases, as c=n/V and while n is larger, V is much larger, resulting in a lower concentration. pH can also be found by using 14- pOH, as pH + pOH = 14 at 25ēC. I don't know about LCP, but if the same ion concentration is maintained, that is, if [H₃O⁺] remains the same, the pH and therefore acidity will remain the same also. It only becomes more basic if [H₃O⁺] decreases.
If [H₃O⁺] decreases, such as through dilution, it becomes more basic / less acidic and the pH rises. A substance can be acidic while being "more basic" than another acid. More basic = less acidic.
Also, something can be basic even if there are H₃O⁺ ions. To be basic and have a higher pH than 7, the [H₃O⁺] must be very low, but this cannot happen by adding water to an acid as there will always be more H₃O⁺ than OH- ions and water will react with itself to create more H₃O⁺, therefore keeping the pH below 7. In any acidic or basic solution that contains water at 25ēC, there will be H₃O⁺ and OH- ions.

Hope this helps a bit

[anotherworld2b]

Oh I see
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?

Exothermic means heat is produced (feels hot) and the products have less energy than the reactants. Endothermic means heat is absorbed (feels cold) and the products have more energy than the reactants.
Burning kerosene -> produces heat -> exothermic
Melting ice -> heat absorbed & water has more energy than ice -> endothermic
See if you can do the rest and post any you can't do

[exit]

Oh I see
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?

endothermic.

[Shadowxo]

Oh I see
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?

Gases have lots of kinetic energy, liquids have a bit and solids have the least. so solid -> liquid -> gas are all increasing energy, therefore as the product has more energy than the reactant, the reaction is endothermic.

[RuiAce]

Oh I see
I was also wondering would solid -> liquid -> gas be endothermic or exothermic?

A brief (maybe not clear - sorry) explanation as to why it's endothermic:

For something to change states in that order, the temperatures need to be HIGHER. This is because the chemicals capture the heat energy around them to go into the next state. If the temperatures were lower, the energy the chemical (say, water) contains will be released back out as it reverts down a state.

[beau77bro]

Hey struggling on a couple of conquering chem questions. I'll post them together but thankyou.

15.a. Hahaha could someone explain this?



And then 23. I have 0 clue, a worked solution would be amazing and massively appreciated

[thenerdygangster]

15a) Le Chatelier's principle is basically a principle of equilibrium in a system. So if you were to change something in a system, the equilibrium would shift to counteract the change. In this case, by adding sodium hydrogen carbonate, there would be a net reaction to the left thus producing more H₂CO₃. I would say because you are producing H₂CO₃, you are using up H⁺ ions. Therefore, the H⁺ ion concentration would decrease.

[kiwiberry]

Hey struggling on a couple of conquering chem questions. I'll post them together but thankyou.

15.a. Hahaha could someone explain this?

(Image removed from quote.)

And then 23. I have 0 clue, a worked solution would be amazing and massively appreciated

(Image removed from quote.)

For 15a), in solution, NaHCO₃ --> Na⁺ + HCO₃^-. Therefore, the concentration of HCO₃^- will increase, and by Le Chatelier, the equilibrium will shift left to decrease the concentration of HCO₃^-. Thus the concentration of H⁺ will decrease as well.

[beau77bro]

Thanks guys I get it now I think

[Mathew587]

Wait why does H+ ions decrease if HCO3 decreases? I get why that decreases but not why H+ ions do.

[thenerdygangster]

why does H+ ions decrease if HCO3 decreases? I get why that decreases but not why H+ ions do.

You're producing H₂CO_3, in this question. In order to form H₂CO_3,, H⁺ ions need to be used up. This means that the amount of H⁺ ions decrease. Hope that helps 

[Mathew587]

*facepalm* haha yes i see. very obvious indeed :/