HSC Chemistry Question Thread

[anotherworld2b]

i was wondering is oil sustainable? what effects could it have on local resources?

[winstondarmawan]

i was wondering is oil sustainable? what effects could it have on local resources?

I'm assuming you are talking about crude oil and it's fractions.
Crude oil is NOT sustainable, because it is not a renewable resource and takes millions of years to form.
This is why we are looking for alternate sources of fuel/petrochemicals, such as biomass and ethanol - which are indeed sustainable and renewable resources (however, we do not yet have the means to use them in a more commercial setting, i.e. they are not efficient enough).
By effect on local resources, I'm not sure what you mean.
Obviously the use of fossil fuels will diminish underground supply because it is not a renewable resource. It also has impacts on the environment, such as the release of CO2 which is a greenhouse gas > global warming or potentially CO, C, SOx which are air pollutants and pulminary irritants.
Hope this helps.

[J.B]

Hi,
So I understand that in the natural buffer solution, there is a weak acid and it's conjugate base, so that if acid is added, the base acts as a proton acceptor, and if base is added, the acid acts as a proton donor.
and so the equation is: H2CO3 <> H+  +  HCO3-
But then in our study notes, we have been told that NaHCO3 is used for neutralising chemical spills, and when the HCO3- ion acts as a base it neutralises the H+, in an acid + carbonate > co2 + water:
H+ + HCO3- > H20 + CO2.

So I was just wondering why the reverse reaction of the natural buffer system is not the same as the neutraliser? & vise versa.
I hope that makes sense.
Thanks.

[MisterNeo]

Hi,
So I understand that in the natural buffer solution, there is a weak acid and it's conjugate base, so that if acid is added, the base acts as a proton acceptor, and if base is added, the acid acts as a proton donor.
and so the equation is: H2CO3 <> H+  +  HCO3-
But then in our study notes, we have been told that NaHCO3 is used for neutralising chemical spills, and when the HCO3- ion acts as a base it neutralises the H+, in an acid + carbonate > co2 + water:
H+ + HCO3- > H20 + CO2.

So I was just wondering why the reverse reaction of the natural buffer system is not the same as the neutraliser? & vise versa.
I hope that makes sense.
Thanks.

When the carbon dioxide and water is produced by neutralisation in a closed system, it will partially dissolve to form carbonic acid.
It works in a similar way to the buffer but the carbonic acid is separated into its components.

[justwannawish]

I was wondering how to do this question:

The exothermic reaction between nitrogen and hydrogen forms ammonia with the enthaply being -100kJ/mole. The initial potential energy of the reactants was 200kJ and the activation energy is 50kJ per mole. What would be the energy change if 12.4L of nitrogen was reacted at 25 degrees and 100kPa?

Thank you!

[Mymy409]

I need help with this question:
Compare the following solutions
Soln 1: 0.25 mol/L H2SO4
Soln 2: 0.50 mol/L HCl

The H+ ion conc. and pH of soln 1 would be
A) Hydrogen ion conc - lower than soln 2 / pH - higher than soln 2
B) Hydrogen ion conc - lower than soln 2 / pH - lower than soln 2
C) Hydrogen ion conc - the same as soln 2 / pH - lower than soln 2
D) Hydrogen ion conc - the same as soln 2 / pH - the same as soln 2


_______________________________________________

The pH of a solution of CH3COOHNa was measured to be 9.05.
Which of the following best explains the measured pH?
A) Sodium ions donate protons to the water
B) Acetate ions donate protons from the water
C) Sodium ions accept protons from the water
D) Acetate ions accept protons from the water

I got this qn correct but I took a sort of guess when answering it. What's the easiest way to do it?

Mod edit: Merged posts

[MisterNeo]

I need help with this question:
Compare the following solutions
Soln 1: 0.25 mol/L H2SO4
Soln 2: 0.50 mol/L HCl

The H+ ion conc. and pH of soln 1 would be
A) Hydrogen ion conc - lower than soln 2 / pH - higher than soln 2
B) Hydrogen ion conc - lower than soln 2 / pH - lower than soln 2
C) Hydrogen ion conc - the same as soln 2 / pH - lower than soln 2
D) Hydrogen ion conc - the same as soln 2 / pH - the same as soln 2

The H⁺ concentration would be lower than Solution 2 because sulfuric acid does not fully ionise both hydrogens. The second protonation is an equilibrium and will not fully ionise, thus the 0.25M will not be doubled into a 0.5M and will be lower than Solution 2.


Since the H⁺ is lower, the pH will be HIGHER (don't get confused), so the answer is A.

The pH of a solution of CH3COOHNa was measured to be 9.05.
Which of the following best explains the measured pH?
A) Sodium ions donate protons to the water
B) Acetate ions donate protons from the water
C) Sodium ions accept protons from the water
D) Acetate ions accept protons from the water

So the answer is D, but why?

Firstly, cations never accept protons, and they only donate protons if it's something like the ammonium ion.
You should know that sodium acetate is a basic salt because it is made from a strong base (NaOH) and a weak acid (CH3COOH). When the salt dissociates in water, it releases the sodium ions and acetate ions.
The easiest way to do these questions is if you know the equilibriums of the acids/bases and their conjugates.

Acetate is the conjugate base of acetic acid. Bases accept protons.
When the salt is placed into water, there is no initial acetic acid concentration, so the equilibrium will immediately shift to the right according to LCP. This creates hydroxide ions and increases pH.

[Mymy409]

The H⁺ concentration would be lower than Solution 2 because sulfuric acid does not fully ionise both hydrogens. The second protonation is an equilibrium and will not fully ionise, thus the 0.25M will not be doubled into a 0.5M and will be lower than Solution 2.


Since the H⁺ is lower, the pH will be HIGHER (don't get confused), so the answer is A.
 

So the answer is D, but why?

Firstly, cations never accept protons, and they only donate protons if it's something like the ammonium ion.
You should know that sodium acetate is a basic salt because it is made from a strong base (NaOH) and a weak acid (CH3COOH). When the salt dissociates in water, it releases the sodium ions and acetate ions.
The easiest way to do these questions is if you know the equilibriums of the acids/bases and their conjugates.

Acetate is the conjugate base of acetic acid. Bases accept protons.
When the salt is placed into water, there is no initial acetic acid concentration, so the equilibrium will immediately shift to the right according to LCP. This creates hydroxide ions and increases pH.

Thanks a lot, MisterNeo!
Another question:

Calculate the pH of the resultant solution when 25.0mL of 0.750mol/L HCl is added to 10.0mL of 0.500mol/L Ba(OH)2 solution.
A) 0.456
B) 0.602
C) 1.862
D) 2.058

[MisterNeo]

Thanks a lot, MisterNeo!
Another question:

Calculate the pH of the resultant solution when 25.0mL of 0.750mol/L HCl is added to 10.0mL of 0.500mol/L Ba(OH)2 solution.
A) 0.456
B) 0.602
C) 1.862
D) 2.058

To do these types of questions:
-Moles of HCl is conc. multiplied by volume equals 0.01875mol.
-Since it's a 2:1 mole ratio, Ba(OH)₂ is the limiting reagent and will leave excess H⁺ (0.00875mol).
-Divide these excess moles by the total volume of the two solutions (0.035L), which will give you the concentration of H⁺ (0.25M).
-Use the pH log formula and you will get pH 0.602 (B).
Hopefully I got it right..

[anotherworld2b]

I was wondering what the balanced equation for copper nitrate + sodium hydroxide is. I was also wondering what effect would adding sodium hydroxide to copper nitrate in ice water surroundings do?

[Natasha.97]

I was wondering what the balanced equation for copper nitrate + sodium hydroxide is. I was also wondering what effect would adding sodium hydroxide to copper nitrate in ice water surroundings do?

Hi!

Cu(NO₃)_2(aq) + 2NaOH_(aq) = Cu(OH)_2(s) + 2NaNO_3(aq)
Not sure about the second question, sorry!

[Mymy409]

To do these types of questions:
-Moles of HCl is conc. multiplied by volume equals 0.01875mol.
-Since it's a 2:1 mole ratio, Ba(OH)₂ is the limiting reagent and will leave excess H⁺ (0.00875mol).
-Divide these excess moles by the total volume of the two solutions (0.035L), which will give you the concentration of H⁺ (0.25M).
-Use the pH log formula and you will get pH 0.602 (B).
Hopefully I got it right..

Yup, that's correct! Just one thing to correct: there should be a 2 before the H20 to balance both sides of the equation.

[MisterNeo]

Yup, that's correct! Just one thing to correct: there should be a 2 before the H20 to balance both sides of the equation.

Silly me. Thanks for that.

Cu(NO₃)₂ + 2NaOH = Cu(OH)₂ + 2NaNO₃ (All in solution)

Isn't copper hydroxide insoluble? 

I was also wondering what effect would adding sodium hydroxide to copper nitrate in ice water surroundings do?

Not too sure about this, but solubility of salts decreases as temperature decreases. So maybe more copper hydroxide precipitate will form? Or some sodium nitrate will also precipitate because of low temperatures.

[Natasha.97]

Isn't copper hydroxide insoluble? 

Yes it is! Nice catch, fixed

[winstondarmawan]

They become oxidised preferentially to the metal they are intended to protect. They will donate their electrons to whatever the oxidant is (e.g. molecular oxygen).

Hope this helps

This is what I had thought originally, but after a question in my trials and the solutions from a HSC past paper I am led to believe that the electrons are donated to the metal they are trying to protect...
Very confused by this.