HSC 2012-2015 Chemistry Marathon (archive) (2 Viewers)

[BlueGas]

re: HSC Chemistry Marathon Archive

New Question: (kudos to addoil) [4 marks]

A student titrated 0.14 M barium hydroxide solution with 22.35mLs of 0.08M nitric acid to the equivalence point.

The student accidentally spilt an excess 13.5mL of barium hydroxide into the mixture.

Calculate the pH of the resulting solution.

Guys, how would I know what the products are for this reaction? I know barium hydroxide is Ba(OH)2, and nitric acid is HNO3, but how would I combine them to get the products?

 

[rand_althor]

re: HSC Chemistry Marathon Archive

Guys, how would I know what the products are for this reaction? I know barium hydroxide is Ba(OH)2, and nitric acid is HNO3, but how would I combine them to get the products?

Acid + Base -> Salt + Water. Or do you mean how do you know it is Ba(NO3)2 and not BaNO3 that is produced?

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Acid + Base -> Salt + Water. Or do you mean how do you know it is Ba(NO3)2 and not BaNO3 that is produced?

Yes this part.

 

[rand_althor]

re: HSC Chemistry Marathon Archive

Yes this part.

Read through this.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Read through this.

Are you able to explain it?

 

[leehuan]

re: HSC Chemistry Marathon Archive

Barium ion has 2+ charge
Nitrate ion has 1- charge

So to balance out charges

Ba(NO3)2

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Question 25
(4 marks)
An indicator is placed in water. The resulting solution contains the green ion, Ind-, and the red molecule, HInd. Explain why this solution can be used as an indicator. In your response, include a suitable chemical equation that uses Ind- and HInd.

 

[rand_althor]

re: HSC Chemistry Marathon Archive

Question 25
(4 marks)
An indicator is placed in water. The resulting solution contains the green ion, Ind-, and the red molecule, HInd. Explain why this solution can be used as an indicator. In your response, include a suitable chemical equation that uses Ind- and HInd.

When the solution is added to an acid, the high hydronium concentration causes the equilibrium to shift to the left, as per Le Chatelier's Principle. This increases the concentration of HInd and thus makes the solution red. When the solution is added to a base, the hydroxide ions and hydronium ions react, increasing the concentration of water. According to Le Chatelier's Principle, this causes the equilibrium to shift to the right, increasing the concentration of Ind^- and thus making the solution green. The intensity of the colour is based on the degree of ionisation of the acid or base. In stronger acids or bases, the intensity of the colour will be greater. Therefore the solution can be used as an indicator as it is changes colour in acids and bases.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

When the solution is added to an acid, the high hydronium concentration causes the equilibrium to shift to the left, as per Le Chatelier's Principle. This increases the concentration of HInd and thus makes the solution red. When the solution is added to a base, the hydroxide ions and hydronium ions react, increasing the concentration of water. According to Le Chatelier's Principle, this causes the equilibrium to shift to the right, increasing the concentration of Ind^- and thus making the solution green. The intensity of the colour is based on the degree of ionisation of the acid or base. In stronger acids or bases, the intensity of the colour will be greater. Therefore the solution can be used as an indicator as it is changes colour in acids and bases.

I get how to answer the question once i have the equation, but how do you know the equation?

The question says an indicator is placed in water and the resultin solution contains Ind- and HInd.

Like i thought it meant something like this: Indicator + H2O --> Ind- + HInd
and i thought they were askin why: Ind- + HInd could be used as an indicator.

 

[rand_althor]

re: HSC Chemistry Marathon Archive

I get how to answer the question once i have the equation, but how do you know the equation?

The question says an indicator is placed in water and the resultin solution contains Ind- and HInd.

Like i thought it meant something like this: Indicator + H2O --> Ind- + HInd
and i thought they were askin why: Ind- + HInd could be used as an indicator.

The HInd is the indicator. Note that Ind is short for Indicator. For this question you have to realise that the indicator is similar to a buffer system. The resulting solution contains Ind^- and HInd as it is in equilibrium. I figured that HInd would be reacting with water, and since the question says Ind^- is present, that means HInd loses a proton to form Ind^-.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

The HInd is the indicator. Note that Ind is short for Indicator. For this question you have to realise that the indicator is similar to a buffer system. The resulting solution contains Ind^- and HInd as it is in equilibrium. I figured that HInd would be reacting with water, and since the question says Ind^- is present, that means HInd loses a proton to form Ind^-.

yes. thankyou! I didn't see the connection between Indicator an Ind, I feel so dumb

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Read through this.

Okay, I've got a better understanding on the reactions, I know that for example Na is 1+ and Ca is 2+, since they are in groups 1 and 2 respectively, how about elements such as titanium or nickel?

 

[leehuan]

re: HSC Chemistry Marathon Archive

Titanium is usually 4+ and nickel is usually 2+, but these ones appear very infrequently. If they appear, you can either consult your table of standard reduction potentials and only look at the charge, or more likely it will be something like Titanium (IV) Oxide, which you automatically know its 4+.

But if they throw at you copper, lead or tin without that, ASSUME 2+. If they give you Copper (I) then copper is 1+. Iron they have to say 2+ or 3+.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Titanium is usually 4+ and nickel is usually 2+, but these ones appear very infrequently. If they appear, you can either consult your table of standard reduction potentials and only look at the charge, or more likely it will be something like Titanium (IV) Oxide, which you automatically know its 4+.

But if they throw at you copper, lead or tin without that, ASSUME 2+. If they give you Copper (I) then copper is 1+. Iron they have to say 2+ or 3+.

Also this might seem like a really nooby question, but since oxygen is in group 6, why doesn't it have 6+? Like howcome Ca in group 2 has 2+, but elements like oxygen don't have their group number?

 

[leehuan]

re: HSC Chemistry Marathon Archive

Because oxides are 2-

It goes
Li. Be. B. C . . . N. O. F
+1 +2 +3 +4OR-4 -3 -2 -1

Apologies, I can't get them to align properly

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Because oxides are 2-

It goes
Li. Be. B. C . . . N. O. F
+1 +2 +3 +4OR-4 -3 -2 -1

Apologies, I can't get them to align properly

Thank you so much!

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Also this might seem like a really nooby question, but since oxygen is in group 6, why doesn't it have 6+? Like howcome Ca in group 2 has 2+, but elements like oxygen don't have their group number?

Skipped year 8 chemistry I see. Nah jks, don't worry about it, so many nooby questions i have asked over the past year.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Skipped year 8 chemistry I see. Nah jks, don't worry about it, so many nooby questions i have asked over the past year.

Dw guys it took me 3 months to understand what a "mole" was. Furthermore i still don't know what avagadro's number is... My year 11 teacher called me "Mr avagadro" he also started the "sonic" bs...

 

[BlueGas]

re: HSC Chemistry Marathon Archive

This might seem even more of a noobier question but I was recently looking at videos on "how to identify the products of a reaction", and basically what helped me is ab + cd = ad + bc

But this general equation doesn't always work like for example the reaction of sodium carbonate in water, how would I know the products on this case?

 

[leehuan]

re: HSC Chemistry Marathon Archive

CO3(2-) + 2H2O <--> HCO3(-) + OH(-)

Acid base reaction -> Type: Conjugate acid formation

There are some you really should just know.

e.g. getting ethylene from ethanol by dehydration? Using that analogy would be kinda bad
C2H5OH -(conc. H2SO4)-> CH2=CH2 + H2O