HSC 2012-2015 Chemistry Marathon (archive) (1 Viewer)

[bangladesh]

Re: HSC 2012 Chemistry Marathon

Yes! Reason? Because I thought molar heat of combustion was defined as the amount of energy 1g of substance blah blah...?

It's MOLAR heat of combustion, Therefor it refers to one mole of the substance.

 

[ocatal]

Re: HSC 2012 Chemistry Marathon

Yes! Reason? Because I thought molar heat of combustion was defined as the amount of energy 1g of substance blah blah...?

It isn't 1g of the substance, it is 1 mole.

Since the molar weight of propane is about 44, 44 / 44 = 1, meaning 1 mole of the substance releases 2200 kJ of heat.

 

[AnimeX]

Re: HSC 2012 Chemistry Marathon

Yes! Reason? Because I thought molar heat of combustion was defined as the amount of energy 1g of substance blah blah...?

Okay, so it's 2200kj /mol
divide it by the molar mass you get 2200/ 44 kj/g

which is 50kj/g

so now you have 50kj/g, times top and bottom by 44 [because from MC it's either 44 or 1, try both]

so you get 2200kj* per 40 g.

And note, because it's exothermic reaction (cause combuston releases heat) it will be B

 

[ForOneEon]

re: HSC Chemistry Marathon Archive

It's multiple choice, don't worry about the reason too much.

 

[abdog]

Re: HSC 2012 Chemistry Marathon

Thank you so much for your input, especially the effects of acid rain!

Now, for mine!
Assess the need to monitor levels of lead ions in substances used in society and describe appropriate technologies that can be used for such measurements. - [7 marks][/QUOTE]

Lead, a heavy metal, is a severe neurotoxin, retards neurological development in children and is extremely harmful to humans even in low concentrations. Lead was used as a fuel additive until recently and this caused it to be released in high amounts near highways by motor vehicles. It was also used in paint in the past and when houses with this kind of paint were demolished, lead was released into the atmosphere.

Technologies, such as Atomic Absorption Spectroscopy(AAS) is a highly accurate method used to measure Lead ions in the environment. Firstly, a sample containing lead ions is fed into the nebuliser, and it is made into a mist and mixed with fuel and oxidants. The mist is then combusted, turning the lead ions into atoms and a current is passed through a lead hollow cathode lamp, which causes it to produce light in a spectrum specific to lead. As this light passes through the mist, the lead atoms absorb this light. This changed light is then detected by a detector and a reading, called absorbance, is produced. From there, the concentration of lead ions can be measured.

Assessment:
It is very important to constantly monitor the levels of lead ions in substances used in society to make sure that they are within the parameters set by health organisations, so that humans are not exposed to dangerous levels of lead.

 

[abdog]

Re: HSC 2012 Chemistry Marathon

It isn't 1g of the substance, it is 1 mole.

Since the molar weight of propane is about 44, 44 / 44 = 1, meaning 1 mole of the substance releases 2200 kJ of heat.

I can't believe I forgot its MOLAR..... thanks guys!

 

[someth1ng]

re: HSC Chemistry Marathon Archive

http://www.digipac.ca/chemical/mtom/contents/chapter3/chap3_7_3.htm

how come here it works for solids aswell?
I think it works for solids but DOESN'T affect it if the others are gases and you're dealing with pressure

That's because it's equilibrium is dissolution. It's the equilibrium that exists between a saturated solution and solid of the solute. In this situation, increasing the amount of solid won't affect equilibrium.

Oh and, just because something is in the equation, doesn't mean that it affects equilibrium. You don't include the solvent and you don't include solids in equilibrium expressions.

Yeah, if the reactants and products are all gases in a closed system, by adding a solid, you are decreasing the volume of the system, hence an increase in pressure and causing the equilibrium to shift to the side with less moles of gas.

That's making an assumption because that's not the solid affecting the equilibrium per se, it's the decrease in volume that's affecting it.

The molar heat of combustion of propane is given in the data book as 2200 kJ mol-1
What does this mean?

(A) 1 g of propane releases 2200 kJ of heat
(B) 44 g of propane releases 2200 kJ of heat
(C) 1 g of propane absorbs 2200 kJ of heat
(D) 44 g of propane absorbs 2200 kJ of heat

All are wrong because propane does not absorb or release heat unless combusted.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

Most pH indicators are weak acids or bases which change colour at a certain pH.

Explain how pH indicators work.
In your answer, include relevant chemical equations.
(4 marks)

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Most pH indicators are weak acids or bases which change colour at a certain pH.

Explain how pH indicators work.
In your answer, include relevant chemical equations.
(4 marks)

An indicator solution consists of a weak acid and it's conjugate base in equilibrium.
Consider the following equation: HIn + H₂O <--> H₃O⁺ + In^-

HIn and In^- are weak conjugate acid/base pairs
Depending on the [H⁺], the colour will change. It will have different colours in the acidic form and it's conjugate base form. The resultant colour after mixing with a solution can be compared with a table which shows the colour a solution takes on at different pH's to approximate a pH for the solution.

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Most pH indicators are weak acids or bases which change colour at a certain pH.

Explain how pH indicators work.
In your answer, include relevant chemical equations.
(4 marks)

Weak acid's conjugate base = weak base or strong base?
Or is it strong base when the conjugate acid is an extremely weak acid?

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Weak acid's conjugate base = weak base or strong base?
Or is it strong base when the conjugate acid is an extremely weak acid?

^This idea is very weird, i see different things in different sets of notes and textbooks

 

[someth1ng]

re: HSC Chemistry Marathon Archive

An indicator solution consists of a weak acid and it's conjugate base in equilibrium.
Consider the following equation: HIn + H₂O <--> H₃O⁺ + In^-

HIn and In^- are weak conjugate acid/base pairs
Depending on the [H⁺], the colour will change. It will have different colours in the acidic form and it's conjugate base form. The resultant colour after mixing with a solution can be compared with a table which shows the colour a solution takes on at different pH's to approximate a pH for the solution.

A little dodgy, to be honest.

You started with "An indicator solution consists of a weak acid and it's conjugate base in equilibrium" when that is clearly wrong by the provided statement. You don't explain how it works - why does the colour change? (one conjugate becomes more dominant at certain pH levels).

Also, you only have one equation when it asked for equations.

Mark: 1/4

Weak acid's conjugate base = weak base or strong base?
Or is it strong base when the conjugate acid is an extremely weak acid?

Think of it like a scale.

The stronger the acid (weaker the base), the weaker the conjugate base.
The stronger the base (weak the acid), the weak the conjugate acid.

A weak base, itself, does not mean that the conjugate will be "strong" but it does mean that it will be stronger (the weaker the base is).

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

A little dodgy, to be honest.

You started with "An indicator solution consists of a weak acid and it's conjugate base in equilibrium" when that is clearly wrong by the provided statement. You don't explain how it works - why does the colour change? (one conjugate becomes more dominant at certain pH levels).

Also, you only have one equation when it asked for equations.

Mark: 1/4

So what other equations could you write?

 

[psychotropic]

re: HSC Chemistry Marathon Archive

Most pH indicators are weak acids or bases which change colour at a certain pH.

Explain how pH indicators work.
In your answer, include relevant chemical equations.
(4 marks)

An indicator is a substance that, in solution, changes colour depending on the substance that is added to it. This colour can be used to approximate the pH of the solution, as indicators are known to have certain colours at different pH levels
pH indicators, such as HIn, are commonly made of a weak acid and its conjugate weak base, such as the HIn/In- indicator. HIn/In- is a buffer that will resist a change in pH.This means that the rate of change of pH when a base or acid is added will follow a logarithmic shape, which is in agreement with the definition pH = -log10[H30+]
When an acid is added
H30+ + In- <------> HIn + H20
this means that if an acidic substance i,e, one contianing hydronium ions, is added to this indicator, by le Chatelier's principle, the system will oppose the change in pH, meaning it will favour the foward reaction to reduce the change in pH.
The exact same logic can be applied when a base is added: HIn + OH- <------> In- + H20
This means that when a weak, or very small amount of acid or base is added to this indicator, by LCP, there will be very little change in pH. However, it means that when larger amounts are added such the pH change will become larger, which will be evidenced by a colour change. This will mimic a logarithmic scale, where a ten fold increase in concentration of hydronium equates to a change in pH of 1

 

[abdog]

re: HSC Chemistry Marathon Archive

How do we know if a substance in going to be basic, acidic or neutral in water? For example, HCO3-, takes an H+ from water and OH- ions are formed, making the solution basic. How do we know if a substance is accepting or donating protons to water?

 

[rednellav]

re: HSC Chemistry Marathon Archive

How do we know if a substance in going to be basic, acidic or neutral in water? For example, HCO3-, takes an H+ from water and OH- ions are formed, making the solution basic. How do we know if a substance is accepting or donating protons to water?

Write the equation and look at which literally accepts the hydrogen and which donates it. (will give you your conjugates)

 

[abdog]

re: HSC Chemistry Marathon Archive

Write the equation and look at which literally accepts the hydrogen and which donates it. (will give you your conjugates)

So how do we know if its accepting or donating? Again, HCO3- is a good example. HCO3- in water can be written as HCO3- + H20 ---> H2C03 + OH- (solution becomes basic) or HCO3- + H20 ---> C032- + H30+(solution becomes acidic)

Unless you know by heart its basic, both reactions are possible. How do we know which one to use?

 

[bedpotato]

re: HSC Chemistry Marathon Archive

So how do we know if its accepting or donating? Again, HCO3- is a good example. HCO3- in water can be written as HCO3- + H20 ---> H2C03 + OH- (solution becomes basic) or HCO3- + H20 ---> C032- + H30+(solution becomes acidic)

Unless you know by heart its basic, both reactions are possible. How do we know which one to use?

HCO₃^- is amphiprotic though, so it can act as either a base or an acid by either accepting or donating a proton.

Acting as an acid:
HCO₃^- + OH^- --> CO₃^2- + H₂O

Acting as a base:
HCO₃^- + H₃O⁺ --> H₂CO₃ + H₂O

Don't really understand what you're asking :s

 

[someth1ng]

re: HSC Chemistry Marathon Archive

So what other equations could you write?

A basic indicator.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

An indicator is a substance that, in solution, changes colour depending on the substance that is added to it. This colour can be used to approximate the pH of the solution, as indicators are known to have certain colours at different pH levels
pH indicators, such as HIn, are commonly made of a weak acid and its conjugate weak base, such as the HIn/In- indicator. HIn/In- is a buffer that will resist a change in pH.This means that the rate of change of pH when a base or acid is added will follow a logarithmic shape, which is in agreement with the definition pH = -log10[H30+]
When an acid is added
H30+ + In- <------> HIn + H20
this means that if an acidic substance i,e, one contianing hydronium ions, is added to this indicator, by le Chatelier's principle, the system will oppose the change in pH, meaning it will favour the foward reaction to reduce the change in pH.
The exact same logic can be applied when a base is added: HIn + OH- <------> In- + H20
This means that when a weak, or very small amount of acid or base is added to this indicator, by LCP, there will be very little change in pH. However, it means that when larger amounts are added such the pH change will become larger, which will be evidenced by a colour change. This will mimic a logarithmic scale, where a ten fold increase in concentration of hydronium equates to a change in pH of 1

The bolded is wrong.