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March 28, 2024, 10:13:53 pm

Author Topic: VCE Chemistry Question Thread  (Read 2313057 times)  Share 

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soNasty

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Re: VCE Chemistry Question Thread
« Reply #825 on: May 05, 2014, 12:25:44 am »
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Thank for that man!

hobbitle

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Re: VCE Chemistry Question Thread
« Reply #826 on: May 05, 2014, 03:51:27 pm »
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Hi guys

Is it possible to do these problems without knowing the Kb values?

Q11.83
What amount of NH3 must be dissolved in water to give 500mL solution with a pH of 11.22?

Q11.84
Calculate the pH of 0.20 M NaCN.  What is the concentration of HCN in the solution?

Cheers.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #827 on: May 05, 2014, 04:40:09 pm »
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Also, could anyone explain what exactly the difference is between Ka/Kb values and Kc?  Does Ka/Kb refer specifically and only to a weak acid or base in water reaction?  But Kc can refer to the equilibrium constant of any reaction?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #828 on: May 05, 2014, 08:16:09 pm »
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Hi guys

Is it possible to do these problems without knowing the Kb values?

Q11.83
What amount of NH3 must be dissolved in water to give 500mL solution with a pH of 11.22?

Q11.84
Calculate the pH of 0.20 M NaCN.  What is the concentration of HCN in the solution?

Cheers.

You definitely need acidity constants.

Also, could anyone explain what exactly the difference is between Ka/Kb values and Kc?  Does Ka/Kb refer specifically and only to a weak acid or base in water reaction?  But Kc can refer to the equilibrium constant of any reaction?

Kc is the equilibrium constant in terms of concentrations (generally used when referring to gas reactions). Ka is the acidity constant while Kb is the basicity constant; Ka * Kb = Kw for a conjugate acid base pair
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #829 on: May 06, 2014, 07:33:06 am »
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Does the addition reaction of ethene and water need a catalyst?

Aren't alkenes so reactive that a cataylst isn't needed?
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #830 on: May 06, 2014, 09:12:44 am »
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Does the addition reaction of ethene and water need a catalyst?

Aren't alkenes so reactive that a cataylst isn't needed?

Ethene, when reacted with water, produces ethanol. In this hydration (addition) reaction, phosphoric acid catalyst in 300oC conditions is required.

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Re: VCE Chemistry Question Thread
« Reply #831 on: May 06, 2014, 05:50:54 pm »
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does the benzene ring in acetylsalicylate count as a functional group?
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Capristo

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Re: VCE Chemistry Question Thread
« Reply #832 on: May 06, 2014, 07:30:05 pm »
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What is it that causes salicylic acid to have higher rf values than aspirin, considering salicylic acid is more polar?

Thanks  :)
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #833 on: May 06, 2014, 08:36:13 pm »
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Does the addition reaction of ethene and water need a catalyst?

Aren't alkenes so reactive that a cataylst isn't needed?

You are confusing two types of reactivity. There is kinetic reactivity (lability), which describes how fast something reacts. There is also thermodynamic reactivity, in which physics predicts a reaction will occur to a great extent.  The difference can be shown in the following examples.
If you add an indicator to a solution and then add an acid, the acid will QUICKLY change the colour of the indicator (assuming you swirl the solution). Acid-base reactions are generally quite fast, even though they don't proceed to completion. So this is something that is fast kinetically but not favoured thermodynamically.

Now, let's look at the hydrogenation of ethene to form ethane. Thermodynamically, this reaction should proceed almost to completion. However it's not very fast. Catalysts and high temperatures are used if you need an acceptable reaction rate for some reasons, such as the hydration of ethene to form ethanol. Just because an alkene is reactive doesn't mean it'll react fast.

Another example. Lithium is the strongest reductant on your standard electrode potential table. It isn't, though, the fastest thing to react with water. Sodium will fizz, potassium will fizz violently and produce hydrogen gas so vigorously the hydrogen gas burns, while lithium does like nothing.

You'll learn more about the differences between reaction rate and extent of reaction.

What is it that causes salicylic acid to have higher rf values than aspirin, considering salicylic acid is more polar?

Thanks  :)

Rf values in what solvent and stationary phase?
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Capristo

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Re: VCE Chemistry Question Thread
« Reply #834 on: May 06, 2014, 10:23:51 pm »
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Rf values in what solvent and stationary phase?

Ethyl acetate and silica gel restrospectively. I'm not entirely sure if the results were correct. I was mainly researching some ways in which TLC could be used to qualitatively determine the purity of aspirin.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #835 on: May 07, 2014, 10:46:45 am »
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Hi guys,

I don't understand whats happening here.

When people have a 'Barium Meal' i.e. BaSO4 (s) <--> Ba2+ + SO42-...
They make it less soluble (because Ba2+ is toxic to the body) by making it up in a solution with lots of extra SO4 ions.
This forces the backwards reaction to occur (Le Chateliers principle)...
But the Ksp doesn't change? 
I can't get my head around this.....
Doesn't the Ksp have to change in order to reduce the solubility of Ba2+?
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thushan

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Re: VCE Chemistry Question Thread
« Reply #836 on: May 07, 2014, 10:59:11 am »
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Nup. So the solubility of barium sulfate is known as the maximal amount of barium sulfate that can be dissolved into a solution. It is not synonymous with Ksp.

Ksp = [Ba2+][SO4 2-], where the solution is already saturated.
Qsp = [Ba2+][SO4 2-], where the solution may or may not be saturated. Qsp < Ksp if the solution is not saturated, Qsp = Ksp if the solution is saturated.

Suppose we had a solution already containing sulfate ions (eg. K2SO4). The solubility of barium sulfate in this solution is the amount that can be dissolved into this solution to saturate it - i.e. bring Qsp up to Ksp. Since we already have sulfate ions in the solution, when we start adding BaSO4, it requires less BaSO4 to bring Qsp up to Ksp. In other words, less BaSO4 can be dissolved into this solution to achieve saturation.

Hence, the solubility of BaSO4 in a solution containing sulfate ions from other sources is less than that in water. In either case Ksp is the same.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #837 on: May 07, 2014, 11:33:13 am »
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Hence, the solubility of BaSO4 in a solution containing sulfate ions from other sources is less than that in water. In either case Ksp is the same.

Thanks thushan. 

Is it wrong for me to be thinking of Kw, Ka, Kb, Kc, Ksp etc as all basically the same thing, but the subscript just refers to the specific kind of reaction it is referring to (e.g. Kc is for gases only, Ka is for acid in water, Ksp is for salt in water)?

ie. are they all just an equilibrium constant for a reaction - which will only change with a change in temperature - and the 'formula' is:

Kwhatever = [Product 1]n[Product 2]m / [Reactant 1]p

Where the concentration of a solid or a liquid is always 1, and n, m, p are the stoiciometric constants for that species?

We have been kind of taught them all over the last couple of weeks and they haven't been terribly concise about it (IMO - but I am probably just not understanding it yet).
« Last Edit: May 07, 2014, 11:36:15 am by hobbitle »
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thushan

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Re: VCE Chemistry Question Thread
« Reply #838 on: May 07, 2014, 05:55:59 pm »
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Thanks thushan. 

Is it wrong for me to be thinking of Kw, Ka, Kb, Kc, Ksp etc as all basically the same thing, but the subscript just refers to the specific kind of reaction it is referring to (e.g. Kc is for gases only, Ka is for acid in water, Ksp is for salt in water)?

ie. are they all just an equilibrium constant for a reaction - which will only change with a change in temperature - and the 'formula' is:

Kwhatever = [Product 1]n[Product 2]m / [Reactant 1]p

Where the concentration of a solid or a liquid is always 1, and n, m, p are the stoiciometric constants for that species?

We have been kind of taught them all over the last couple of weeks and they haven't been terribly concise about it (IMO - but I am probably just not understanding it yet).

You're absolutely right.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #839 on: May 07, 2014, 06:49:16 pm »
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You're absolutely right.

God that's so much less confusing. They could have taught it so much better haha. Thankyou!
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