Qualification > Sciences
IGCSE CHEMISTRY DOUBTS
Ivo:
--- Quote from: ~VIN1094~ on June 06, 2010, 11:49:35 pm ---OK here, look at the reaction. Its the decomp. of H2O2. Doubling the conc. of H2O2 increases the amount of oxygen in H2O2 by twice. Hence when
it reacts with H2O2 decomposes there is now twice the number of O2 produced.. got it?
Also when amount of catalyst is halved the reaction is slower. As you know a catalyst never takes part in a reaction so it really doesn't affect the amount of O2 produced.
--- End quote ---
OK, I kinda got it. So for this example, increasing the concentration means increasing the amount of hydrogen peroxide dissolved in the water, so if there is double the the amount of hydrogen peroxide dissolved, then double volume of oxygen produced and in faster time.
However, would this be the same for excess. Say sodium hydroxide reacted with hydrochloric acid. Hydrochloric acid was used as the excess, would doubling the concentration of acid also double the amount of sodium chloride? If not, why?
And is it true that increasing the volume of something decreases the rate?
Vin:
--- Quote from: Ivo on June 06, 2010, 11:54:35 pm ---OK, I kinda got it. So for this example, increasing the concentration means increasing the amount of hydrogen peroxide dissolved in the water, so if there is double the the amount of hydrogen peroxide dissolved, then double volume of oxygen produced and in faster time.
However, would this be the same for excess. Say sodium hydroxide reacted with hydrochloric acid. Hydrochloric acid was used as the excess, would doubling the concentration of acid also double the amount of sodium chloride? If not, why?
And is it true that increasing the volume of something decreases the rate?
--- End quote ---
I guess you are mixing up two concepts. If you add excess HCl ALL the NaOH would be reacted BUT this doesn't mean that it always doubles the amount of NaCl. This depends on how much NaCl is present.
Yes, increasing the vol. decreases the rate as you've added more molecules to be reacted and this would take more time.
Anyway I'm off now its 4:30 am!
GOOD LUCK! :D
Ivo:
--- Quote from: ~VIN1094~ on June 05, 2010, 11:17:36 pm ---Potassium manganate (VII) is an oxidising agent, purple compound. The oxidation state of manganese is +VII. BUt it is much more stable in oxi. state +II. So it is strongly driven to gain electrons and reduce its oxi. state to +II.
So it takes electrons from other substances, in the presence of a little acid. It itself is reduced causing a colour change..
(reduction)
MnO4- ----------> Mn2+
manganate(VII)ion manganese ion(II)
purple colourless
This means KMnO4 can be used to test for a reducing agent.
--- End quote ---
Just one more thing, here you said potassium manganate is an oxidising agent. So does that mean when it oxidises something, it changes from purple to colourless? So what do you mean by test for a reductant? You mean when the other thing reduces the potassium manganate, the colour changes or what?
Ivo:
--- Quote from: Ivo on June 07, 2010, 12:05:18 am ---Just one more thing, here you said potassium manganate is an oxidising agent. So does that mean when it oxidises something, it changes from purple to colourless? So what do you mean by test for a reductant? You mean when the other thing reduces the potassium manganate, the colour changes or what?
--- End quote ---
You'll also be rewarded for your hard work by me +repping you like a million times! :D
Vin:
--- Quote from: Ivo on June 07, 2010, 12:05:18 am ---Just one more thing, here you said potassium manganate is an oxidising agent. So does that mean when it oxidises something, it changes from purple to colourless? So what do you mean by test for a reductant? You mean when the other thing reduces the potassium manganate, the colour changes or what?
--- End quote ---
This is right..w09 paper, Q 6 a) b) ii) talks about this.
Also an oxidising agent brings about oxidation if another substance and itself reduces. vise versa for reducing agent.
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