IGCSE CHEMISTRY DOUBTS

[Ivo]

Please can somone answer my doubtful doubt! 

For varying concentrations of acid in reactions and looking at rates, why sometimes when they sketch a graph, increasing the concentration of acid still has same amount of product (gas) produced, but other times, when double concentration, the gas volume doubles?

Majorly confused.  How about with surface area and temperature, do they have an effect on the final amount of gas produced?

[Vin]

http://www.xtremepapers.net/CIE/Cambridge%20IGCSE/0620%20-%20Chemistry/0620_w09_qp_31.pdf
5c(ii) can ny 1 help wid de equation

4

OK, Ge4H10 is like any other Alkane which forms CO2 and water.. but it this case its GeO2
Complete combustion

C₄H₁₀ + 13 O₂ --> 4CO₂  +  5H₂O

Ge₄H₁₀ + 13 O₂----> 4 GeO₂+ 5H₂O

[Vin]

Please can somone answer my doubtful doubt!  

For varying concentrations of acid in reactions and looking at rates, why sometimes when they sketch a graph, increasing the concentration of acid still has same amount of product (gas) produced, but other times, when double concentration, the gas volume doubles?

Majorly confused.  How about with surface area and temperature, do they have an effect on the final amount of gas produced?

OK, I get a part of your ques. Is this what you are looking for?..

Zn + 2HCl  --->  ZnCl2  +  H2

Conc. for acid doubled, but same amount of Zn used reaction faster but results in same volume of H2 gas.

Doubling the amount of both reactants results in doubling of product (gas).

Surface area and Temperature doesn't have anything to do with the yield, they only influenced the rate of reaction. Posting a q. as an example would do good as I still cannot properly get your question. Usually in the ques it is mentioned right, when the conc of reactants in increased/decreased.

[Ivo]

OK, I get a part of your ques. Is this what you are looking for?..

Zn + 2HCl  --->  ZnCl2  +  H2

Conc. for acid doubled, but same amount of Zn used reaction faster but results in same volume of H2 gas.

Doubling the amount of both reactants results in doubling of product (gas).

Surface area and Temperature doesn't have anything to do with the yield, they only influenced the rate of reaction. Posting a q. as an example would do good as I still cannot properly get your question. Usually in the ques it is mentioned right, when the conc of reactants in increased/decreased.

Hmm, I'm still a bit confused.  Maybe a question example might help.  w02, Q2) c) ii) and iii)

[Vin]

Hmm, I'm still a bit confused.  Maybe a question example might help.  w02, Q2) c) ii) and iii)

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.

[Ivo]

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.

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]

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?

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!

[Ivo]

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.

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]

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?

You'll also be rewarded for your hard work by me +repping you like a million times! 

[Vin]

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?

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.

[Ivo]

This is right..w09 paper, some q. talks about this.

Thanks for all your help.  I have added the reputation, and a very long comment!  Enjoy reading it, or shall I say enjoy your good night's sleep!  Wow, 4am, you better go!

[Lariemeeva]

On which bases can we determine that a metal would react with dilute HCL?

[Dana]

2 questions,
1st, there was a qs in stoichiometry and the answer was that there were 0.03 moles of O2 molecules then they asked how many moles of oxygen atoms will there be. the answer is 0.06. i dont get why :/ isnt it supposed to be the opposite? like you half it instead of twice it? :/

and second question, in CO2 are the C and the O bonded by a double bond? cuz if theres only a single bond then there are only gonna be 6 e's around C

[Vin]

On which bases can we determine that a metal would react with dilute HCL?

Reacts with metals which are higher than hydrogen in the reactivity series.



^^Why? Because a metal lower than hydrogen (Copper/Silver/Gold/Platinum) wont be able to displace H2 from HCl

[Lariemeeva]

Right, thanks a lot!