rates of reaction: justify concentration of liquids

[lboogy]

Hi everyone,

 

I've just done a science experiment - a fairly simple one testing rates of reaction using sodium thiosulphate (0.1 concentration 50ml) & hydrochloric acid (also 0.1 concentration 50ml). I wish I'd asked my tutor at the time but I didn't, and now I need to know why I decided to use a 0.1 concentration of each liquid and why I decided to use 50mls vloume of each liquid.

 

Can anyone think of any scientific justifications?

 

thanks for your help in advance!

[insane_alien]

yeah. i can think of one. i'll help you figure it out. first, write down the balanced reaction equation. then look at the ratios again.

[lboogy]

Hey Insane alien!

 

I think I've balanced it:

 

Na2S2O3(aq) + 2HCl(aq) → S(s) + SO2(g) + 2NaCl(aq)

 

2Na2S2O3(aq) + 4HCl(aq) → S(s) + 3SO2(g) + 4NaCl(aq)

 

 

is this correct? How does this tell me why I should use 0.1 concentrate of both liquids and 50mls of each also?

[insane_alien]

should have asked this before, but did you do other experiments along side of this?

 

like with different quantities or different chemicals?

 

 

specifically you should be comparing the concentrations of H+ and Na+ and

[lboogy]

no, this was the only experiement I did & I had to pay £70 so I didn't have time to do lots of different experiementd! Now though I have to make it seem like there was scientific reasoning behind me using an equal concentration of 0.1 concentrate for both liquids!

[insane_alien]

ahh. now it gets difficult. let me sleep on it.

[lboogy]

lol! ok, thanks for your help too!

[lboogy]

Hi...ok I think I'm making some improvement - can anyone tell me f this sounds acceptable?

 

when testing rates of reaction, the speed of a chemical reaction is proportional to the quantity of the reacting substances (law of mass action) so the higher the concentration of HCl acid, the faster the reaction, or the higher the concentration of sodium thiosulphate, the faster the reaction.

 

Any quantities could have been chosen - but what is important is that if quantity is not the variable then the quantity should be kept the same in all the tests. Does this sound fair enough?

[Darkblade48]

Sounds fine, though when you say "quantity" you must clarify that you are referring to moles and not to volume.

[lboogy]

Ok, thanks for the tip.

 

I've just read that the volume of the substance (in terms of liquids) also effects the rate of reaction, in that the greater the volume, the faster the reaction - based on that there are ultimately more particles so the reaction will be faster.

 

Does anyone think this is true - I don't, based on the fact that it is concentration that is important, and if all variables (including concentration) were the same except for volume, the reaction would take place in the same amount of time.

 

What do you think on this?

[Darkblade48]

What you said is true; volume does not play a role because of the number of particles (i.e. a 10 ml solution of 1 M HCl would react with metal slower than a 10 ml solution of 10 M HCl would).

 

However, in terms of surface area, volume might matter. For example, imagine a small tube filled with a 1 M solution of HCl making contact with a piece of metal (small surface area) vs. a large diameter tube filled with the same volume of 1 M HCl making contact with another piece of metal.

 

Obviously, the larger diameter tube will have more surface area available for the metal/acid interaction to occur (i.e. the volume of acid is spread out over more surface area)

[lboogy]

Yeah, I can see that with a solid and a liquid, the bigger the surface area of the solid the quicker the rate of the reaction, as the interface between the reactants is bigger. So it is 100% correct to say with liquids, volume doesn't matter?