acids question! (1 Viewer)

[Dumbarse]

Q. A student placed a 0.5g strip of magnesium ribbon in 100mL of
(i) 0.10 mol/L ethanoic acid
(ii) 0.10 mol/L sulfuric acid

Compare the chemistry of the above two reactions in terms of the strength of the acid, rate of reaction and volume of gas formed. Use equations in your answer

???

 

[BlackJack]

0.5g Mg = 0.20567667...mol Mg

2CH3COOH + Mg(s) => (CH3COO)2Mg + H2(g)
mol ethanoic acid = 0.01
mol H+ in this = 0.01

H2SO4 + Mg(s) => MgSO4 + H2(g)
mol sulfuric acid = 0.01
mol H+ in this = 0.02

Since ethanoic acid is weak and has low dissociation, the rate of reaction will be slower than the reaction with sulfuric acid, which fully dissociates within water.
Also, since sulfuric aicd is diprotic, there will be twice as much Mg that reacts and becomes aqueous. Note how the acid is the limiting agent in both cases. Therefore... in ATP the amount of H2 gas evolved is 0.122L for ethanoic acid and 0.245L for sulfuric.

 

[Dumbarse]

Originally posted by BlackJack
0.5g Mg = 0.20567667...mol Mg

2CH3COOH + Mg(s) => (CH3COO)2Mg + H2(g)
mol ethanoic acid = 0.01
mol H+ in this = 0.01

H2SO4 + Mg(s) => MgSO4 + H2(g)
mol sulfuric acid = 0.01
mol H+ in this = 0.02

Since ethanoic acid is weak and has low dissociation, the rate of reaction will be slower than the reaction with sulfuric acid, which fully dissociates within water.
Also, since sulfuric aicd is diprotic, there will be twice as much Mg that reacts and becomes aqueous. Note how the acid is the limiting agent in both cases. Therefore... in ATP the amount of H2 gas evolved is 0.122L for ethanoic acid and 0.245L for sulfuric.

thanx bjack
how did u get, "mol H+ in this = 0.01"
did u work out "mol H+ in this = 0.02" because there are 2 H's in H2SO4
also how did u calculate the gases evolved

 

[Dumbarse]

what about strength of acid , do u have to calculate pH for this?, how do u show it?

 

[Ozz^E]

Nah...no need to calculate pH. Strength of acid just want u to show that u appreciate the fact that becuase sulfuric acid is stronger, it will ionise fully and thus produce a fster reaction and more product H2 gas. Though u could add there that sulfuric acid would have a lower pH and thus more H+ ions.

 

[BlackJack]

Ethoanoic acid, when you look at its structur, has only one H atom that'll detach and form an acid... since there is only 0.01 moles of ethanoic acid in solution...

One mole of gas at 25 degrees and 101.3 kPa wil take up 24.47L, but every 2 moles of ethanoic acid will give you 1 mole of H2 from the equation. Therefore the 0.01 moles will give you 0.005 moles of gas. Multiply.
In the sulfuric acid equation, one mole of sulfuric acid gives you one. Hence, there will be 0.01 moles of gas evolved in this case.

(trivia: of course, everyone knows the ideal gas law doesn't hold for most gases as you increase the pressure... )

 

[McLake]

Originally posted by Dumbarse
what about strength of acid , do u have to calculate pH for this?, how do u show it?

Either talk about degree of inosation or use equations ...