Question

[H3O^+(aq)]		Drops of Stock Solutions			Reaction Time
Test Tube	KMnO4	H2O	H3O^+/H2SO4	C2O4H2
C-1	15	25	5	5	14mins 3 sec
C-2	15	15	15	5	8mins 10sec
C-3	15	5	25	5	3mins 8 sec

a)     What are the molecules and ions doing in solution – are they static or moving around? 


b)    Can ions or molecules that are isolated from one another react? What must happen for them to react? 


c)     Why should the permanganate disappear faster if the concentration of H3O+(aq) is higher? 


d)    Devise a general, qualitative, statement about the effect that an increase in the concentration of a reactant has on the rate of a reaction, and an explanation of that effect. 


Answer 1

Ans. a. Molecules in the solution are moving around constantly.

Ans. b. No, the ions or molecules (say, two ions or molecules that need to react to form a product.) isolated from each other can’t react. For example, H₃O⁺ (H₂SO₄) ions and C₂O₄H₂ when kept in different test tubes (physically isolated from each other in different test tubes) do not react.

For ions/molecules to react they must be brought into a solvent that dissolves both of them.

Ans. c.               3 H₂C₂O₄+ 2 MNO₄^- -----> 6 CO_2(g) + 2 MnO_2(s) + 2 OH^- + 2 H₂O

            Color:        clear         purple                              Yellow

Presence of higher [H₃O⁺] in solution forms oxalic acid more rapidly. Oxalic acid thus formed reacts with KMnO₄ decolorizing it more rapidly. Thus, faster decolorization indicates greater rate of synthesis of oxalic acid.

Ans. d. Compare reaction time between [H₃O⁺] and reaction time while keeping [₂C₂O₄] constant- reaction time decrease with increase in [H₃O⁺]. It means rate of reaction increases.

Thus, it can be generalized that- “Increase in concentration of reactants increase the rate of reaction”.