Answer to Homework 1

Dear 1SB4,

Here are the suggested answers and comments on your answers. If you have any queries, you could either approach me in school or drop me a note. In addition, feel free to revisit the earlier post for your classmates' answer.

Regards
Mr Kwok

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(a) Write a balance equation showing H2O2 decomposing into water and oxygen gas.

2H2O2 -> 2H2O + O2

Comments:

Generally, well answered.

(b) By quoting relevant data from the data booklet, show how MnO2 functions as a catalyst.


From the answer in (a), it is observed that H2O2 is both oxided and reduced. Hence, using the following half equations: (where E refers to standard reduction potentials)

oxidation half-cell: MnO4- + 4H+ + 3e- < - > MnO2 + 2H2O E = +1.67 V
reduction half-cell: H2O2 + 2H+ + 2e- < - > 2H2O E = +1.77V

Reaction 1: 3H2O2 + 2MnO2 -> 2MnO4- + 2H+ + 2H2O E (reaction) = +0.10v

To regenerate the catalyst:

oxidation half-cell: O2 + 2H+ + 2e- <-> H2O2 E = +0.68V
reduction half-cell: MnO4- + 4H+ + 3e- < - > MnO2 + 2H2O E = +1.67 V

Reaction 2: 2MnO4- + 3H2O2 + 2H+ -> 2MnO2 + 4H2O + 3O2 E (reaction) = + 0.99V

Both E's of reaction 1 and reaction 2 are greater than zero. Both steps are spontaneous. Reaction 2 shows that MnO2 is regenerated.

Comments:

First point:
If you have chosen MnO2 to oxidise H2O2 first, you aren't quite correct. That is because you will get an equation that looks like:

MnO2 + 2H+ + 2H2O2 -> Mn2+ + 2H2O + O2 E (reaction) = +0.55V

The above equation implies that H2O2 will only be oxidised in the pressence of acid medium. However, you are given an aqueous solution of H2O2. It is neutral and should therefore contain very small amount of H+ ions. Hence, this reaction is unlikely to occur.

Second point:
A few of you fail to quote the half equations. Marks are awarded for you to quote and for you to apply. Therefore, you are required to show the Ecell of the two reaction steps (like what I have shown).

Third point:
The catalyst lowers the activation energy by participating in the reaction such that a reaction pathway with a lower energy barrier is obtained. As a result, you have to show MnO2 reacting with the reactant and not MnO2 undergoing a disproprotionation reaction.

(c) Suggest the identity of the white fumes.

Water droplets.

The reaction is extremely exothermic, hence water is evaporated. The water vapour mixes with cool air and thus condenses to give water droplet and that produces the white fumes.

Comments:

Numerous gave me O2. But O2 is always colourless. Water vapour is also colourless, thus it is also not a suitable answer. But it is good that you have paid attention to video to realise that liquid water will vaporise.