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- Jan 20, 2014
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I had a question regarding oxidation states.
There is a TBR question that asks:
Which compound is a chemical by-product in Stage II of the firework in Figure 1?
a. NO3
b. K2O
c. SO3
d. CL2
The answer is SO3, but i did not really understand the logic behind picking this answer choice.
(The figure is not necessary by the way, all we know is that stage 2 is exothermic)
Do they get this answer because you are just supposed to know the combustion reactions and what is produced? And how can you tell if a compound is fully oxidized or not?
Here is the explanation given:
C is the best answer. Stage II in Figure 1 is labeled as highly exothermic. The role of that stage is to generate a great amount of heat. This is done by reacting the oxygen gas produced by the oxidizer with a reducing agent. The passage mentions that common reducing agents include sulfur and charcoal. The complete combustion of sulfur and charcoal produces sulfur trioxide, SO3, and carbon dioxide, CO2, respectively. The oxides of nitrogen and potassium are not products of the combustion reaction in Stage II, because both nitrogen and potassium start and finish in their highest oxidation state, meaning that neither was oxidized or reduced. Both NO3 and K2O are fully oxidized, so they are eliminated. If the oxidizer in Stage II is perchlorate, ClO4-, then the product has chloride in its fully reduced form (with an oxidation state of -1, not 0). The product when potassium chromate is used is potassium chloride, KCl. Choices A, B, and D are thus eliminated, leaving choice C as the best answer. The best answer is C.
There is a TBR question that asks:
Which compound is a chemical by-product in Stage II of the firework in Figure 1?
a. NO3
b. K2O
c. SO3
d. CL2
The answer is SO3, but i did not really understand the logic behind picking this answer choice.
(The figure is not necessary by the way, all we know is that stage 2 is exothermic)
Do they get this answer because you are just supposed to know the combustion reactions and what is produced? And how can you tell if a compound is fully oxidized or not?
Here is the explanation given:
C is the best answer. Stage II in Figure 1 is labeled as highly exothermic. The role of that stage is to generate a great amount of heat. This is done by reacting the oxygen gas produced by the oxidizer with a reducing agent. The passage mentions that common reducing agents include sulfur and charcoal. The complete combustion of sulfur and charcoal produces sulfur trioxide, SO3, and carbon dioxide, CO2, respectively. The oxides of nitrogen and potassium are not products of the combustion reaction in Stage II, because both nitrogen and potassium start and finish in their highest oxidation state, meaning that neither was oxidized or reduced. Both NO3 and K2O are fully oxidized, so they are eliminated. If the oxidizer in Stage II is perchlorate, ClO4-, then the product has chloride in its fully reduced form (with an oxidation state of -1, not 0). The product when potassium chromate is used is potassium chloride, KCl. Choices A, B, and D are thus eliminated, leaving choice C as the best answer. The best answer is C.
