Material Balance With Reaction Questions and Answers

Answer: A

A species may be created or destroyed by the reaction, so input and output no longer match. The total mass balance still holds without such terms, because mass itself is conserved. Only the molecular species balances gain the extra terms.

Answer: B

Correct answer: True

Atoms are neither created nor destroyed, so total mass is conserved whatever reactions occur. Moles are not: \( \mathrm{N_2 + 3H_2 \rightarrow 2NH_3} \) turns four moles into two. That is why a mass balance can always be written as input equals output with no generation term, while a mole balance on a reacting species cannot.

Answer: C

Correct answer: atoms are conserved, so no reaction terms appear

An atomic balance is written as input equals output for each element, with no generation or consumption term and no need for the reaction extents. It is therefore the shorter route when several reactions occur at once or when the stoichiometry is not fully known - a combustion problem with an unspecified fuel composition being the classic case.

Answer: D

Each answers a different question, and quoting one where another is meant misstates the performance of a reactor. Conversion can be high while selectivity is poor, which is the situation a designer most wants to avoid.

Answer: A

Correct answer: is in the smallest stoichiometric proportion

The limiting reactant is identified by dividing each reactant’s available moles by its stoichiometric coefficient and taking the smallest result - not by comparing raw amounts, and certainly not by mass. A reactant present in the largest quantity can still be limiting if the equation demands proportionally more of it.

Answer: B

Dividing by the coefficients gives two point five for hydrogen against four for oxygen, so hydrogen runs out first. Comparing the raw mole numbers would wrongly suggest oxygen is limiting. The coefficients must divide the feed amounts first.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the definition. The theoretical requirement is whatever is needed to consume the limiting reactant entirely, and the excess is what is supplied beyond that, expressed as a fraction of the requirement. Quoting an excess against anything else would be meaningless, since the requirement is what defines the reference point.

Answer: D

Correct answer: 25 per cent

\[ \frac{5-4}{4} = 0.25 = 25\%. \] The denominator is the theoretical requirement, not the amount supplied - dividing by 5 instead gives the 20 per cent offered. That is exactly the point the neighbouring true/false item makes, and it is the single commonest error in excess air calculations.

Answer: A

Correct answer: True

Percentage excess is \[ \frac{\text{supplied}-\text{theoretical}}{\text{theoretical}}\times100, \] so the theoretical requirement is the base. Using the supplied amount as the base gives a smaller figure and one that can never exceed 100 per cent, which is a quick way of spotting the error - 200 per cent excess air is entirely ordinary and impossible on the wrong base.

Answer: B

It runs from zero to one and refers to a specific reactant, usually the limiting one. The third option confuses conversion with yield, which brings the stoichiometric ratio into it. Conversion always names the reactant it refers to.