State whether the following is true or false.
Lowering the temperature always reduces selectivity towards the wanted product.
Answer: C
It depends entirely on which reaction has the higher activation energy, and if the unwanted one does then cooling improves selectivity. The direction has to be worked out from the kinetics. Activation energies decide the direction.
Please log in or register to report this question.
For a series reaction where the intermediate is wanted, the key variable is
Answer: D
Correct answer: the residence time chosen for the run
In \( A \rightarrow B \rightarrow C \) the intermediate builds up and is then consumed, so its concentration passes through a maximum. Stopping at that time gives the best yield, and going beyond it destroys what has been made. Everything else about the reactor matters less than getting that time right - which is also why plug flow beats a stirred tank here, since it gives every element the same time rather than a spread of them.
Please log in or register to report this question.
Assertion (A): A plug flow reactor gives a higher yield of a wanted intermediate than a stirred tank.
Reason (R): Every element in plug flow spends the same time in the reactor while a stirred tank spreads the times widely.
Answer: A
Both are true and the reason gives the mechanism: some material in a stirred tank leaves almost at once and some stays far too long, and both extremes lose the intermediate. A narrow residence time distribution is what helps.
Please log in or register to report this question.
For a series reaction A to B to C with equal first order rate constants of 0.1 per minute, at what time is B at its maximum in a batch reactor?
Answer: B
When the two constants are equal the maximum occurs at the reciprocal of the constant. With unequal constants the optimum is given by the logarithm of their ratio divided by their difference. The maximum is sharper when the constants differ widely.
Please log in or register to report this question.
All of the following improve the yield of a wanted intermediate EXCEPT
Answer: C
Correct answer: running the reaction to complete conversion
Complete conversion is the one thing guaranteed to destroy the intermediate, since with no reactant left the second step continues unopposed and converts B into C. The others all help: stopping at the optimum time, removing the intermediate as it forms so it cannot react further, and using plug flow so that no material lingers past the optimum.
Please log in or register to report this question.
State whether the following is true or false.
Reactive distillation can raise the yield of an intermediate by removing it as it forms.
Answer: D
Taking the intermediate out of the reacting mixture stops it from going on to the next product. The same idea underlies membrane reactors and other schemes that combine reaction with separation. Removing the product stops the second step.
Please log in or register to report this question.
Three of the following help protect a wanted intermediate. Which is the odd one out?
Answer: A
Complete conversion is exactly what destroys an intermediate. The other three all interrupt the sequence before the second step can take hold. Running to completion destroys everything. The three all stop the sequence early.
Please log in or register to report this question.
Yield of a wanted product is normally defined as
Answer: B
It measures how much of the potential was realised, whereas selectivity compares the two products with each other. Conversion, the third option, says nothing about which product was formed. The three measures answer different questions.
Please log in or register to report this question.
Consider the following statements about the three performance measures.
1. Conversion measures how much reactant was consumed.
2. Selectivity measures which product it went to.
3. Yield is simply another name for conversion.
Answer: C
Correct answer: 1 and 2 only
Statements 1 and 2 define conversion and selectivity correctly. Statement 3 confuses yield with conversion. The three are distinct and related by \[ \text{yield} = \text{conversion} \times \text{selectivity}, \] so a reactor can convert everything and yield almost nothing if the selectivity is poor.
Please log in or register to report this question.
A reactor converts 80 per cent of its feed with a selectivity of 75 per cent to the wanted product. What is the yield?
Answer: D
Correct answer: 60 per cent
\[ 0.80 \times 0.75 = 0.60 = 60\%. \] Yield is the product of conversion and selectivity, so it can never exceed either of them - which immediately rules out the 155 per cent offered, and shows why quoting a high conversion or a high selectivity alone tells you little about how much product was actually made.
Please log in or register to report this question.
