Elements of
Chemical Reaction Engineering
6th Edition



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Essentials of
Chemical Reaction Engineering
Second Edition

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Chapter 1: Mole Balances

Self Test - The Convention for Rates of Reaction

 

Consider the reaction

in which the rate of disappearance of A is 5 moles of A per dm3 per second at the start of the reaction.

At the start of the reaction

(a)   What is -rA?

(a) +5   (b) -5   (c) -10   (d) +10  

(b)   What is the rate of formation of B?

(a) +5   (b) -5   (c) -10   (d) +10  

(c)   What is the rate of formation of C?

(a) +15   (b) -15   (c) -7.5   (d) +7.5  

(d)   What is the rate of disappearance of C?

(a) +15   (b) -15   (c) -7.5   (d) +7.5  

(e)   What is the rate of formation of A, rA?

(a) +5   (b) -5   (c) -10   (d) +10  

(f)    What is -rB?

(a) +10   (b) -10   (c) -5   (d) +5  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Answers

(a)   -rA is the rate of disappearance of A is

-rA = 5 mol/dm3/s

(b)   For every one mole of A that disappears, two moles of B disappear. Reactant B is a is disappearing twice as fast as reactant. I.e., A.

                                                      -rB = 2 x -rA = 10 moles/dm3/s

        Multiplying by minus one (-1) we get the rate of formation of B

rB = -10 mol/dm3/s

        B is being used up therefore its rate of formation is a negative number.

(c)   C is a product that is being formed three times as fast as A is disappearing

rC = 3 x (-rA) = 15 mol/dm3/s

        Because C is a product is being formed, its rate of formation is positive.

(d)   The rate of disappearance of C is -rC. Therefore we multiply the rate of formation of C, rC, by minus one (-1) to get

-rC = -15 mol/dm3/s

        Because C is a product, its rate of disappearance, -rC, is a negative number.

(e)   A is a reactant that is being used up therefore its rate of formation is negative

rA = -5 mol/dm3/s

(f)    -rB is the rate of disappearance of B

-rB = 2 x (-rA) = 10 mol/dm3/s

 

Summary

            Rate of disappearance of A = -rA = 5 mole/dm3/s

            Rate of disappearance of B = -rB = 10 mole/dm3/s

            Rate of disappearance of C = -rC = -15 mole/dm3/s

            Rate of formation of A = rA = -5 mole/dm3/s

            Rate of formation of B = rB = -10 mole/dm3/s

            Rate of formation of C = rC = 15 mole/dm3/s

 

For reactants the rate of disappearance is a positive (+) number.

For products the rate of disappearance is a negative (-) number because they are being formed and not disappearing.

For reactants the rate of formation is a negative (-) number because they are disappearing and not being formed.

For products the rate of formation is a positive (+) number.



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