# Chapter 1 - 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?

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

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

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

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

(f)    What is -rB?

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

(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

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

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

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

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

## 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 rte of formation is a positive (+) number.