Consider the reaction

_{}

in which the rate of disappearance of A is 5 moles of A per
dm^{3} per second at the start of the
reaction.

At the start of the reaction

(a) What is -r_{A}?

(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,
r_{A}?

(f) What is -r_{B}?

(a) -r_{A} 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.

-r_{B} = 2 x -r_{A} = 10 moles/dm^{3}/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
-r_{C}. Therefore we multiply the rate of
formation of C, r_{C}, by minus one (-1) to
get

_{}

Because C is a product, its
rate of disappearance, -r_{C}, is a negative
number.

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

_{}

(f) -r_{B} is the rate of disappearance of B

_{}

Rate
of disappearance of A = -r_{A} = 5
mole/dm^{3}/s

Rate
of disappearance of B = -r_{B} = 10
mole/dm^{3}/s

Rate
of disappearance of C = -r_{C} = -15
mole/dm^{3}/s

Rate
of formation of A = r_{A} = -5
mole/dm^{3}/s

Rate
of formation of B = r_{B} = -10
mole/dm^{3}/s

Rate
of formation of C = r_{C} = 15
mole/dm^{3}/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.