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 -r_A?

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

(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

-r_A = 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.

-r_B = 2 x -r_A = 10 moles/dm³/s

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

r_B = -10 mol/dm³/s

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

r_C = 3 x (-r_A) = 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 -r_C. Therefore we multiply the rate of formation of C, r_C, by minus one (-1) to get

-r_C = -15 mol/dm³/s

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

r_A = -5 mol/dm³/s

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

-r_B = 2 x (-r_A) = 10 mol/dm³/s

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

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

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

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

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

Rate of formation of C = r_C = 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.