Chemistry 30
Reaction Rates and Chemical Equations
We have learned that the speed of a chemical reaction can be determined by measuring the
change in the concentration of amount of product / reactant as a function of time.
The rate of a chemical reaction can be calculated in two different ways:


Looking at the rate of appearance of products over a change in time
Looking at the rate of disappearance of reactants over a change in time
Examine the following chemical reaction, and notice how the molar ratio in the balanced
chemical reaction plays a major role in the rate of formation of products / reactants:

2 HI(s)
1. When 2 moles
decomposes then
2. When 1000
H2(g)
+
I2(g)
1. Reactions forms one
mole of I2.
2. Reaction forms 500
molecules
molecules of the
decompose then
product
3. When HI decomposes
at 2.00 mol/L/s
4. If the rate of HI
decomposition is 4.6
mol/L/s
3. Reactions forms I2 at
1.00 mol/L/s
4. Reactions forms I2 at
2.3 mol/L/s
Chemistry 30
The Big Idea:
The balances in the equation allow us to determine the relative rate of
decomposition of reactants to the formation of products.
Practice Question
Examine the following reaction:
N2 (g) + 3H2 (g)
2 NH3 (g)
If the rate of decomposition of nitrogen gas is 0.03 mols per liter per second, what is the
rate of formation of ammonia gas?
Chemistry 30
Assignment: Balanced chemical equations and rate.
*** If the rate of a reaction is known in terms of one particular substance in a reaction, the rate
of the reaction in terms of the other reactants and products can also be determined. These
relative rates can be expressed as ratios.
Examine the following unbalanced reaction below.
___N2O5 (g)

___NO2 (g)
+
___O2 (g)
Balance the above reaction
1. Examine the mole ratios What is the relative rate of (expressed as a ratio)
o
o
formation of O2 to the rate of decomposition of N2O5 ?
formation of O2 to the rate of formation of N02?
2. If the rate of formation of O2 was 8.6 * 10 - 4 mol*L-1*s-1, what is the rate of
decomposition of N2O5 ?
3. If the rate of formation of O2 was 4.3 * 10 - 4 mol*L-1*s-1, what is the rate of formation
of N02 ?
3. In the following decomposition reaction,
2 N2O5 → 4 NO2 + O2
oxygen gas is produced at the average rate of 9.1 × 10-4 mol · L-1 · s-1. Over the same period,
what is the average rate of the following:


4.
the production of nitrogen dioxide
the loss of nitrogen pentoxide
Consider the following reaction:
N2(g) + 3 H2(g) → 2 NH3(g)
If the rate of loss of hydrogen gas is 0.03 mol · L-1· s-1, what is the rate of production of
ammonia?