1. Warm Up #? What is a rate? A ratio of two different units
Give some examples of rates.
What do you think might change the rate of a reaction?
A ratio of two different units
Temperature, concentration, a catalyst, an inhibitor, or particle size (surface area)

2. Rate of a Reaction

3. Reaction Rate
Reactant → products
A B

4. *Because [A] decreases with time, D[A] is negative.
rate = -
D[A] Dt
rate =
D[B] Dt
D[A] = change in concentration of A over
time period Dt
*Because [A] decreases with time, D[A] is negative.
D[B] = change in concentration of B over
time period Dt

5. Average Instantaneous Rate averaged over a specific period of time
Rate for a specific instance in time
Rate =
Δ concentration
Δ time
Rate = —
[A]final – [A]initial
tfinal - tinitial
Units: M or mol/L
s s

6. Example
Hydrogen reacts with chlorine to produce hydrochloric acid. Calculate the average reaction rate for all reactants. Write the balanced equation.

7. Example
H2 + Cl2 → 2HCl
average reaction rate expressed in moles H2 consumed per liter per second
average reaction rate expressed in moles Cl2 consumed per liter per second
Rate = -[H2] = -[0.025 – 0.060]
t – 0.00
= M/s, rate will always be positive
Rate = -[Br2] = – 0.070
t – 0.00
= M/s, rate will always be positive

8. Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)
Example
Determine the average rate for the consumption of molecular bromine for the following time intervals.
a. first 50 sec b. first 100 sec
red-brown
Br2 (aq) + HCOOH (aq) Br- (aq) + 2H+ (aq) + CO2 (g)
time

9. Slope of the tangent is the instantaneous rate

10. Avg. Reaction Rate and Stoichiometry
2A B
Two moles of A disappear for each mole of B that is formed.
rate = -
D[A] Dt 1 2
rate =
D[B] Dt
aA + bB cC + dD
rate = -
D[A] Dt 1 a
= -
D[B] Dt 1 b =
D[C] Dt 1 c =
D[D] Dt 1 d

11. Review of Concepts
Write a balanced chemical equation for a gas-phase reaction whose rate is given by

12. Collision Theory rate  number of collisions sec
Summary: 1) reacting substances must collide
2) must collide with the correct orientation
3) must collide with sufficient energy, Ea

13. A + B AB C + D +
Exothermic Reaction
Endothermic Reaction
The activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.

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