Rates of reaction (Chapter 6)

If a reaction produces a gas then measure….

pressure or volume of gas produced

If a reaction involves ions then measure….

changes in conductivity

If a reaction produces a colour then measure…

Colour intensity, using a
"Spectrophotometer" for
precise measurements

If reaction involves change of mass then messure

change in mass with a scale

chemical kinetics

the study of the rate at which chemical reactions occur

reaction rate

the rate at which reactants change into products over time

average rate of reaction

change in concentration/change in time

how to calculate average reaction rate on a graph

slope between two points on a secant line

instantaneous rate of reaction

The rate of reaction at any one particular time during the reaction

how to calculate instantaneous rate of reaction on a graph

slope of tangent line

Importance of collisions

Collisions must occur for a reaction to happen

Collisions theory

1. Collisions must occur between two or more reactant particles
2. Orientation: the particles must collide with the correct orientation
3. activation energy: the particles must have a certain minimum energy to meet the activation energy requirement

Activation energy

the minimum amount of energy required to overcome repulsion forces between reactants. Essential for a chemical reaction to start.

Why is activation energy needed?

1. overcome repulsive forces between reactants
2. weaken bonds of reactants

activated complex (transition state)

a high-energy/unstable intermediate state between reactant and product

Factors that affect particle collisions

1. concentration
2. temperature
3. Surface area
4. Catalysts
5. Nature of Reaction

How does the nature of reactants affect rate of collisions

-reactions involving ions are faster than molecules due to greater force of attractions
-more bonds increases activation energy

How does temperature affect the rate of reaction?

• lowers activation energy
• greater proportion of particles with sufficient energy to react and form products
  -increases number of products

How does concentration affect collisions

More collisions will occur with a higher concentration of reactant particles

How does surface area affect rate of collisions

Greater SA = greater chance of a reaction to occur

How do catalysts affect the rate of reactions?

provide alternative pathways for reactants lowering the activation energy required for the reaction to occur

• does not affect the amount of products produced or delta H

rates of reaction depend only on the _ concentrations

reactant

reaction rates can be calculated at a _ temperature

fixed

rate law equation

rate = k [A]^m [B]^n

what does k represent in rate law equation:

rate constant

Rate Constant Units apply to

one specific reaction at a specific temperature

sum of exponents of rate equation =

order of reaction

reaction mechanism

Series of steps that occur as reactants are converted to products

reaction intermediate

• substance that used up during the reaction sequence
  -does not appear in the overall equation.

rate determining step

the slowest step in a reaction mechanism (highest activation energy)

Plausible Reaction Mechanism

1. Have elementary steps that sum to form overall balanced equation
2. align with experimentally determined rate law

on a graph how to tell if exothermic or endothermic

endothermic - reactant< product exothermic - reactant >product

Zero order

concentration of reactant has no effect on the rate

• A^0

first order

• A^1

Second order

• A^2