Lecture 9: Rate Laws - Orders of Reaction - Rate Constants

Rate of reaction

The speed of different chemical reactions varies hugely, some are fast some are slow

When do reactions take place?

When particles collide with a certain amount energy.

Activation energy

The minimum amount of energy needed for particles to react

What does the rate of reaction depend on?

1. Frequency of collisions

2. Energy with which the particles collide

What happens if there is not enough energy for the activation energy?

The particles bounce off of eachother

What speeds up reactions?

Anything that increases the number of successful collisions between reactant particles

What factors affect the rate of reactions?

• Temperature

• Concentration of dissolved reactants

• Increased pressure of gaseous reactant

• Increased surface area of solid reactants

How are temp and ROR related?

Increase in temperature increases the rate of reaction

Why does a raise in temp increase ROR?

The particles speed put when they are heated, which causes more collisions

How are ROR and concentration related

An increase in concentration increases the rate of reaction

Why does an increase in concentration increase ROR?

Because there are more particles, so there are more chances to collide

What happens with ROR when reactants are the same state?

Reactions tend to occur faster

What happens to ROR when solid has a larger surface area (crushed)

The reaction proceeds faster

How are ROR and pressure related?

When pressure increases, so does the rate of reaction

Why does ROR increase with pressure?

Gas particles come closer together the more pressure, therefore collisions are more frequent and more likely to react

Why are surface area and ROR directly related?

The larger the surface area, the smaller the pieces therefore more collisions and a greater chance for reactions to occur.

Catalyst

Something that speeds up a reaction by changing the mechanism that leads to the products

Are catalysts part of the reaction?

No, they are not reactants or products, but are listed in the yield sign. Think of them has helping the reaction without changing anything about themselves.

How do catalysts impact activation energy?

A catalyst lowers the activation energy, so more particles can collide with the energy they have

Do catalysts produce more product?

No, they only make the same amount of product more quickly

Examples of catalysts

• Nickel

• Iron

• Platinum

The rate law

An expression or equation that relates the rate of reaction to the concentrations of reactants at constant temperature

What is “n” in Rate Law?

• The exponent that represents order of reaction

• Dependent on mechanisms of reaction

• Must be determined by experiement

Differential Rate Law

Shows how the rate of a reaction depends on concentrations

Integrated Rate Law

Shows how the concentrations of species in the reaction depend on time

Reaction Order

A reaction has an individual order “with respect to” or “in” each reactant

First order

If the rate doubles when [A] doubles, the rate depends on [A]¹

Second Order

If the rate quadruples when [A] doubes, the rate depends on [A]²

Zero Order

If rate does not change when [A] doubles, the rate does not depend on [A]

Properties of Reaction Orders

• Cannot be deduced from the balanced chemical equations

• Usually positive integers or zero

• Can be fractional or negative

Zero order on a graph on [A] vs. time

Linear negative

1st and 2nd order on a graph [A] vs. time

non-linear, still negative

0 order on rate vs. [A] graph

flat line

1st order on rate vs. [A] graph

linear - positive

2nd order on rate vs. [A] graph

exponential - positive

What do the units of k depend on?

The order of the reaction

units of k for 0 order

mol/L*s

units of k for 1 order

1/s

units of k for 2 order

L/mol*s

units of k for 3 order

L²/mol²*s