Honors Chemistry- Kinetics

Kinetics

Tell us how quickly a reaction will occur

Collision Theory

A reaction occurs with the reactant bonds breaking. However to do this, the reactants must collide with enough energy and in the right spot

Activation energy

minimum energy required to start the reaction

“Activation complex”

Bonds broke but have not formed, unstable and the highest energy point on a graph

What is special about the activation complex on an endothermic graph?

Equal to the enthalpy

How does a reaction happen faster?

If the reactants are able to collide more or lower the activation energy

Faster Reaction: Higher reactant concentration

reactants collide more often, breaking bonds more often

Faster Reaction: Heat

move faster, so they collide more often which create more broken bonds

Faster Reaction: Catalyst

lowers activation energy

Faster Reaction: Increase reactant surface

allow more space for collisions to occur between eachother

What happens to the reaction rate overtime?

Reactants are used up and products are made, so reactants go down and products go up. As particles collide faster at the start of a reaction instead of the end.

How is reaction rate measured?

M S^-1

Coefficients and relative rates of chemical equations

Gives us info about the rates of consumption of reactants and production of products, BUT NOT ABSOLUTE AMOUNTS

Rate law

An equation that describes how a change in concentration affects the reaction rate. Rate=K(A)^m (B)^n (m and n are orders of the reaction, which must be determined)

Orders

Tells us exactly how a change in the concentration will change the rate of reaction

A bigger k value(rate constant) will…

Imply a faster reaction

How do you solve for K?

First pick a certain trial and determine the order of the reactants. Next, plug the values into the equation and rearrange it to where k = rate / ([A]^m [B]^n)

Units of (K) rate important patterns

• Zero-order reaction: rate = k, so k has units of M/s. 

• First-order reaction: rate = k[A], so k has units of s⁻¹. 

• Second-order reaction: rate = k[A]², so k has units of M⁻¹s⁻¹. 

• Third-order reaction: rate = k[A][B]², so k has units of M⁻²s⁻¹. 

First	rate = k[A]	s⁻¹
Second	rate = k[A]²	M⁻¹s⁻¹
Third	rate = k[A]³	M⁻²s⁻¹

Slowest reaction step is also called…

Determine the rate step

What is special about reaction mechanisms?

The coefficents can be used to determine the order, ONLY IN THIS CASE

Catalysts

Appears in RDS, and rate law, but NOT OVERALL(R—>P)

Intermediates

(P—>R) Does not appear in the rate law, could be in overall

Radioactive decay

Change in the nucleus that results in nuclear radiation based on the release of particles and energy from the nucleus

Alpha decay

4/2 a

Beta decay

0/-1 B

Half Life

Length of time where 12 of radioactive atoms decay

Shorter half life equals…

Faster reaction

Half life equation for first order

t(1/2)=0.693/k

Half life equation for second order reaction

t(1/2)=1/k(reactant)”o” o is a subscript, not an actual zero

All radioactive decay follows…

FIRST ORDER KINETICS