Chemical Kinetics Intro (Rates of Reaction)

Unit 3: Kinetics and Equilibrium

What actually happens during a chemical reaction? Reactant molecules must collide for a

reaction to occur

Chemical Kinetics The study of rates of

chemical reactions and the mechanisms (or steps) by which a chemical reaction takes place.

Reaction rates vary greatly – some are very fast (burning) and some are very slow (disintegration of a plastic bottle in sunlight).

Definition:

Rate of reaction: the change in concentration of a reactant or product per unit of time.

Rxn Rate (avg) = Δ [reactant or product] Δ time

Note: [square brackets] = mol/L = concentration

Example :

If the concentration of reactant X after 50 s in a reaction was 0.0079 mol/L and after 100 s was 0.0065 mol/L, calculate the average reaction rate.

Rate = Δ[X] Δt

= (0.0065 – 0.0079 M) (100-50 s)

= - 0.000028 M/s

Think about it!

Concentration of reactant decreases with time, as it is being used up

The rate is fastest when concentration of reactants is greatest; and slows when concentration of reactants are less

Still thinking…..

The reactants are being used up as the reaction takes place.

What’s happening to the concentration of products over time? It is increasing.

Concentration rate of rxn In the beginning of

the reaction, product is formed quickly

But slows over time because less reactants means less reactants collisions producing the new product.

Time (s)Con

cent

ratio

n of

pro

duct

(m

ol/L

)

Factors that affect the Rate:

1. Concentration (and pressure)2. Temperature3. Amount of surface area4. Catalysts5. Nature of reactants

Concentration

A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate

Note: We are not increasing the amount being made for a given balanced equation with limiting reactants, we are only speeding up how quickly those products are made.

Pressure (Gases)

Affects the rate of reaction, particularly gases. When you increase the pressure, the molecules

have less space in which they can move. That greater concentration of molecules increases the number of collisions.

Temperature

Remember: Temperature is a measure of the average kinetic energy of a system

So higher temperature implies higher average kinetic energy of molecules and more collisions per unit time.

Surface Area

Reducing the size of particles increases the rate of a reaction because it increases the surface area available for collisions to take place. This increases the number of collisions.

Catalysts

A catalyst is a substance that speeds up a reaction without being used up itself.

Some reactions have catalysts that can speed them up, but for many reactions there is no catalyst that works.

How do they work? (We will talk about this later!)

Nature of Reactants

Reactants with large number of chemical bonds that have to be broken, or with strong chemical bonds that have to be broken, will lead to a slower rate.

(Later….. The strong bonds will cause a high activation

energy, leading fewer collisions being successful.

More complex reactant molecules with more atoms, will have more difficulty lining up properly to have a successful reaction. This will also lead to a lower reaction rate.)

Some good rules of thumb… Ions will react more quickly, because already

broken up. Simple molecules (non-metals covalently bonded)

will take more time Complex molecules and ionic compounds (metal

and non-metal bonded) will take the most time because the strong ionic bonds need lots of energy to be broken, and the complex molecules will be hard to have proper geometry (line up)

Summary (fast to slower): Ions (solutions)>simple molecular>ionic and complex molecular

Try it :

Place the following reactions from fastest to slowest, based on the previous slides.

1. 2Cu+2 + 2I- 2 Cu+ + I22. 2NaCl 2Na + Cl23. Cl2 2Cl

Page 467 #1, 3, 4, 5

Answer from previous slide:

• 1 is fastest, then 3, and 2 is slowest