We are going to look at a reaction named after elephant toothpaste and you’ll see why

In graduated cylinder # 1 – 20 mL 8% H2O2, 1 mL of detergent and a 0.5g of KI

In graduated cylinder #2 - 20 mL 30% H2O2, 1 mL of detergent and a tip of of KI

What we observed was the decomposition of hydrogen peroxide using potassium iodide as a catalyst (the soap is just for fun)

H2O2(l) -> H2O(l) + O2 (g)

As you saw the rate of reaction can change quite drastically!

Chemical Kinetics – the study of ways to make a chemical reaction go faster or slower

Let’s Think… What might be some reasons why

chemists might want to know about reaction rates?Designing new medications Improving techniques to control pollutionDevelop more efficient methods in the food

processing industry

is found by measuring the rate at which a product is formed or the rate at which a reactant is consumed over time

Properties of the reactants that can be measured are:mass conductivityconcentration volumecolor pressure

we look at how these properties change over time

Units for reaction rates can vary:

kg/h, mol/min, mol/Ls, etc

How we find a reaction rate is similar to the speed of a car. If a car travels 700 km in 7 hours to get from Toronto to Montreal, the average speed would be 100 km/h

With a reaction, if 700 mols of product aremade in 7 minutes, the rate of reaction would be

100 mol/min

So that we can compare many different reaction types, we are going to use change in concentration to measure reaction rate

Rate = change in concentration

change in time r = Δc

Δt mol/Ls

What is the rate of production for nitrogen dioxide in the system: 2N2(g) + O2(g) -> 2NO2(g) if the concentration of nitrogen dioxide changes from 0.32 mol/L to 0.80 mol/L in 3 s?

G: t1 = 0 s, t2 = 3 s, c1 = 0.32 mol/L, c2 = 0.80 mol/L

R : rA: r = Δc Δt S: r = Δc = c2 – c1 = 0.80 mol/L – 0.32 mol/L

Δt t2 – t1 3 s – 0 s

= 0.16 mol/Ls P: Therefore the rate of reaction for the production

of nitrogen dioxide is 0.16 mol/Ls

What is the rate of production of ammonia for the system, between 1.0 min and 4.0 min, if the concentration of ammonia is 3.5 mol/L at 1.0 min and 6.2 mol/L after 4.0 min?

N2 (g) + 3H2 (g) 2NH3 (g)

G: t1 = 1.0 s, t2 = 1.0 min = 60.0 s, c1 = 3.5 mol/L, c2 = 6.2 mol/L

R : rA: r = Δc S: r = Δc = c2 – c1 = 6.2 mol/L –

3.5 mol/L Δt Δt t2 – t1 60.0 s – 1.0 s

= 0.046 mol/Ls P: Therefore the rate of reaction for the production

of ammonia is 0.046 mol/Ls

• The average rate of reaction over time is found by finding the slope of the line between two points on a graph

•The instantaneous rate of reaction can also be found by drawing a tangent line on any given point in time

• For most reactions, concentration changes are more rapid near the beginning of the reaction, while the rate decreases as time elapses

• If you plot the data for concentration of a product vs. time, you get a positive slope

• Therefore, when we measure rates off a graph, we describe them using a negative value for the rate of consumption of the reactants, and a positive value for the rate of production of the products!

What is the rate of consumption of the reactants in a reaction that produces ammonia, when the rate of production of ammonia is 4.0 mol/L·s?

N2 + 3H2 2 NH3

-[ΔN2] = 4.0 mol/L·s x 1 mol N2 = 2.0 mol/L·s

Δt 2 mol NH3

-[ΔH2] = 4.0 mol/L·s x 3 mol H2 = 6.0 mol/L·s

Δt 2 mol NH3

What factors do you think would affect reaction rates?

1. Nature of Reactants 2. Concentration 3. Temperature 4. Surface Area 5. Catalyst

1. Nature of Reactants : the more reactive a reagent is the faster the reaction

2. Concentration: if concentration increases, reaction rate increases

3. Temperature: if temperature increases, reaction rate increases

4. Surface Area: if surface area increases,

reaction rate increases

5. Catalyst: are used to increase the rate of reaction and are not used up in the reaction

provide a reaction site for the molecules to collide

Karlos Dansby