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Rates of reaction

Rates of reaction. Elephant toothpaste. We are going to look at a reaction named after elephant toothpaste and you’ll see why In graduated cylinder # 1 – 20 mL 8% H 2 O 2 , 1 mL of detergent and a 0.5g of KI In graduated cylinder #2 - 20 mL 30 % H 2 O 2 , 1 mL of detergent and a tip of of KI.

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Rates of reaction

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  1. Rates of reaction

  2. Elephant toothpaste • 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

  3. A Look at The reaction • 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!

  4. Rate OF a Reaction • 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 pollution • Develop more efficient methods in the food processing industry

  5. Rate OF A Reaction • 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  conductivity  concentration  volume  color  pressure • we look at how these properties change over time

  6. Rate of A Reaction :Calculations, OF COURSE! • Units for reaction rates can vary: kg/h, mol/min, mol/Ls, 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

  7. Rate of A Reaction :Calculations, OF COURSE! • 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/L s

  8. Examples • 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 : r A: 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/Ls P: Therefore the rate of reaction for the production of nitrogen dioxide is 0.16 mol/Ls

  9. Time for you to Try! • 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 : r A: r = ΔcS: 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/Ls P: Therefore the rate of reaction for the production of ammonia is 0.046 mol/Ls

  10. Related Rates • 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

  11. 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

  12. 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!

  13. Let’s try one together! • 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

  14. Factors that affect reaction rates • What factors do you think would affect reaction rates? • 1. Nature of Reactants • 2. Concentration • 3. Temperature • 4. Surface Area • 5. Catalyst

  15. Karlos Dansby Summary : Factors That affect reaction rates • 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

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