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Chemical kinetics

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  1. Chemical kinetics Chemistry 11 - Kinetics Source: Brown and LeMay’s Chemistry the Central Science

  2. Objectives

  3. Change • How does change affect us? • What are some factors that we think about when we discuss change? • Today is Earth Day, what changes should be discussed?

  4. Chemical kinetics • Chemistry, by its very nature, is concerned with change. • What changes have we discussed / observed this semester in Chemistry?

  5. Chemical kinetics • Chemical reactions convert substances into new substances with entirely different properties. • Aside from studying what changes occur, it is also important to study how these changes occur.. • What are some things that might describe how a chemical reaction occurs?

  6. Chemical kinetics • Chemical kinetics is concerned with how fast—or the rate at which—a reaction occurs. • There is an enormous range of rates of reaction. • Give an example of a quick reaction (fast rate). • Give an example of a slow reaction (slow rate).

  7. Chemical kinetics • Some examples of the application of kinetics include pharmaceuticals, environmental science, and industrial chemistry. • Our goals for this topic are: • to understand how to determine the rates at which reactions occur • to consider the factors that control these rates

  8. Factors that affect reaction rates • What factors determine how rapidly food spoils? • What factors affect how quickly steel rusts?

  9. Factors that affect reaction rates • How does a chemical reaction occur? • What factors might affect how quickly a reaction occurs?

  10. Factors that affect reaction rates • On a molecular level, chemical reactions occur when molecules collide. • The more collisions, the faster the reaction. • There are four factors that allow us to change the rates at which particular reactions occur: • the physical state of the reactants • the concentrations of the reactants • the temperature at which a reaction occurs • the presence of a catalyst

  11. Factors that affect reaction rates • Reactants must come together in order for a reaction to occur. • The more contact there is between molecules, the more rapidly they will react. • Of the three states of matter, which states are most ideal for molecules to interact with each other? Physical state

  12. Factors that affect reaction rates • Most of the reactions we will consider are either gases or liquid solutions because these states are most ideal. • Aside from the state (of matter), what other factors of physical state might affect the rate of reaction? Physical state

  13. Factors that affect reaction rates • How do you think concentration is related to reaction rates (e.g. burning of steel wool)? Concentration

  14. Factors that affect reaction rates • The higher the concentration of the reactants, the faster a reaction occurs. Concentration

  15. Factors that affect reaction rates • What effect does temperature have on molecules? • How does this affect a chemical reaction? Temperature

  16. Factors that affect reaction rates • Reactions occur more quickly at higher temperatures. • Recall that temperature is related to the average kinetic energy of molecules. • If molecules are moving around more, they will collide more frequently, which leads to increased reaction rates. • Think of spoiling milk. Temperature

  17. Factors that affect reaction rates • What does the word “catalyst” mean? Catalysts

  18. Factors that affect reaction rates • In chemistry, catalysts are agents / substances that increase reaction rate without being used up. • A very important example of a catalyst are enzymes in our bodies. Catalysts

  19. Reaction rates • What does the word “rate” imply?

  20. Reaction rates • A rate describes how something changes over time. • In chemical reactions, what is changing over time?

  21. Reaction rates • The rate of a reaction describes the in concentration of reactants or products over time.

  22. Reaction rates • As time progresses, what happens to: • the concentration of reactants? • the concentration of products?

  23. Reaction rates Rate data for reaction of C4H9Cl with water • Is C4H9Cl a reactant or product? • What is happening to its concentration? Why? • Calculate the rate of reaction at each time interval. • What is the unit of measurement for reaction rate?

  24. Reaction rates Rate data for reaction of butyl chloride (C4H9Cl) with water • Construct a graph of the concentration of butyl chloride over time. • How can we get the rate of reaction from the graph? • Is the rate of reaction constant?

  25. Reaction rates Rate data for reaction of butyl chloride (C4H9Cl) with water • Let’s find the rate of reaction at • t = 600s • Now, find the rate of reaction at t = 50s. • Determine the rate of reaction at t = 900s. • Compare the rate at t = 50s and • t = 900s.

  26. Stoichiometry • The reaction in the previous example is represented as: C4H9Cl(aq) + H2O(aq) C4H9OH(aq) + HCl(aq) • How is the rate of disappearance of a reactant related to the rate of appearance of a product?

  27. Stoichiometry C4H9Cl(aq) + H2O(aq) C4H9OH(aq) + HCl(aq) • For this reaction, • However, notice that all the stoichiometric relationships are 1:1

  28. Stoichiometry • What if the reaction were as follows? 2HI(g) H2(g) + I2(g) • Would the relationship between disappearance of reactants and appearance of products be the same?

  29. Stoichiometry 2HI(g) H2(g) + I2(g)

  30. Stoichiometry In general, for the reaction: aA + bB  cC + dD The rate is given by:

  31. Stoichiometry The reaction at which ozone (in the ozone layer of the atmosphere) disappears is expressed as: 2O3(g)  3O2(g) • How is the rate of disappearance of ozone related to the rate of appearance of oxygen? • If the rate at which O2appears is 6.0 x 10-5 M/s, what is the rate at which ozone disappears?

  32. Concentration • How does increasing the concentration of reactants affect rate?

  33. Concentration Consider the data for the following reaction: NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l)

  34. Concentration • The effect of concentration on rate of reaction can be expressed using a rate law: Rate = k[A]m[B]n where: k is the rate constant (depends on temperature) [A] and [B] are the concentrations of reactants A and B m and n are reaction orders

  35. Concentration • For the rate law: Rate = k[A]m[B]n the exponents m and n are called reaction orders. They describe how the concentration of the reactant affects rate. • 1: first order 2: second order 3: third order, and so on…

  36. Concentration • For the rate law: Rate = k[A]m[B]n the exponents m and n are called reaction orders. They describe how the concentration of the reactant affects rate. • 0: zeroth order 1: first order 2: second order 3: third order, and so on… • Reaction orders are determined experimentally.

  37. Concentration Rate = k[A]2[B] • What will happen if [A] is doubled and [B] is kept constant? • If [B] is doubled and [A] is held constant? • If [A] and [B] are doubled simultaneously?

  38. Concentration Consider the following data:

  39. Concentration Consider the following data:

  40. Temperature • How does temperature affect reaction rate?

  41. Temperature • If the temperature of a reaction is increased, reaction rate increases. • Why is that?

  42. Temperature • In 1888, a Swedish scientist (Svante Arrhenius) suggested that molecules must possess a certain minimum amount of energy in order to react. • Golf analogy

  43. Temperature • The minimum amount of energy required is called the activation energy. • Activation energy can also be determined experimentally and mathematically but we will not concern ourselves with that now. Energy Reaction pathway

  44. Catalysts • Catalysts increase reaction rate without being used up. • In terms of energy: Energy Uncatalyzed reaction Catalyzed reaction Reaction pathway

  45. Catalysts • Examples of catalysts include platinum catalytic converters and enzymes. Energy Uncatalyzed reaction Catalyzed reaction Reaction pathway