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Chemical kinetics

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

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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

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