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Chemical Kinetics The speed with which chemical reactions occur depends on external conditions The area of chemistry concerned with the speed at which.

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Presentation on theme: "Chemical Kinetics The speed with which chemical reactions occur depends on external conditions The area of chemistry concerned with the speed at which."— Presentation transcript:

1 Chemical Kinetics The speed with which chemical reactions occur depends on external conditions The area of chemistry concerned with the speed at which reactions occur is called chemical kinetics

2 Reaction Rates The speed, or rate of any event is measured by the amount of change that occurs in a given interval of time The rate of a chemical reaction (reaction rate) is the change in concentration of reactants and products in a certain amount of time

3 Reaction Rates The rate of a reaction must be determined experimentally Rate Equation Average rate of rxn = Δ[reactant or product] Δtime

4 Reaction Mechanisms A series of steps that leads from reactants to products is called a reaction mechanism Some reactions have very simple mechanisms, while others are multi-step & complex

5 Reaction Mechanisms When you create a chemical reaction, you observe only the overall, or net, chemical change For some reactions, chemists have devised experiments that reveal the sequence of steps in each mechanism

6 Reaction Mechanisms Each individual reaction step (elementary step) is usually a simple process The elementary steps in a multi- step reaction mechanism must always add to give the chemical equation of the overall process

7 Reaction Mechanisms Substances that are produced in one step of a reaction but consumed in a later step are called intermediate products (intermediates) Multi-step reaction mechanisms always involve one or more intermediate products

8 Reaction Mechanisms Reaction mechanisms explain why rates of chemical reactions must be found experimentally (chemical equations do not indicate the elementary steps) Each elementary step proceeds at its own rate

9 Reaction Mechanisms The rate of the overall reaction is limited by the slowest elementary step, called the rate-determining step

10 Reaction Mechanisms Example: Take the following rxn: 2NO + F 2  2NOF It proceeds as follows: Step 1: NO + F 2  NOF 2 Step 2: NOF 2 + NO  2NOF

11 Reaction Mechanisms The intermediate product (NOF 2 ) was used up in the second step and is not seen in the net reaction

12 Rate Laws The effect of concentration on the rate of reaction is described using a rate law The rate law can be determined by keeping all but one reactant concentration constant while measuring reaction rate for various concentrations of that reactant

13 Rate Laws The rate equation is Rate = k[A] x [B] y [A] & [B] represent molar concentrations of reactants A & B x & y are the powers of the concentrations of the reactants (always determined experimentally) k, the proportionality constant, is called a rate constant (has a fixed value for a reaction at a particular temperature

14 Rate Laws Example: NO 2 + O 3  NO 3 + O 2 Experiments have shown the rate law to be: Rate = k[NO 2 ][O 3 ]

15 Rate Laws Suppose the initial concentration of each reactant is 1.00 M If the concentration of either reactant is increased to 2.00 M, the rate increases by a factor of 2

16 Rate Laws The overall order of the reaction is equal to the sum of the exponents for the individual reactants

17 Rate Laws Rate = k[A] a [B] b If each of the exponents equals 1, then the reaction is said to be first-order in A, first-order in B, and second-order overall

18 Collision Theory On a molecular level, for two substances to combine, they must hit each other A theoretical model (collision theory) states that molecules must collide in order to react

19 Collision Theory An effective collision leads to the formation of products An ineffective collision does not lead to the formation of products

20 Collision Theory If every collision lead to a reaction, all chemical reactions would have rates hundreds of times faster than they actually are

21 Collision Theory Two factors determine if a collision is effective: Orientation Must be in a position that allows bonds to break and atoms to rearrange Energy of Colliding Molecules Sufficient energy to break bonds

22 Energy in Reactions Remember, there are two kinds of energy (potential and kinetic) Also, energy is conserved in a system

23 Energy in Reactions Energy required to break bonds comes from kinetic energy The amount of kinetic energy of a particle is dependent on its mass and velocity

24 Activation Energy Particles must possess a certain amount of kinetic energy to react Energy diagrams show the changes in energy that occur during a chemical reaction

25 Activation Energy

26 The difference between the energy at the peak of the reaction and the energy of the reactants is the activation energy (energy needed to start the reaction

27 Activated Complex There is a brief transition state where there is a complex that is neither reactant nor product called the activated complex The activation energy is the energy required to achieve the transition state and form the activated complex

28 Factors Affecting Reaction Rates There are five general factors that affect the rate of reactions: Nature of Reactants The rate of a reaction depends on the particular reactants and the complexity of the bonds to be broken

29 Factors Affecting Reaction Rates Single bonds will break rapidly, but multiple bonds will take more time The state of reactant can also affect the reaction rate

30 Factors Affecting Reaction Rates Reactions where the reactants are in different states are heterogeneous reactions Reactions where the reactants are in the same state are homogeneous reactions

31 Factors Affecting Reaction Rates Temperature The higher the temperature, the faster molecules will move, which makes collisions more likely

32 Factors Affecting Reaction Rates Concentration The rate of reaction is defined as the change in concentration over time As you increase the concentration of a molecule, you increase the likelihood of collisions

33 Factors Affecting Reaction Rates Surface Area The larger the surface area of a reactant, the greater the number of particles that are exposed for reaction, which increases the frequency of collisions

34 Factors Affecting Reaction Rates Catalysts Catalysts are substances that increase the rate of reaction without being used up They work by lowering the activation energy of the reaction. With lower E a, more collisions will have sufficient energy to pass the energy barrier


36 Factors Affecting Reaction Rates It is not always desirable to speed up reactions. Inhibitors are substances that slow the rate of reaction They work by removing catalysts or by causing the reaction to take other routes that produce side products and do not lead to the regular products

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