Ch 17 reaction rates
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Ch 17: Reaction Rates. We define a rate as a change in a quantity divided by the change in time: rate = ∆quantity ∆time Examples of types of rates: Speed of a car Points scored in a game Hot dogs eaten in 5 minutes Pages printed by a printer in 1 minute.

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Ch 17: Reaction Rates

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Ch 17: Reaction Rates

  • We define a rate as a change in a quantity divided by the change in time:

    rate = ∆quantity

    ∆time

    Examples of types of rates:

    • Speed of a car

    • Points scored in a game

    • Hot dogs eaten in 5 minutes

    • Pages printed by a printer in 1 minute


Reaction Rates

  • Chemicals reactions can also be measured in how they change over time.

  • The reaction rate of a chemical reaction is the change in concentration (M, molarity) of a reactant or product per unit time (s, second).

  • Given the chemical equation:

    CO(g) + NO2(g)  CO2(g) + NO(g)

    The average reaction rate of the formation of NO(g) over a period of time t1 and t2 would be

    [NO]t2 - [NO]t1

    avg reaction rate = t2 - t1

    if t1 = 0.0s and t2 = 2.0s and initially there is no NO

    and at 2.0s 0.010M NO forms,then:


Collision Theory

  • Reactions occur when molecules collide together

  • The collision theory says that:

    • atoms, ions, and molecules must collide in order to react.

    • Reacting substances must collide with the correct orientation

    • Reacting substances must collide with sufficient energy to form the activated complex

  • The activated complex (or transition state) is a temporary, unstable arrangement of atoms that may form products or may break apart to reform the reactants.


Activation Energy

  • In order for a reaction to occur, the reacting atoms, ions, and molecules must have sufficient energy when they collide in the right orientation.

  • The minimum amount of energy required for the activated complex to form and for the reaction to take places is called the activation energy (Ea).

  • Although important in determining if a reaction will occur, this tells us little about the actual speed/rate of reaction.


Factors Affecting Reaction Rates

  • The compounds themselves

  • Concentration

    • Reactions speed up when the concentrations of reactants are increased. (Increasing the concentration increases the number of particles available to collide)

  • Surface Area

    • Reactions can occur faster when more area is exposed to take part in a reaction. (Increasing surface area increased the number of particles available to collide)

  • Temperature

    • Reactant particles require activation energy in order to react. Raising the temperature increases the average number of particles with sufficient activation energy, increasing the rate of reaction.


Catalysts and Inhibitors

  • A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed in the reaction.

  • Essentially, a catalyst helps lower the activation energy of the reaction. This means that more collisions will then have sufficient energy to react.

  • An inhibitor on the other hand is a substance that slows down reaction rates or prevents reactions from occurring at all.


Reversible Reactions (Ch18)

  • Given a chemical equation:

    N2 + 3H2 2NH3

    We say that the reaction goes to completion as all the reactants react and turn into products.

  • But, sometimes, the reverse reaction can also occur:

    2NH3  N2 + 3H2

  • A reversible reaction is one that can occur in both the forward and reverse directions.

  • Chemical equations that are reversible use a double arrow instead of the standard single arrow to show that it is reversible:

    N2 + 3H2 2NH3


Chemical Equilibrium (Ch 18)

  • In the beginning of the reaction, the forward and reverse reactions occur at different rates.

  • Eventually, the rate of product formation in the forward reaction will be balanced by the decomposition of product in the reverse reaction. This is called chemical equilibrium.

  • Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates

    Rate forward reaction= Rate reverse reaction

  • Equilibrium is a dynamic process. The reaction does not “stop” even though it seems like nothing is changing.


Changes in Equilibrium (Ch 18)

  • Equilibrium of chemical reactions can “shift” in one direction or another.

  • Le Châtelier’s Principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

    • Increase reactants – forward reaction favored, making more product. The reaction “shifts to the right”

    • Increase products – reverse reaction favored, making more reactants. The reaction “shifts to the left”

    • Decrease reactants – reverse reaction favored, making more reactants. The reaction “shifts to the left”

    • Decrease producs – forward reaction favored, making more products. The reaction “shifts to the right”

    • Increase temperature – Endothermic reaction is favored. Reaction shifts towards the endothermic process

    • Add catalyst – no effect


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