Chapter 6: Reaction Rates

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25 Terms

1

Collision theory / requirements for effective collisions

chemical reactions can occur only if reactants collid with….

  • Favourable orientation/angle

  • Enough energy ( activation energy)

<p>chemical reactions can occur only if reactants collid with….</p><ul><li><p>Favourable orientation/angle </p></li><li><p>Enough energy ( activation energy)</p></li></ul><p></p>
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2

Activation Energy (Ea)

  • Minimum amount of collision energy required for a reaction to occur

  • Energy used to break the bonds in the reactants

  • Distance between the reactant and highest point on energy-time graph

<ul><li><p>Minimum amount of collision energy required for a reaction to occur</p></li><li><p>Energy used to break the bonds in the reactants</p></li><li><p>Distance between the reactant and highest point on energy-time graph </p></li></ul><p></p>
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3

What happens as the Ea increases? decreases?

reaction rate slows, reaction rate speeds up

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4

Activated complex

  • Short lived molecular configuration that exists at the top of the energy barrier that the reactants must overcome to become products

  • Reactants are partially broken, products are partially formed

  • High in energy and unstable due to bonds rearranging

    • Can go forward (products) or backwards (re form reactants)

<ul><li><p>Short lived molecular configuration that exists at the top of the energy barrier that the reactants must overcome to become products</p></li><li><p>Reactants are partially broken, products are partially formed</p></li><li><p>High in energy and unstable due to bonds rearranging</p><ul><li><p>Can go forward (products) or backwards (re form reactants)</p></li></ul></li></ul><p></p>
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5

Enthalpy change (ΔH)

  • The difference between the potential energy of the reactants and that of the products

  • Determined after product formed

  • Has no effect on the rate of reaction

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6

Forward and reverse reactions

  • Possible if chemical equation has this symbol: “⇌”

  • Mirror images of one another

  • Exothermic and Endothermic reverse

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7

Reaction Rate

  • Measure of how quickly or slowly reactants are consumed / products are formed

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8

Reaction rate is NOT constant. Explain.

  • Beginning of reaction is fastest as concentration of reactants is high, reactants consumed to form products quickly

  • Slows down as reactant concentration decreases

  • Eventually stops as no more reactant is left

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9

Factors that affect reaction rates

RECALL: Cool People Stay Till Chemistry Night

  • Concentration

  • Pressure (Increased Pressure)

  • Surface Area

  • Temperature

  • Catalyst

  • Nature of Reactant

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10

How does concentration affect reaction rates

  • Increases frequency of collisions

  • High concentrations increase rate

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11

How does pressure increase reaction rate

  • Only for gaseous systems

  • Increases frequency of collisions

  • Increased pressure increases rate

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12

How does surface area increase reaction rate

  • Increases frequency of collisions

  • Only in heterogenous systems (reactants in different shapes)

  • Increase in surface area increases rate

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13

How does temperature increase reaction rate

  • Increases frequency of collisions and energy of collisions

  • Increase of temperatures increases rate

    • Molecules collide more often with more energy due to increased KE

    • DOESNT CHANGE Ea, more particles are just able to overcome it.

<ul><li><p>Increases frequency of collisions and energy of collisions</p></li><li><p>Increase of temperatures increases rate</p><ul><li><p>Molecules collide more often with more energy due to increased KE</p></li><li><p>DOESNT CHANGE E<sub>a</sub>, more particles are just able to overcome it.</p></li></ul></li></ul><p></p>
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14

Maxwell Boltzmann distribution of molecules

Gaussian/Bell curve

Temperature

  • Area under curve represents all particles in reaction

  • Shaded region represents all particles that pass that have high enough energies to react

  • As temp increases, shaded region becomes larger

  • Y axis: # of particles, X axis: energy

<ul><li><p>Area under curve represents all particles in reaction</p></li><li><p>Shaded region represents all particles that pass that have high enough energies to react</p></li><li><p>As temp increases, shaded region becomes larger</p></li><li><p>Y axis: # of particles, X axis: energy</p></li></ul><p></p>
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15

How does catalyst increase rate of reaction

  • Provides an alternative pathway with a lower activation barrier (Ea) which increases reaction rate

  • Not consumed in a reaction - Regenerated at the end of the reaction and can be used again

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16

Maxwell Boltzmann distribution of molecules

Gaussian/Bell curve

Catalyst

  • Catalyst doesn’t change the curve

  • Ea is lowered and moved to the left

<ul><li><p>Catalyst doesn’t change the curve</p></li><li><p>E<sub>a</sub> is lowered and moved to the left</p></li></ul><p></p>
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17

Potential energy diagrams with catalyst

  • Catalyst will provide alternative pathway that has a smaller Ea

  • X: progress of reaction, Y: energy

  • Enthalpy is unchanged

<ul><li><p>Catalyst will provide alternative pathway that has a smaller E<sub>a</sub></p></li><li><p>X: progress of reaction, Y: energy</p></li><li><p>Enthalpy is unchanged</p><p></p></li></ul><p></p>
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18

How does the nature of reactant affect the rate of reaction?

  • similar elements react similarly but at different rates (depending on reactivity series of metal)

    • COLLISION ENERGY

  • Reactions in gas or aqueous system react more quickly than solids

    • Due to higher KE

    • COLLISION ENERGY AND FREQUENCY

  • Small simple ions and molecules react more quickly than larger molecules

    • Small molecules have less friction due to less surface area —> move more quickly —> increases rate

    • COLLISION ENERGY AND FREQUENCY

  • charged particles in solution react more quickly than neutral molecules

    • Neutral molecules collide by chance, whereas charged particles gravitate towards eachother due to electrostatic attraction —> higher change of collision —> increases rate’

    • COLLISION FREQUENCY

  • Weak bonds are easier to break (small Ea), strong bonds are harder (higher Ea)

    • Bonds in reactants must be broken first before product formation

    • Bond dissociation energies

    • COLLISION ENERGY

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19

Reaction mechanisms

  • The series of steps that take place as a reaction proceeds from reactants to products

  • Refers to all the steps together

  • Mechanisms are proposed as there is no way to observe reactions at a molecular level

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20

Elementary steps

A single step in a reaction mechanism

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21

Reaction intermediate

  • Compound produced in one elementary step and used up in subsequent elementary step

  • Does not show up in overall equation

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22

Rate Determining Step (RDS)

  • Elementary step with slowest rate

    • Highest Ea

  • Determines rate of overall chemical reaction

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23

Overall activation energy

  • Distance between the start to the highest point on the graph on reaction mechanism

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24

Rate law

  • calculation of reaction rate as a function of reactant concentration

  • depends on the RDS of a reaction mechanism

  • Must be determined experimentially

  • r = k[A]m[B]n

    • k is rate constant (depends on temp)

    • m/n number of molecules of A/B collide

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25

Methods of measuring reaction rates

  • Volume or pressure over time

    • if reaction involves gases

    • gas syringe(V), gas sensor (P)

  • Change in temperature over time

    • endo/exothermic

    • calorimeter

  • Change in absorbance over time (colour)

    • spectrophotometer

  • Reactions involving ions?:

    • Conductivity over time (ions involved and electricity produced)

    • pH over time (if H or OH ions involved)

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