reaction rates chm 1: chapter 18 chm hon: chapter 17 & 18

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Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

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Page 1: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction Rates

CHM 1: Chapter 18

CHM Hon: Chapter 17 & 18

Page 2: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

PART 1RATES OF REACTION

Page 3: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Reactions occur at different rates

• Speed = Distance in a given amount of time– Shelia runs 10 meters in 4 seconds

• 10 m / 4s • 5 m / 2s • 2.5 m/s

Page 4: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Rate = a measure of the speed of any change that occurs within an interval of time.– Rate of a Chemical Change or reaction =

amount of reactant changing per unit time

Page 5: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Collision Theory: atoms, ions and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy– Kinetic energy = the energy possessed by a

body because of its motion

Page 6: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

Particles lacking the necessary kinetic energy to react bounce apart unchanged with they collide

Page 7: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• If enough energy is applied to a molecule, the bonds holding the molecule together can break!

• Substances supplied with enough energy decompose to simpler substance or reorganize themselves into new substances.

Page 8: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory• Activation Energy = the minimum energy

that colliding particles must have in order to react

Page 9: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• When 2 reactant particles with enough activation energy collide, an activated complex (new entity) is formed

Page 10: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18
Page 11: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

Page 12: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory• Activated Complex = an unstable

arrangement of atoms that forms momentarily at the peak of the activation-energy barrier (Brief existence!)– Forms only if:

• Colliding particles have sufficient energy • Atoms are oriented properly

– Also called… Transition state• Reformation of the reactants or formation of

products. Both outcomes equally likely!

Page 13: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Pop Quiz!

Page 14: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Particle size– Surface area of reactant affects the

reaction rate• Smaller particle size larger the surface area• High surface area

– High amount of reactant exposed– High collision frequency– High reaction rate

Page 15: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Increasing surface area– Solid: dissolve into solution, grind into

powder

Page 16: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

Page 17: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Catalysts = a substance that increases the rate of reaction without being used up during the reaction– Permit reactions to proceed along a lower

energy path– More reactants have the energy to form

products within a given time

Page 18: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction with catalyst vs. no catalyst

Page 19: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Catalysts– Important to the human body– Enzymes = catalysts– Without enzymes, digestion

would take years!!

Page 20: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory• Inhibitor = a substance that interferes

with the action of a catalyst– Reduces the amount of functional catalyst

available– Reactions slow or stop

Page 21: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Collision Theory

• Heterogeneous catalyst = the catalyst exists in a different physical state that the reaction it catalyzes– Ex. Catalytic converter

• Homogeneous catalyst = the catalyst exists in the same physical state as the reaction it catalyzes– Ex. Both the enzyme and reaction are in

aqueous solution

Page 22: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

PART 2

REVERSIBLE REACTIONS

&

EQUILIBRIUM

Page 23: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reversible reactions

• Reversible reaction = a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously

Page 24: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reversible reactions• Chemical Equilibrium = a state of

balance; when rates of the forward and reverse reactions are equal

Page 25: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reversible reactions

• Chemical Equilibrium– No net change occurs in the amounts of the

components of the system– Dynamic state: both forward and reverse

reactions continue

Page 26: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reversible reactions

• Equilibrium position = the relative concentrations of the reactants and products at equilibrium– Indicates whether the reactants or

products are favored• If A reacts to give B, eq. mixture contains more

B, then…

Page 27: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reversible reactions

Page 28: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reversible reactions

• Catalysts– Speeds up both forward and reverse

reactions equally

Page 29: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

• When the equilibrium of a system is disturbed, the system makes ajustments to restore equilibrium– Shift in Equilibrium position

Page 30: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s PrincipleLe Chatlier’s Principle

If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

Stresses that upset the equilibriumchanges in the concentration of reactants or

productschanges in temperaturechanges in pressure

Page 31: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

• Concentration– System adjusts to minimize the effects of the

change– Adding product to a reaction pushes in the

direction of reactants and vice a versa

Page 32: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

• Temperature– Increasing the temperature causes the

equilibrium position of a reaction to shift in the direction that absorbs heat

• Heat is considered a product!!

Page 33: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle• Pressure

– Change in pressure only affects gaseous equilibria that have an unequal number of moles of reactants and products

Page 34: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

Page 35: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

Page 36: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

Page 37: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

Page 38: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Le Chatelier’s Principle

Page 39: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

• Equilibrium constant = (Keq) the ratio of product to reactant concentrations at equilibrium– Each concentration raised to a power

equal to the # of moles of that substance in the balanced chemical equation

Page 40: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

Page 41: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

• Brackets indicate concentration (mol/L)

• Value of Keq depends on the temperature

• The size of Keq shows whether products or reactants are favored at equilibrium

Page 42: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

Page 43: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

Keq=12

Page 44: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

Page 45: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Equilibrium Constants

Page 46: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

PART 3

Solubility Equilibrium

Page 47: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Solubility Product Constant

• Ionic compounds have different solubilities

• Most salts are somewhat soluble– Most salts of the alkali metals are soluble in

water (slightly / sparingly soluble)– Compounds that contain phosphate, sulfide,

sulfite or carbonate ions are generally insoluble

Page 48: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18
Page 49: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Solubility Product Constant

• When salts completely dissolve it is a one way reaction…

• When salts do no completely dissolve it becomes a reversible reaction and an equilibrium is established…

Page 50: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Solubility Product Constant• Solubility product Constant

Ksp = an equilibrium constant for the dissolving of a sparingly soluble ionic compound in water

The product of the concentrations of the ions each raised to the power of the coefficient of the ion in the dissociation equation

Page 51: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18
Page 52: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Solubility Product Constant

Page 53: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Solubility Product Constant

Page 54: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

The Common Ion Effect

• Common Ion = an ion that is found in both salts in a solution

• Common Ion effect = the lowering of the solubility of an ionic compound as a result of the addition of a common ion

Page 55: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

The Common Ion Effect

Page 56: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

The Common Ion Effect

Page 57: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

The Common Ion Effect

Page 58: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

The Common Ion Effect

• The Ksp can be used to predict whether a precipitate will form when solutions are mixed.

• If the product of the concentrations of two ions in the mixture is greater than Ksp of the compound formed from the ions, a precipitate will form.

Page 59: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

The Common Ion Effect

Page 60: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Part 4

Entropy & Free Energy

Page 61: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Free Energy and spontaneous reactions

• Many chemical and physical processes release energy that can be used to bring about other changes

• Free energy = energy that is available to do work– Not necessarily used efficiently!

• Example: internal-combustion engine (cars) only 30 % efficient No process 100% efficient

Page 62: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Free Energy and spontaneous reactions

• Spontaneous reaction = occurs naturally and favors the formation of products at the specified conditions.

• Nonspontaneous reaction = a reaction that does not favor the formation of products at the specified conditions

Page 63: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Free Energy and spontaneous reactions

• Spontaneous reactions – Produce substantial amounts

of products at equilibrium and

release free energy

• Nonspontaneous reactions– Do not give substantial amounts of products

at equilibrium

Page 64: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Free Energy and spontaneous reactions

Spontaneous or nonspontaneous?

SPONTANEOUS

Page 65: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Free Energy and spontaneous reactions

• Note: – Spontaneous and nonspontaneous do not

refer to rate of reaction!• May take 100’s of years, but still my be considered

spontaneous if products are favored!• May speed up reaction by introducing energy as

heat (increase temperature)

Page 66: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Free Energy and spontaneous reactions

• Note:– Some nonspontaneous reactions may be

made to occur if it is coupled to a spontaneous reaction (one that releases free energy)

• Common in Biological processes taken place in living organisms

Page 67: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

• Enthalpy = the heat content of a system at constant pressure– Heat changes accompany most chemical and

physical processes

• Exothermic = release of heat

• Endothermic = absorption of heat

Page 68: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

• The Combustion of carbon is exothermic and spontaneous– Heat is released during the reaction

393.5 kj / mole of carbon burned

Page 69: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

• Sometimes a reaction may be spontaneous but absorb heat– As this reaction turns from solid to liquid

(melts) 1 mol of ice at 25 C absorbs 6.0 kj/mol of heat from its surroundings

Page 70: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy• Entropy = a measure of the disorder of a

system• Law of Disorder: (entropy change) the

natural tendency is for systems to move in the direction of maximum disorder or randomness– An increase in entropy favors the

spontaneous chemical reaction; a decrease favors the nonspontaneous reaction

Page 71: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

Page 72: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

Page 73: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

Page 74: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Entropy

Page 75: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Enthalpy, Entropy & Free Energy

• In every chemical reaction, heat is either released or absorbed and entropy or randomness either increases or decreases

• The size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous (whether it favors products and releases free energy)

Page 76: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Enthalpy, Entropy & Free Energy

~ Entropy and enthalpy changes affect the spontaneity of chemical reactions!

~ Either of the two (not both) can be unfavorable for a spontaneous process

Page 77: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18
Page 78: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18
Page 79: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Gibbs Free-Energy

• Gibbs free-energy change = the maximum amount of energy that can be coupled to another process to do useful work.

Temperature in K

Change in entropy

Change in enthalpy

Page 80: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Gibbs Free-Energy

• All spontaneous processes release free energy

• The numerical value of is negative in spontaneous processes because the system loses free energy!

Page 81: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Gibbs Free-Energy

• Solid calcium carbonate decomposes to give calcium oxide and carbon dioxide– Entropy increases: one of the products formed from

the (s) reactant is a (g)– Endothermic reaction– Nonspontaneous reaction

Page 82: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

PART 5

Progress of Chemical Reactions

Page 83: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• Rate of reaction depends partly on the concentrations of the reactants.

• The rate at which A yields B is expressed as the change in A over time ( ).

Page 84: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• A is the reactant– Reactant is decreasing– The concentration of A is smaller at a later

time than initially, therefore will always be negative

Page 85: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• Rate law = an expression for the rate of a reacion in terms of the concentration of reactants

• Specific rate constant = (k) a proportionality constant relating the concentrations of reactants to the rate of the reaction

Page 86: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• The value of the specific rate constant, k, is large if the products form quickly– Less time!

• The value is small if the products form slowly– More time!

Page 87: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws• Order of a reaction = the power to which

the concentration of a reactant must be raised to give the experimentally observed relationship between concentration and rate

• First-order reaction = one in which the reaction rate is directly proportional to the concentration of only one reactant.– Reaction rate is directly proportional to the

concentration of only one reactant

Page 88: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• First-order reaction – Example: Conversion of A to B in a one step

reaction

Page 89: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• When two substances react to give products:– Example, double-replacement reaction

Page 90: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• Note:

–When each of the exponents a and b in the rate law equals 1, the reaction is said to be… • First-order in A• First-order in B• Second-order over-all

Page 91: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws• Example problem: Finding the order of a

reaction from experimental data

The rate law for the one-step reaction aA B is of the form: Rate=k[A]a. From the data in the following table, find the order of the reaction with respect to A and the overall order of the reaction.

Page 92: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

Page 93: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Rate Laws

• Practice problem:– Show that the unit of k for a first-order

reaction is a reciprocal unit of time, such as a reciprocal second (s -1)

– Answer:

Page 94: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction Mechanism

• Elementary reaction = a reaction in which reactants are converted to products in a single step– Has only one activation-energy peak between

products and reactants and therefore only one activated complex

– Most chemical reactions consist of a number of elementary reactions!

Page 95: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction Mechanism

• Reaction mechanism = The series of elementary reactions or steps that take place during the course of a complex reaction

Page 96: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction Mechanism

• The peaks correspond to the energies of the activated complexes. Each valley corresponds to the energy of an intermediate

Reaction progress curve

Page 97: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction Mechanism• Intermediates do not appear in the overall

chemical equation for a reaction.

• Example: The reaction mechanism for the decomposition of nitrous oxide

Page 98: Reaction Rates CHM 1: Chapter 18 CHM Hon: Chapter 17 & 18

Reaction Mechanism

• IMPORTANT: The overall chemical equation for a complex reaction fives no information about the reaction mechanism. – Reaction mechanisms must be determined

experimentally.