rate of reaction and chemical equilibrium. 2 collision theory molecules must collide to react...
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Rate of Reaction and Rate of Reaction and Chemical EquilibriumChemical Equilibrium
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Collision TheoryCollision Theory
Molecules must collide to reactMolecules must collide to react
Effective collisions lead to products being Effective collisions lead to products being formedformed
Ineffective collisions do not form productsIneffective collisions do not form products
Reactants will form products if they collide Reactants will form products if they collide with adequate kinetic energywith adequate kinetic energy
Reactants will bounce apart if they do not Reactants will bounce apart if they do not have adequate kinetic energyhave adequate kinetic energy
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A) Two BrNO molecules approach each A) Two BrNO molecules approach each other at high speedother at high speed
B) The collision occurs. B) The collision occurs.
C) The energy of the collision causes Br-N C) The energy of the collision causes Br-N bonds to break and Br-Br bonds to form.bonds to break and Br-Br bonds to form.
D) The products: one BrD) The products: one Br22 and two NO and two NO
molecules.molecules.
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Reaction progress of two BrNO molecules.Reaction progress of two BrNO molecules.
Activation Energy
Activation Energy: The minimum amount of energy particles must have in order to react
Activated complex:•Reactants that have attained the activation energy level.•Unstable with short lifetime ~ 10-13s•Sometimes called transition state
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Factors Affecting Reaction RateFactors Affecting Reaction Rate
TemperatureTemperature
ConcentrationConcentration
Particle SizeParticle Size
CatalystCatalyst
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Comparison of the activation energies for Comparison of the activation energies for an uncatalyzed reaction and for the same an uncatalyzed reaction and for the same
reaction with a catalyst present.reaction with a catalyst present.
The relationship between the rate of a reaction and the concentration of one reactant is determined experimentally.
A series of experiments reveals how the concentration of each reactant affects the reaction rate.
An equation that relates reaction rate and concentrations of reactants is called the rate law for the reaction.
Rate Laws for Reactions
Chapter 17 Section 2 Reaction Rate
It is applicable for a specific reaction at a given temperature.
The general form for the rate law is given by the following equation where A and B are reactants:
R = k[A]n[B]m
The rate law is applicable for a specific reaction at a given set of conditions and must be determined from experimental data.
The power to which a reactant concentration is raised is called the order in that reactant.
The value of n is said to be the order of the reaction with respect to [A], so the reaction is said to be “nth order in A.”
Rate Laws for Reactions, continuedUsing the Rate Law
Chapter 17 Section 2 Reaction Rate
The value of k usually increases as the temperature increases, but the relationship between reaction rate and concentration almost always remains unchanged.
An order of one for a reactant means that the reaction rate is directly proportional to the concentration of that reactant.
An order of two means that the reaction rate is directly proportional to the square of the reactant.
An order of zero means that the rate does not depend on the concentration of the reactant, as long as some of the reactant is present.
Rate Laws for Reactions, continuedUsing the Rate Law, continued
Chapter 17 Section 2 Reaction Rate
The sum of all of the reactant orders is called the order of the reaction, or overall order.
Rate Laws for Reactions, continuedUsing the Rate Law, continued
Chapter 17 Section 2 Reaction Rate
NO2(g) + CO(g) NO(g) + CO2(g)
R = k[NO2]2
• second order in NO2
• zero order in CO• second order overall• The orders in the rate law may or may not
match the coefficients in the balanced equation.
The specific rate constant (k) is the proportionality constant relating the rate of the reaction to the concentrations of reactants.
1. Once the reaction orders (powers) are known, the value of k must be determined from experimental data.
2. The value of k is for a specific reaction; k has a different value for other reactions, even at the same conditions.
Rate Laws for Reactions, continuedSpecific Rate Constant
Chapter 17 Section 2 Reaction Rate
3. The units of k depend on the overall order of the reaction.
4. The value of k does not change for different concentrations of reactants or products. So, the value of k for a reaction remains the same throughout the reaction and does not change with time.
Rate Laws for Reactions, continuedSpecific Rate Constant, continued
Chapter 17 Section 2 Reaction Rate
5. The value of k is for the reaction at a specific temperature; if we increase the temperature of the reaction, the value of k increases.
6. The value of k changes (becomes larger) if a catalyst is present.
Rate Laws for Reactions, continuedSpecific Rate Constant, continued
Chapter 17 Section 2 Reaction Rate
Sample Problem BSample Problem BThree experiments that have identical conditions were Three experiments that have identical conditions were
performed to measure the initial rate of the reaction performed to measure the initial rate of the reaction
2HI(2HI(gg) ) HH22((gg) + I) + I22((gg))
The results for the three experiments in which only the HI The results for the three experiments in which only the HI concentration was varied are as follows:concentration was varied are as follows:
Write the rate law for the reaction. Find the value and units Write the rate law for the reaction. Find the value and units of the specific rate constant.of the specific rate constant.
Experiment [HI] (M) Rate (M/s)
1 0.015 1.1 × 10−3
2 0.030 4.4 × 10−3
3 0.045 9.9 × 10−3
Chapter 17 Section 2 Reaction Rate
Sample Problem B SolutionSample Problem B SolutionRate Laws for Reactions, continued
2
1
[HI] 0.030 M2.0
[HI] 0.015 M
2
1
R
R
3
3
4.4 10 M/ s4.0
1.1 10 M/ s
Chapter 17 Section 2 Reaction Rate
The general rate law for this reaction is R = k[HI]n
Concentration ratio:
rate ratio:
rate law: R = k[HI]2
Sample Problem B Solution, Sample Problem B Solution, continuedcontinuedRate Laws for Reactions, continued
R =
k
3
2 2-1 11.1 10 M/ s
[HI] (0.015 M)4.9 M s
Chapter 17 Section 2 Reaction Rate
Sample Problem CSample Problem CThree experiments were performed to measure the initial rate Three experiments were performed to measure the initial rate
of the reactionof the reaction
A + B A + B CC
Conditions were identical in the three experiments, except that Conditions were identical in the three experiments, except that the concentrations of reactants varied. The results are as follows:the concentrations of reactants varied. The results are as follows:
Write the rate law for the reaction. Find the value and unitsWrite the rate law for the reaction. Find the value and unitsof the specific rate constant.of the specific rate constant.
Experiment [A] (M) [B] (M) Rate (M/s)
1 1.2 2.4 8.0 × 10–8
2 1.2 1.2 4.0 × 10−8
3 3.6 2.4 7.2 × 10−7
Chapter 17 Section 2 Reaction Rate
Sample Problem C SolutionSample Problem C Solution
• The general rate law for this reaction is The general rate law for this reaction is R = k[A]R = k[A]nn[B][B]mm..
• To find To find m, m, compare Experiments 1 and 2, which have compare Experiments 1 and 2, which have the same [A].the same [A].
Rate Laws for Reactions, continued
1
2
[B] 2.4 M2.0
[B] 1.2 M
1
2
R
R
8
8
8.0 10 M/ s2.0
4.0 10 M/ s
Chapter 17 Section 2 Reaction Rate
Concentration ratio:
rate ratio:
m is 1, and the reaction is first order in B
Sample Problem C Solution, Sample Problem C Solution, continuedcontinued
•To find To find n, n, compare Experiments 1 and 3, which compare Experiments 1 and 3, which have the same [B].have the same [B].
Rate Laws for Reactions, continued
3
1
[A] 3.6 M3.0
[A] 1.2 M
3
1
R
R
7
8
7.2 10 M/ s9.0
8.0 10 M/ s
Chapter 17 Section 2 Reaction Rate
Concentration ratio:
rate ratio:
n is 2, and the reaction is second order in A
The rate law is R k[A]2[B].
Sample Problem C Solution, Sample Problem C Solution, continuedcontinuedRate Laws for Reactions, continued
R =
k
–8 –2 –
8
21
2
8.0 10 M/ s
[A] [B] (1.22.3 10
M) (M
2 4 )s
. M
Chapter 17 Section 2 Reaction Rate
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Catalysts & InhibitorsCatalysts & InhibitorsCatalysts increase the rate of a chemical Catalysts increase the rate of a chemical reaction without being used up in the reactionreaction without being used up in the reaction– Permit the reaction to occur with lower activation Permit the reaction to occur with lower activation
energy than normalenergy than normal
PtPt
2H2H22(g) + O(g) + O22(g) (g) 2H 2H22O(l) O(l)
– Your body makes extensive use of catalysts or Your body makes extensive use of catalysts or enzymes to allow bodily functions to progress enzymes to allow bodily functions to progress rapidlyrapidly
Inhibitors are substances that interfere with Inhibitors are substances that interfere with the action of a catalystthe action of a catalyst– Poisons the catalystPoisons the catalyst
2222
The amount of the substance in the vapor The amount of the substance in the vapor state becomes constant and reaches state becomes constant and reaches equilibriumequilibrium
How would we reverse this process?How would we reverse this process?
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Reversible ReactionsReversible ReactionsA A reversible reactionreversible reaction is one in which the is one in which the conversion of reactants to products and the conversion of reactants to products and the conversion of products to reactants occur conversion of products to reactants occur simultaneously.simultaneously.When the rates of the forward and reverse When the rates of the forward and reverse reactions are equal, the reaction has reached reactions are equal, the reaction has reached a state of balance called a state of balance called chemical chemical equilibriumequilibrium..At chemical equilibrium, no net change At chemical equilibrium, no net change occurs in the actual amounts of the occurs in the actual amounts of the components of the system.components of the system.The relative concentrations of the reactants The relative concentrations of the reactants and products at equilibrium constitute the and products at equilibrium constitute the equilibrium positionequilibrium position of a reaction. of a reaction.
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Reversible ReactionsReversible Reactions
At equilibrium, all three types of molecules are present.
SO2 and O2 react to give
SO3
SO3 decomposes
to SO2 and O2
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Changes in ConcentrationChanges in Concentrationof Reactants and Productsof Reactants and Products
Initially SO2 Initially SO3 Present O2 Present
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Le Chatelier’s PrincipleLe Chatelier’s Principle18841884
When a system at chemical equilibrium is When a system at chemical equilibrium is disturbed, the system will adjust itself to disturbed, the system will adjust itself to minimize the disturbanceminimize the disturbance– Excellent guide for estimating how a chemical Excellent guide for estimating how a chemical
reaction will shift in response to changesreaction will shift in response to changes
Henri Le Chatelier
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Factors Affecting Equilibrium: Factors Affecting Equilibrium: Le ChLe Châtelier’s Principleâtelier’s Principle
ConcentrationConcentration
Rapid breathing during and after vigorous Rapid breathing during and after vigorous exercise helps reestablish the body’s correct exercise helps reestablish the body’s correct COCO22:H:H22COCO33 equilibrium, keeping the acid equilibrium, keeping the acid concentration in the blood within a safe range.concentration in the blood within a safe range.
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Factors Affecting Equilibrium: Factors Affecting Equilibrium: Le ChLe Châtelier’s Principleâtelier’s Principle
Temperature Temperature
Dinitrogen tetroxide is a colorless gas; nitrogen Dinitrogen tetroxide is a colorless gas; nitrogen dioxide is a brown gas. The flask on the left is in dioxide is a brown gas. The flask on the left is in a dish of hot water; the flask on the right is in a dish of hot water; the flask on the right is in ice.ice.
Warm Cool
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Factors Affecting Equilibrium: Factors Affecting Equilibrium: Le ChLe Châtelier’s Principleâtelier’s Principle
PressurePressure
Pressure affects a mixture of nitrogen, Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibriumhydrogen, and ammonia at equilibrium
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Equilibrium ConcentrationEquilibrium Concentration
aA + bB aA + bB cC + dD cC + dD
KKeqeq = [C] = [C]cc[D][D]dd
[A][A]aa[B][B]bb
KKeq eq > 1 products favored at equilibrium> 1 products favored at equilibrium
KKeq eq < 1 reactants favored at equilibrium< 1 reactants favored at equilibrium
Square bracketsindicate concentration or “molarity”
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The Solubility Product ConstantThe Solubility Product Constant
KKsp, sp, solubility product constant solubility product constant equals the product of the equals the product of the
concentrations of the ions, each concentrations of the ions, each raised to a power equal to the raised to a power equal to the coefficient of the ion in the coefficient of the ion in the dissociation equation. dissociation equation.
The smaller the numerical value The smaller the numerical value of the solubility product constant, of the solubility product constant, the lower the solubility of the the lower the solubility of the compound.compound.
AgCl (s) ↔ Ag+ (aq) + Cl- (aq)
Ksp = [Ag+]1 [Cl-]1
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The Solubility Product ConstantThe Solubility Product Constant
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The Solubility The Solubility Product ConstantProduct Constant
18.3
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System initially at equilibrium.System initially at equilibrium.CaCOCaCO33 CO CO22 + CaO + CaO
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a) A mixture of NHa) A mixture of NH33((gg), N), N22((gg), and H), and H22((gg) at equilibrium.) at equilibrium.
b) The volume is suddenly decreased.b) The volume is suddenly decreased.c) The new equilibrium position.c) The new equilibrium position.
N2 + 3H2 <--> 2NH3
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Le Chatelier’s Principle and TemperatureLe Chatelier’s Principle and Temperature
Exothermic ReactionExothermic Reaction
HH22 + I + I22 2HI + heat 2HI + heatHeating drives reaction to the leftHeating drives reaction to the left
Cooling drives reaction to the right Cooling drives reaction to the right
Endothermic ReactionEndothermic Reaction
heat + NHheat + NH44Cl Cl NH NH33 + HCl + HClHeating drives reaction to the rightHeating drives reaction to the right
Cooling drives reaction to the left Cooling drives reaction to the left
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Rules for assigning oxidation states and Rules for assigning oxidation states and catalyzed reactions.catalyzed reactions.
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Some Reaction are ReversibleSome Reaction are Reversible
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(a)(a) Initial equilibrium mixtureInitial equilibrium mixture(b) Addition of N(b) Addition of N22..
(c) New equilibrium position. (c) New equilibrium position.
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(a) Equal numbers of moles of H(a) Equal numbers of moles of H22O and CO are O and CO are
mixed in a closed container.mixed in a closed container.(b) The reaction begins to occur.(b) The reaction begins to occur.(c) The reaction continues, and more reactants are (c) The reaction continues, and more reactants are changed to products.changed to products.(d) No further changes are seen as time continues (d) No further changes are seen as time continues to pass.to pass.