Reaction Rates and Reaction Rates and EquilibriumEquilibrium

Chapter 19Chapter 19C.SmithC.Smith

I. Rates of ReactionI. Rates of ReactionA. Collision TheoryA. Collision Theory• 1. Rate is defined as the measure of speed 1. Rate is defined as the measure of speed

change that occurs over a period of time.change that occurs over a period of time.• 2. In chemistry, rates of chemical change usually 2. In chemistry, rates of chemical change usually

are expressed as the amount of reactant are expressed as the amount of reactant changing per unit time.changing per unit time.

• 3. Visible changes caused by chemical reactions 3. Visible changes caused by chemical reactions are related to changes in the properties of are related to changes in the properties of individual atoms, ions, and molecules through a individual atoms, ions, and molecules through a model called collision theory.model called collision theory.

• 4. Atoms, ions, and molecules can react to form 4. Atoms, ions, and molecules can react to form products when they collide provided that the products when they collide provided that the particles have enough kinetic theory.particles have enough kinetic theory.

I. Rates of ReactionI. Rates of ReactionA. Collision TheoryA. Collision Theory

• 5. The minimum amount of energy that particles must 5. The minimum amount of energy that particles must have in order to react is called activation energy.have in order to react is called activation energy.

• 6. Activation energy is a barrier that reactants must 6. Activation energy is a barrier that reactants must cross to be converted into productscross to be converted into products• a. During a reaction, unstable particles that are neither a. During a reaction, unstable particles that are neither

products or reactants are temporarily formed.products or reactants are temporarily formed.• b. The particle is called an activated complex and exist for a b. The particle is called an activated complex and exist for a

very short time. (approximately 10very short time. (approximately 10-13-13 sec.) sec.)• c. This period is called the transition state in where the c. This period is called the transition state in where the

activated complex can either reform reactants or form activated complex can either reform reactants or form products. products.

I. Rates of ReactionI. Rates of ReactionA. Collision TheoryA. Collision Theory• 7. Collision theory explains why some naturally 7. Collision theory explains why some naturally

occurring reactions are immeasurably slow at occurring reactions are immeasurably slow at room temperature and essentially at a rate of room temperature and essentially at a rate of zero.zero.

• 8. If the activation energy of the products is lower 8. If the activation energy of the products is lower than the energy of the reactants, the reaction is than the energy of the reactants, the reaction is exothermic.exothermic.

• 9. If the activation energy of the products is 9. If the activation energy of the products is higher than the energy of the reactants, the higher than the energy of the reactants, the reaction is endothermic.reaction is endothermic.

I. Rates of ReactionI. Rates of ReactionA. Collision TheoryA. Collision Theory• Addison-Wesley Chemistry p.535

I. Rates of ReactionI. Rates of ReactionB. Factors Affecting Reaction RatesB. Factors Affecting Reaction Rates

• 1. Temperature1. Temperaturea. At higher temperatures, molecules move a. At higher temperatures, molecules move faster and more chaotic and has an increase faster and more chaotic and has an increase in kinetic energy.in kinetic energy.b. The effect increases the number of b. The effect increases the number of collisions between molecules and the more collisions between molecules and the more colliding particles are energetic enough to slip colliding particles are energetic enough to slip over the activation energy barrier.over the activation energy barrier.

I. Rates of ReactionI. Rates of ReactionB. Factors Affecting Reaction RatesB. Factors Affecting Reaction Rates

• 2. Concentration2. Concentrationa. Increasing the number of particles increase the a. Increasing the number of particles increase the number of collisions between particles.number of collisions between particles.b. Increasing collisions between particles b. Increasing collisions between particles increases the frequency of particles that become increases the frequency of particles that become activated complexes and possible products.activated complexes and possible products.c. Cramming more particles into a fixed volume c. Cramming more particles into a fixed volume increases the concentration of reactants, the increases the concentration of reactants, the collision frequency, and therefore the reaction rate.collision frequency, and therefore the reaction rate.

I. Rates of ReactionI. Rates of ReactionB. Factors Affecting Reaction RatesB. Factors Affecting Reaction Rates• 3. Particle Size3. Particle Size

a. The smaller the particles size, the larger is the a. The smaller the particles size, the larger is the surface area for given mass of particles.surface area for given mass of particles.b. An increase in surface area increases the amount b. An increase in surface area increases the amount of the reactant exposed for reaction, which further of the reactant exposed for reaction, which further increases the collision frequency and the reaction increases the collision frequency and the reaction rate.rate.c. One way to increase the surface area of solid c. One way to increase the surface area of solid reactants is to dissolve them.reactants is to dissolve them.d Another way to increase the surface area is to grind d Another way to increase the surface area is to grind solids into fine powder. (explosion hazard)solids into fine powder. (explosion hazard)

I. Rates of ReactionI. Rates of ReactionB. Factors Affecting Reaction RatesB. Factors Affecting Reaction Rates

• 4. Catalysts4. Catalystsa. A catalyst is a substance that increases the rate of a a. A catalyst is a substance that increases the rate of a reaction without being involved in or used up in the reaction without being involved in or used up in the reaction.reaction.b. Catalysts lower the activation energy of a reaction b. Catalysts lower the activation energy of a reaction which allows more particles to form products.which allows more particles to form products.

• 5. Inhibitors5. Inhibitorsa. An inhibitor interferes with the action of a catalyst.a. An inhibitor interferes with the action of a catalyst.b. An inhibitor “poisons” a catalyst and can slow and b. An inhibitor “poisons” a catalyst and can slow and even stop a reaction.even stop a reaction.

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumA. Reversible ReactionsA. Reversible Reactions• 1. Reactions sometimes do not go to completion.1. Reactions sometimes do not go to completion.• 2. Some reactions are reversible.2. Some reactions are reversible.• 3. In reversible reactions, the formation of products 3. In reversible reactions, the formation of products

occurs along with the decomposition of products and occurs along with the decomposition of products and reforming of reactants.reforming of reactants.

• 4. This is represented by a double arrow.4. This is represented by a double arrow.• 5. When the forward and reverse reactions occur at 5. When the forward and reverse reactions occur at

the the same ratesame rate in a reaction, a chemical equilibrium is in a reaction, a chemical equilibrium is reached.reached.

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumA. Reversible ReactionsA. Reversible Reactions• 6. At a chemical equilibrium, there is no net change in the 6. At a chemical equilibrium, there is no net change in the

actual amounts of the components of the systems.actual amounts of the components of the systems.• 7. The equilibrium position of a reaction is given by the 7. The equilibrium position of a reaction is given by the

relative concentration of the system’s components.relative concentration of the system’s components.• 8. The equilibrium position indicates whether the 8. The equilibrium position indicates whether the

components on the left or right side of a reversible components on the left or right side of a reversible reaction are at a higher concentration.reaction are at a higher concentration.a. If the higher concentration is on the right side of the a. If the higher concentration is on the right side of the reaction, it is a forward reaction.reaction, it is a forward reaction.b. If the higher concentration is on the left side of the b. If the higher concentration is on the left side of the reaction, it is a reverse reaction.reaction, it is a reverse reaction.

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumA. Reversible ReactionsA. Reversible Reactions

• 9. Example:9. Example:

2SO2SO22 + O + O22 2SO 2SO33Forward reaction if more SOForward reaction if more SO33 present present

Reverse reaction if more OReverse reaction if more O22 or SO or SO22

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumB. Factors Affecting EquilibriumB. Factors Affecting Equilibrium

• 1. A delicate balance exist in a system at 1. A delicate balance exist in a system at equilibrium.equilibrium.

• 2. Any change in the system will cause it to 2. Any change in the system will cause it to shiftshift to restore equilibrium. to restore equilibrium.

• 3. Henri Le Châtelier studied shifts in 3. Henri Le Châtelier studied shifts in equilibrium and concluded the following equilibrium and concluded the following principle: If stress is applied to a system in principle: If stress is applied to a system in dynamic equilibrium, the system changes dynamic equilibrium, the system changes to relieve the stress. to relieve the stress.

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumB. Factors Affecting EquilibriumB. Factors Affecting Equilibrium

• 4. Stresses that upset equilibrium can be 4. Stresses that upset equilibrium can be concentration of reactants or products, changes concentration of reactants or products, changes in pressure, and changes in temperature.in pressure, and changes in temperature.

• 5. Changes in concentration5. Changes in concentrationa. If products are removed, the reaction will a. If products are removed, the reaction will shift to replace them and removal of reactants shift to replace them and removal of reactants will cause a shift to replace them.will cause a shift to replace them.b. If products are added, the reaction will shift b. If products are added, the reaction will shift to form reactants and addition of reactants will to form reactants and addition of reactants will cause a shift to form products.cause a shift to form products.

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumB. Factors Affecting EquilibriumB. Factors Affecting Equilibrium

• 6. Affects of Temperature6. Affects of Temperaturea. Increasing temperature will cause a shift in the direction a. Increasing temperature will cause a shift in the direction that absorbs heat.that absorbs heat.b. Decreasing temperature will cause a shift in the direction b. Decreasing temperature will cause a shift in the direction that releases heat.that releases heat.

• 7. Affects of Pressure - Changes in pressure only affect 7. Affects of Pressure - Changes in pressure only affect equilibrium that have an unequal number of moles of gaseous equilibrium that have an unequal number of moles of gaseous reactants and products.reactants and products.a. Increase in pressure will shift to side of the reaction with a. Increase in pressure will shift to side of the reaction with the least number of moles.the least number of moles.b. Decrease in pressure will shift to side the reaction with the b. Decrease in pressure will shift to side the reaction with the greatest number of moles greatest number of moles

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumC. Equilibrium ConstantsC. Equilibrium Constants

• 1. Chemist represent equilibrium as numerical value.1. Chemist represent equilibrium as numerical value.• 2. This value relates the amount of reactants to 2. This value relates the amount of reactants to

products at equilibrium.products at equilibrium.• 3.This is called the equilibrium constant K3.This is called the equilibrium constant Keqeq.. • KKeqeq = = [products][products]

[reactants][reactants]

aA + bB aA + bB cC + dD cC + dD KKeqeq = = [C][C]cc[D][D]dd

[A][A]aa[B][B]bb

II. Reversible Reactions and EquilibriumII. Reversible Reactions and EquilibriumC. Equilibrium ConstantsC. Equilibrium Constants

• 4. If the equilibrium constant (K4. If the equilibrium constant (Keqeq) is greater than one, ) is greater than one, the products are favored at equilibrium.the products are favored at equilibrium.

• 5. A K5. A Keqeq less than one means the formation of less than one means the formation of reactants is favored at equilibrium.reactants is favored at equilibrium.

• 6. Example: In the reaction below, at the equilibrium 6. Example: In the reaction below, at the equilibrium point, dinitrogen tetroxide has a concentration of point, dinitrogen tetroxide has a concentration of 0.0055M and nitrogen dioxide has a concentration of 0.0055M and nitrogen dioxide has a concentration of 0.025M. What is the equilibrium constant for the 0.025M. What is the equilibrium constant for the reaction?reaction?

• NN22OO44 2NO 2NO22

III. Determining Whether a Reaction Will Occur III. Determining Whether a Reaction Will Occur A. Free Energy and Spontaneous ReactionsA. Free Energy and Spontaneous Reactions

• 1. Some chemical and physical processes release 1. Some chemical and physical processes release energy that can be used to bring about other changes.energy that can be used to bring about other changes.

• 2. This energy is called free energy.2. This energy is called free energy.• 3. Free energy is the energy that is available to do 3. Free energy is the energy that is available to do

workwork• 4. Spontaneous reaction is a reaction that will occur 4. Spontaneous reaction is a reaction that will occur

naturally and favors the formation of the product naturally and favors the formation of the product (K(Keqeq>1)>1)

• 5. All spontaneous reactions release free energy!5. All spontaneous reactions release free energy!

III. Determining Whether a Reaction Will Occur III. Determining Whether a Reaction Will Occur A. Free Energy and Spontaneous ReactionsA. Free Energy and Spontaneous Reactions

• 6. Nonspontaneous reaction is a reaction that does 6. Nonspontaneous reaction is a reaction that does not favor formation of the product and does not not favor formation of the product and does not occur naturally.occur naturally.

• 7. Reactions can be spontaneous for one set of 7. Reactions can be spontaneous for one set of conditions but not for another.conditions but not for another.

• 8. Sometimes nonspontaneous reactions can 8. Sometimes nonspontaneous reactions can occur when combined with spontaneous reactions.occur when combined with spontaneous reactions.

• 9. This does NOT refer to speed of reaction, only 9. This does NOT refer to speed of reaction, only whether or not it naturally occurs.whether or not it naturally occurs.

III. Determining Whether a Reaction Will Occur III. Determining Whether a Reaction Will Occur

A. Free Energy and Spontaneous ReactionsA. Free Energy and Spontaneous Reactions• 10. We define enthalpy change (10. We define enthalpy change (H) to be the measure H) to be the measure

of change in heat content for a reaction.of change in heat content for a reaction.• 11. Since all spontaneous reactions release free energy, 11. Since all spontaneous reactions release free energy,

you would think that these reactions or processes would you would think that these reactions or processes would always be exothermic (release heat = - always be exothermic (release heat = - H).H).

• 12. This is NOT the case with the melting of ice to water 12. This is NOT the case with the melting of ice to water which requires energy (endothermic), but it happens which requires energy (endothermic), but it happens spontaneously.spontaneously.

• 13. Therefore, enthalpy change is not the only factor 13. Therefore, enthalpy change is not the only factor that determines whether a reaction will be spontaneous.that determines whether a reaction will be spontaneous.

III. Determining Whether a Reaction Will Occur III. Determining Whether a Reaction Will Occur B. EntropyB. Entropy

• 1. The law of disorder states that all 1. The law of disorder states that all processes occur in such a way that they processes occur in such a way that they move towards maximum disorder move towards maximum disorder (randomness). (randomness).

• 2. Entropy is defined as the measure of the 2. Entropy is defined as the measure of the amount of disorder in a system.amount of disorder in a system.

• 3. A reaction that absorbs energy 3. A reaction that absorbs energy (endothermic) can be spontaneous if entropy (endothermic) can be spontaneous if entropy increases.increases.

III. Determining Whether a Reaction Will Occur III. Determining Whether a Reaction Will Occur

B. EntropyB. Entropy

• 4. How does entropy increase?4. How does entropy increase?a. Changing to a less organized phasea. Changing to a less organized phase

1. Solid changing to liquid or gas1. Solid changing to liquid or gas2. Liquid changing to gas2. Liquid changing to gas

b. Substance is divided into parts: for example NaCl is b. Substance is divided into parts: for example NaCl is separated to Naseparated to Na++ and Cl and Cl-- in solution in solutionc. The product has more molecules than the reactants : c. The product has more molecules than the reactants : for example 2Hfor example 2H22O O 2H 2H22 + O + O22

• 5. Entropy also increases as heat is applied to a system.5. Entropy also increases as heat is applied to a system.

III. Determining Whether a Reaction Will Occur III. Determining Whether a Reaction Will Occur B. EntropyB. Entropy

• 6. You have to look at both the change in enthalpy (heat) 6. You have to look at both the change in enthalpy (heat) and the change in entropy (disorder) to determine if a and the change in entropy (disorder) to determine if a reaction is spontaneous.reaction is spontaneous.a. Exothermic + entropy increase = Spontaneous reactiona. Exothermic + entropy increase = Spontaneous reactionb. Endothermic + entropy increase = Spontaneous b. Endothermic + entropy increase = Spontaneous reaction ONLY if entropy change is more than heat reaction ONLY if entropy change is more than heat absorbedabsorbedc. Exothermic + entropy decrease =spontaneous reaction c. Exothermic + entropy decrease =spontaneous reaction ONLY if heat change is more than increased orderONLY if heat change is more than increased orderd. Endothermic + entropy decrease = nonspontaneous d. Endothermic + entropy decrease = nonspontaneous reactionreaction