KINETICS AND EQUILIBRIUM HOW SUBSTACNCES REACT!

UNIT 6 KINETICS AND EQUILIBRIUM CHEMICAL KINETICS A. Definition: Branch of chemistry concerned with the rate of chemical reactions and the mechanisms by which chemical reactions occur A. Definition: Branch of chemistry concerned with the rate of chemical reactions and the mechanisms by which chemical reactions occur B. Rate- measured in terms of moles of reactants consumed or moles of product formed in a unit of time B. Rate- measured in terms of moles of reactants consumed or moles of product formed in a unit of time

C. Mechanism –describes the sequence of events (reactions) by which an overall reaction takes place C. Mechanism –describes the sequence of events (reactions) by which an overall reaction takes place REACTION RATES AND COLLISION THEORY 1. Chemical reactions depend on collisions between reacting species ( atoms, molecules, ions, or particles)

2. Reactions occur only when reactants collide with enough energy and in the correct orientation to form an activated complex – a high energy intermediate 3. The rate of the reaction is affected by the number of collisions occurring and the fraction of these collisions that are effective.

a. Effective collisions are collisions in which the reacting species collide at the correct angle and at the right speed so that they form an activated complex a. Effective collisions are collisions in which the reacting species collide at the correct angle and at the right speed so that they form an activated complex b. Therefore, any factor that causes more collisions to occur or results in more collisions being effective (leading to an activated complex) will increase the rate of a reaction b. Therefore, any factor that causes more collisions to occur or results in more collisions being effective (leading to an activated complex) will increase the rate of a reaction

FACTORS AFFECTING REACTION RATE 1. Nature of reactants a. During a chemical reaction bonds are broken and new bonds are made a. During a chemical reaction bonds are broken and new bonds are made b. Covalent substance react slower than ionic Ionic = metal + nonmetal b. Covalent substance react slower than ionic Ionic = metal + nonmetal Covalent = nonmetal + nonmetal

c. Ionic substances are generally dissolved in water before being reacted together. When an ionic substance is dissolved in water the crystal lattice is broken and ions are free to move about, collide and react. Meaning no bonds need to be broken for the reaction to occur. c. Ionic substances are generally dissolved in water before being reacted together. When an ionic substance is dissolved in water the crystal lattice is broken and ions are free to move about, collide and react. Meaning no bonds need to be broken for the reaction to occur. 2. Concentration of Reactants a. An increase in the concentration (amount) of one or more reactant results in an increase in the reaction rate a. An increase in the concentration (amount) of one or more reactant results in an increase in the reaction rate

because there are more molecules reacting therefore there is an increase in the frequency of collisions because there are more molecules reacting therefore there is an increase in the frequency of collisions c. Concentration is generally measured in mole/ liters or M c. Concentration is generally measured in mole/ liters or M d. For reactants dissolved in a liquid solvent the concentration is increased by removing some of the solvent through evaporation or by adding more of the solute. Conversely decreases in concentration can be achieved by adding more solvent d. For reactants dissolved in a liquid solvent the concentration is increased by removing some of the solvent through evaporation or by adding more of the solute. Conversely decreases in concentration can be achieved by adding more solvent

2. Temperature a. An increase in temperature will increase the rate of almost all chemical reactions because it increases the KE(speed) of the particles and if the particles move faster the number of collisions will increase but so will the number of effective collisions

3. Pressure a. An increase in concentration for a gas is achieved by decreasing the volume the gas occupies, this is accomplished by increasing the pressure on the gas. a. An increase in concentration for a gas is achieved by decreasing the volume the gas occupies, this is accomplished by increasing the pressure on the gas. 4. Surface Area a. Increase the surface area of reactants increases the rate of the reaction a. Increase the surface area of reactants increases the rate of the reaction

b. Important for heterogeneous reactions( one where the reactants are in different phases) b. Important for heterogeneous reactions( one where the reactants are in different phases) c. To increase the surface area of a solid grind it up. c. To increase the surface area of a solid grind it up. d. Example. d. Example. A given amount of Zinc will react faster with dilute HCl if the zinc is ground up due to increased surface area. A given amount of Zinc will react faster with dilute HCl if the zinc is ground up due to increased surface area.

5. Reaction Mechanism a. Most reactions do not occur in a single step, but in a series of steps known as the reaction mechanism a. Most reactions do not occur in a single step, but in a series of steps known as the reaction mechanism b. More steps required the slower the reaction b. More steps required the slower the reaction c. Reaction Mechanism is not indicated by a chemical equation c. Reaction Mechanism is not indicated by a chemical equation

6. Presence of a Catalyst a. Increase the rate of a chemical reaction by providing an easier path for the reaction b. They take part in the reaction by lowering the activation energy but they are not used up.

POTENTIAL ENERGY DIAGRAMS 1. A diagram that shows the change in potential (stored energy) that occurs during the course of a chemical reaction. 1. A diagram that shows the change in potential (stored energy) that occurs during the course of a chemical reaction. 2. During the course of a chemical reaction if reactants collide with both the proper orientation and sufficient energy an activated complex is formed. 2. During the course of a chemical reaction if reactants collide with both the proper orientation and sufficient energy an activated complex is formed.

a. activated complex-temporary intermediate product that may either break apart and reform the reactants or rearrange the atoms and form a new product. 3. Activation Energy a. Definition –Minimum energy necessary to initiate (start) a chemical reaction. a. Definition –Minimum energy necessary to initiate (start) a chemical reaction. b. All reactions require some “start up” energy. b. All reactions require some “start up” energy.

4. Heat of reaction (ENTHALPY),  H, 4. Heat of reaction (ENTHALPY),  H, a. Heat energy is released or absorbed in the formation of products. That is reactions are either endothermic or exothermic a. Heat energy is released or absorbed in the formation of products. That is reactions are either endothermic or exothermic b. Δ H represents the difference in Potential Energy (PE) between the products and reactants. b. Δ H represents the difference in Potential Energy (PE) between the products and reactants. c.  H = Hproducts – Hreactants c.  H = Hproducts – Hreactants

5. Exothermic Reactions a. Overall Energy is released a. Overall Energy is released b. Products have lower potential energy than reactants b. Products have lower potential energy than reactants c.  H is negative c.  H is negative 6. Endothermic Reactions a. Overall energy is absorbed a. Overall energy is absorbed b. Products have a higher PE than reactants b. Products have a higher PE than reactants c.  H is positive c.  H is positive

7. Sign used when energy is included in a chemical reaction should not be confused with the sign for  H. a. The sign of  H tells us whether a reaction is endothermic or exothermic. a. The sign of  H tells us whether a reaction is endothermic or exothermic. b. If  H is positive the energy term is found on the reactant side the reaction is endothermic. b. If  H is positive the energy term is found on the reactant side the reaction is endothermic. c. conversely if  H is negative the energy term is found on the product side the reaction is exothermic. c. conversely if  H is negative the energy term is found on the product side the reaction is exothermic.

d. The units of  H and the energy term is kJ. 8. Table I a. Heats of reactions for certain chemical reactions and processes b.  H is measured in kJ.

c. the reaction is exactly equal to  H if the equation is identical to the one listed. Examples: CH 4 + 2O 2  CO 2 + 2H 2 O  H = kJ Rewrite the equation with the heat term added CH 4 + 2O 2  CO 2 + 2H 2 O kJ N 2 (g) + O 2 (g)  2NO (g)  H = kJ Rewrite the equation with the heat term added N 2 (g) + O 2 (g) kJ  2NO (g)

d. If it is the same except that the coefficients are multiples of the ones listed then  H and the energy written in the reaction will also be a multiple. Examples 2H 2 (g) + 2I 2 (g)  4HI (g)  H = 2(+53kJ) Rewrite the equation with the heat term added 2H 2 (g) + 2I 2 (g) + 106kJ  4HI (g)

e. If the reactants and products are switched then change the sign of  H 2NO (g)  N 2 (g) + O 2 (g)  H = -(+182.6kJ) Rewrite with the heat term in the equation 2NO (g)  N 2 (g) + O 2 (g) kJ

EQUILIBRIUM A. Definition: Equilibrium is said to exist when any reaction takes place under fixed conditions so that the rate of the forward reaction is equal to the rate of the reverse reaction. A. Definition: Equilibrium is said to exist when any reaction takes place under fixed conditions so that the rate of the forward reaction is equal to the rate of the reverse reaction. B. At equilibrium the rate at which reactants are converted to products is equal to the rate at which products are reconverted to reactants B. At equilibrium the rate at which reactants are converted to products is equal to the rate at which products are reconverted to reactants

C. THE QUANTITIES OF PRODUCTS AND REACTANTS AT EQUILIBRIUM ARE NOT NECESSARILY EQUAL IT IS THE RATES OF THE FORWARD AND REVERSE REACTION THAT ARE EQUAL. D. A closed system is necessary so neither reactant nor product escapes. D. A closed system is necessary so neither reactant nor product escapes. E.  = forward reaction F.  = Reverse reaction

PHYSICAL EQUILIBRIUM A. Phase equilibria A. Phase equilibria 1. In general for a closed system there will exist an equilibrium between phases where the rate of escape (from the phase) = the rate of return (to that phase) 1. In general for a closed system there will exist an equilibrium between phases where the rate of escape (from the phase) = the rate of return (to that phase) 2. Equilibrium existing between a solid and liquid phase at a substances melting point. At the melting point the rate at which a solid melts to become a liquid is equal to the rate at which the liquid freezes to reform the solid. Rate of melting = Rate of freezing 2. Equilibrium existing between a solid and liquid phase at a substances melting point. At the melting point the rate at which a solid melts to become a liquid is equal to the rate at which the liquid freezes to reform the solid. Rate of melting = Rate of freezing

3. Exists between liquid and gas 3. Exists between liquid and gas Rate of evaporation = Rate of condensation Rate of evaporation = Rate of condensation B. Solution equilibrium 1. Solids and liquids 1. Solids and liquids a. Process that goes on in a saturated solution if any additional material is added. We see the added chemical fall to the bottom. What actually occurs is the added particle dissolves and anther particles falls out of solution. a. Process that goes on in a saturated solution if any additional material is added. We see the added chemical fall to the bottom. What actually occurs is the added particle dissolves and anther particles falls out of solution.

b. Solution equilibrium exists when the rate of dissolving equals the rate of crystallization. b. Solution equilibrium exists when the rate of dissolving equals the rate of crystallization. c. A saturated solution is defined as : A solution in which an equilibrium exists between dissolved and undissolved solute. It cannot hold any more solute at a given temperature.

2. Gases in liquids. a. In a closed system, equilibrium may exist between a gas dissolved in a liquid and the undissolved gas above the liquid a. In a closed system, equilibrium may exist between a gas dissolved in a liquid and the undissolved gas above the liquid b. Equilibrium between dissolved and undissolved gas is affected by temperature and pressure b. Equilibrium between dissolved and undissolved gas is affected by temperature and pressure c. Increase in temperature decreases solubility, converse is also true c. Increase in temperature decreases solubility, converse is also true d. Increases in pressure increases solubility, converse is also true d. Increases in pressure increases solubility, converse is also true

C. Chemical Equilibrium 1. State in which the concentration of the reactants and products of a reaction remain constant. 1. State in which the concentration of the reactants and products of a reaction remain constant. 2. When equilibrium is reached all observable properties of the system, such as color, pressure and temperature remain constant. 2. When equilibrium is reached all observable properties of the system, such as color, pressure and temperature remain constant. 3. When equilibrium is reached the rate of the forward reaction is equal to the rate of the reverse reaction 3. When equilibrium is reached the rate of the forward reaction is equal to the rate of the reverse reaction

a. During a certain unit of time, the rate at which the product is made is equal to the rate at which the Product is broken down a. During a certain unit of time, the rate at which the product is made is equal to the rate at which the Product is broken down b. This results in the mass of each substance remaining constant at equilibrium b. This results in the mass of each substance remaining constant at equilibrium c. This results in a system that is dynamic, constantly changing, but appears to be static, unchanging c. This results in a system that is dynamic, constantly changing, but appears to be static, unchanging

d. The mass of each substance may only be changed if the system is disturbed, meaning something is done so that the rate of the forward and reverse reactions are no longer equal.

LE CHATLIER’S PRINCIPLE A. STATEMENT: If a stress (disturbance) such as a change in concentration, pressure or temperature, is applied to a system in equilibrium, the equilibrium is shifted in such a way that tends to relieve the effects of the stress. If a stress (disturbance) such as a change in concentration, pressure or temperature, is applied to a system in equilibrium, the equilibrium is shifted in such a way that tends to relieve the effects of the stress. B. How to predict which direction the equilibrium will shift. 1. Choose the arrow (forward or reverse) that points away from the substance in the reaction that was increased.

2. Choose an arrow (forward or reverse) that points at the substance that was decreased C. How to predict the change in concentration (amount) of the substances in the reaction 1. All substances at the tip of the chosen arrow increase 1. All substances at the tip of the chosen arrow increase 2. All substances at the tail of the chosen arrow decrease 2. All substances at the tail of the chosen arrow decrease

D. Terminology 1. If the FORWARD reaction is favored, the products are favored, or the equilibrium shifts to the right, it means that the forward reaction goes faster in the reverse reaction once the stress is applied 1. If the FORWARD reaction is favored, the products are favored, or the equilibrium shifts to the right, it means that the forward reaction goes faster in the reverse reaction once the stress is applied 2. If the reverse reaction is favored, the reactants are favored, or the equilibrium shifts to the left, it means that the reverse reaction goes faster than the forward once the stress is applied 2. If the reverse reaction is favored, the reactants are favored, or the equilibrium shifts to the left, it means that the reverse reaction goes faster than the forward once the stress is applied

E. Stresses that can be applied to a chemical system 1. Effect of changing the concentration a. Increasing the concentration either a reactant or product in a reaction at a. Increasing the concentration either a reactant or product in a reaction at equilibrium will cause the reaction to go(shift) in such a direction as to consume the increase. This direction is said to be favored. equilibrium will cause the reaction to go(shift) in such a direction as to consume the increase. This direction is said to be favored.

b. Which ever reaction, forward or reverse that consumes the increase will be favored, meaning the rate of that reaction, (forward or reverse) will be increased until the stress is relieved. Eventually a new equilibrium is reached. b. Which ever reaction, forward or reverse that consumes the increase will be favored, meaning the rate of that reaction, (forward or reverse) will be increased until the stress is relieved. Eventually a new equilibrium is reached.

c. Decreasing the concentration of either a reactant or product) in a reaction at equilibrium will cause the reaction to go(shift) in such a direction as to produce more of the substance decreased. Whichever reaction, forward or reverse, that produces more of the substance will be favored,

d. Example: [ ] = reads concentration N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) This process is called the Haber Process it is used in the commercial production of ammonia Stress: [ H 2 ] is increased ► Result: forward reaction is favored because the forward reaction consumes H 2, consumes H 2, therefore [N 2 ] will decrease and [ NH 3 ] will increase. The equilibrium will shift to the right

Stress: [N 2 ] is increased ► Result: forward reaction is favored because the forward reaction consumes N 2,therefore [H 2 ]will decrease and [ NH 3 ] increase. The equilibrium will shift to the right increase. The equilibrium will shift to the right Stress: [ NH 3 ] is increased Result: reverse reaction is favored because the reverse reaction consumes NH 3,therefore the [N 2 ] and [H 2 ] will increase. The equilibrium will shift to the left. Result: reverse reaction is favored because the reverse reaction consumes NH 3,therefore the [N 2 ] and [H 2 ] will increase. The equilibrium will shift to the left.

Stress: [ H 2 ] is decreased Result: reverse reaction is favored because the reverse reaction produces [H 2 ] therefore the [N 2 ] will increase and the [NH 3 ] will decrease. The equilibrium will shift to the left. therefore the [N 2 ] will increase and the [NH 3 ] will decrease. The equilibrium will shift to the left.

Stress: [N 2 ] is decreased ► Result: reverse reaction is favored because N 2 is produced by the reverse reaction, therefore the [H 2 ] is increased and the [ NH 3 ] will decrease. The equilibrium will shift to the left.

Stress [NH 3 ] is decreased ► Result: forward reaction is favored because the forward reaction produces NH 3 therefore the [H 2 ] and [N 2 ] will both decrease. The equilibrium will shift to the right.

g. Removal of a product as it is formed tends to cause the forward reaction to go more toward completion. Continuous removal of the product may destroy the equilibrium system by removing all of the substance necessary for the reverse reaction h. Products are removed from a reaction by h. Products are removed from a reaction by the formation of a gas, an insoluble precipitate (table F), or in an ionic reaction by producing water.

2. Effect of pressure a. A change in pressure effects chemical equilibria in which gases are involved. There is no effect on a system that does not contain gases a. A change in pressure effects chemical equilibria in which gases are involved. There is no effect on a system that does not contain gases b. An increase in pressure will favor a shift in equilibrium toward the side of the reaction that contains the lesser number of moles ( the side of the reaction with a smaller sum of the coefficients) b. An increase in pressure will favor a shift in equilibrium toward the side of the reaction that contains the lesser number of moles ( the side of the reaction with a smaller sum of the coefficients)

c. A decrease in pressure will favor a shift in equilibrium toward the side of the reaction with the greater number of moles ( the side of the reaction with a larger sum of the coefficients) c. A decrease in pressure will favor a shift in equilibrium toward the side of the reaction with the greater number of moles ( the side of the reaction with a larger sum of the coefficients) d. If there is not change in the number of moles than a change in pressure will not shift the equilibrium d. If there is not change in the number of moles than a change in pressure will not shift the equilibrium

e. Example e. Example N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) 4 moles 2moles 4 moles 2moles Stress: Increase in pressure ► Result: The forward reaction is favored because the product side has fewer total moles therefore [N 2 ] and [H 2 ] will decrease and the [NH 3 ] will increase moles therefore [N 2 ] and [H 2 ] will decrease and the [NH 3 ] will increase

Stress: Decrease in pressure ► Result: The reverse reaction is favored because it has a greater number of total moles therefore [N 2 ] and [H 2 ] will increase and the [NH 3 ] will decrease of total moles therefore [N 2 ] and [H 2 ] will increase and the [NH 3 ] will decrease H 2 (g) + Cl 2 (g) ↔ 2HCl (g) 2 moles 2 moles 2 moles 2 moles Stress: increasing the pressure Result: No change the equilibrium will not shift

3. Effect of temperature a. When the temperature of a system in equilibrium is raised, the equilibrium is displaced (shifted) in such a way that heat is absorbed. Meaning the endothermic reaction is favored. a. When the temperature of a system in equilibrium is raised, the equilibrium is displaced (shifted) in such a way that heat is absorbed. Meaning the endothermic reaction is favored. b. When the temperature of a system in equilibrium is lowered, the equilibrium is shifted in such a way as to release heat. Meaning exothermic reaction is favored. b. When the temperature of a system in equilibrium is lowered, the equilibrium is shifted in such a way as to release heat. Meaning exothermic reaction is favored.

c. Note: The rates of all chemical reactions, both endothermic and exothermic are increased when temperature is increased (by increasing the number of effective collisions). However, the opposing reactions are increased unequally, resulting in a shift in the equilibrium. c. Note: The rates of all chemical reactions, both endothermic and exothermic are increased when temperature is increased (by increasing the number of effective collisions). However, the opposing reactions are increased unequally, resulting in a shift in the equilibrium.

d. Example N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + 22 kcal Stress: Increase in temperature ► Result: The reaction is favored because it is the endothermic ► Result: The reverse reaction is favored because it is the endothermic reaction therefore the [N 2 ] and [H 2 ] will increase and the [NH 3 ] will decrease reaction therefore the [N 2 ] and [H 2 ] will increase and the [NH 3 ] will decrease

Stress: Decrease in temperature ► Result: The forward reaction is favored because it is the exothermic reaction therefore the [N 2 ] and [H 2 ] will decrease and the [NH 3 ] will increase.

4. Effect of a Catalyst a. Increases the rate of both the forward and reverse reaction equally and causes no shift in the equilibrium a. Increases the rate of both the forward and reverse reaction equally and causes no shift in the equilibrium b. May cause equilibrium to be reached more quickly but does not affect the point of equilibrium b. May cause equilibrium to be reached more quickly but does not affect the point of equilibrium

SPONTANEOUS REACTIONS? A. Definition: Spontaneous changes are changes that are observed to occur under given conditions B. Example: If the temperature is above 0C ice melts but if the reverse, freezing is not observed at these temperatures. 1. Chemical reactions are also observed to go in one direction (forward or reverse) under one set of conditions and in the opposite direction under other conditions, or to remain in equilibrium under still another set of conditions. 1. Chemical reactions are also observed to go in one direction (forward or reverse) under one set of conditions and in the opposite direction under other conditions, or to remain in equilibrium under still another set of conditions.

C. Factors determining the direction of spontaneous change 1. There are two fundamental tendencies in nature which together determine the direction (forward or reverse) of a spontaneous change. 1. There are two fundamental tendencies in nature which together determine the direction (forward or reverse) of a spontaneous change. a. Minimum Energy a. Minimum Energy b. Maximum Disorder b. Maximum Disorder

2. Energy Changes (Enthalpy Changes) a. At constant temperature and pressure a system tends to under go a reaction that in its final state it has lower energy than in its initial state a. At constant temperature and pressure a system tends to under go a reaction that in its final state it has lower energy than in its initial state b. Therefore the tendency in nature favors the exothermic reaction, -ΔH b. Therefore the tendency in nature favors the exothermic reaction, -ΔH 3. Tendency toward randomness (Entropy) a. Randomness is the disorder or lack of order in a system a. Randomness is the disorder or lack of order in a system

b. Entropy is a measure of the randomness or disorder in a system c. Symbol  S c. Symbol  S d. The more random the higher the entropy d. The more random the higher the entropy e. e. Nature favors more disorder or higher entropy; +ΔS f. Examples of entropy changes f. Examples of entropy changes

Phase changes illustrate entropy changes Changes from S  L  G show an increase in entropy because the particles are moving toward a state of greater disorder. The reverse, cooling, shows a decrease in entropy Changes from S  L  G show an increase in entropy because the particles are moving toward a state of greater disorder. The reverse, cooling, shows a decrease in entropy ► Systems were the number of particles increases show increases in entropy, more particles more disorder 2NO 2 (g)  2NO(g) + O 2 (g) 2NO 2 (g)  2NO(g) + O 2 (g)

The dissolving of a solid in a solvent (solutions) show increases in entropy because dissolving breaks up the crystal lattice of the solid giving the particles increased freedom g. Increase in entropy means that the final state of a system is more disordered than the initial state.