Chapter 18 Notes Reaction Rates and Equilibrium

Chapter 18 Notes Reaction Rates and Equilibrium

18.1

Rates of Reaction Collision Theory

o Rate = The speed of any change that occurs within an interval of time o KEY = In chemistry, the rate of chemical change or the reaction rate is usually expressed as the

amount of reactant changing per unit time o Collision Theory = atoms, Ions, and molecules can react if they collide with one another, provided

that the colliding particles have enough kinetic energy 1) If the colliding particles don't have enough kinetic energy, they will bounce away

o Activation Energy = The minimum energy that colliding particles must have in order to react 1) If particles do not have the energy, no reaction will occur

o Activated Complex = The unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier 1) Only forms if colliding particles have sufficient energy and if they are oriented properly

o Transition State = Another name for the activated complex because the outcome of both the reformation of the reactants is just as likely as the formation of the products at that instant

Factors Affecting Reaction Rates o KEY = The rate of a chemical reaction depends upon 4 things: 1) Temperature Generally reactions will speed up with an increase in temperature and slow down with a decrease in temperature 2) Concentration The more particles there are, the more collisions will occur, so increasing the concentration will generally speed up a reaction 3) Particle Size Small objects will produce more collisions in the same amount of time than big objects will. As the size of the particles gets smaller, the reaction rate will increase because many small particles has more surface area for the solute to interact with than one large particle has 4) Catalyst Catalysts speed up reactions by lowering the activation energy of a reaction, which makes more collisions have enough energy to get to the activated complex. This speeds up the reaction. Inhibitor = Substance that interferes with the action of a catalyst

18.2

Reversible Reactions and Equilibrium Reversible Reactions

o Reversible Reaction = A reaction in which the conversion of reactants to products and the conversion of products to reactants happens at the same time : 22() + 2() 23() : 22() + 2() 23()

o Chemical Equilibrium = When the rates of the forward and reverse reactions are equal o KEY = At chemical equilibrium, there is no net change in the total amount of any component

There are still reactants becoming products and products becoming reactants, but because they are happening at the same speed, the net change is 0

o Equilibrium Position = The relative concentrations of the reactants and products at equilibrium The side of the reaction has the most concentration at equilibrium is the favored direction If there are more products than reactants, the forward reaction is favored If there are more reactants than products, the reverse reaction is favored

Factors Affecting Equilibrium: Le Chatelier's Principle o Le Chatelier's Principle = If stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress o KEY = Stresses that upset the equilibrium of a chemical system include changes in the concentration of reactants/products, changes in temperature, and changes in pressure

 Concentration If you add more reactants, the reaction will move in the forward direction If you add more products, the reaction will move in the reverse direction If you remove any reactants, the reaction will move in the reverse direction If you remove any products, the reaction will move in the forward direction

Temperature If heat is added, the reaction will move in the direction where heat is absorbed If heat is removed, the reaction will move in the direction where heat is released

Pressure Pressure changes will only affect gaseous equilibria that have different number of moles of gas on either side of the reaction Increased pressure moves the reaction in the direction of less moles of gas Decreased pressure moves the reaction in the direction of more moles of gas

Conceptual Problem 18.1 ? Applying Le Chatlier's Principle What effect does each of the following changes have on the equilibrium position for this reaction?

5() + 3() + 2() Addition of Cl2 = Shifts away from Cl2, pushing it to the left, makes more reactants Increase in Pressure = Shifts to side with less moles of gas, pushing it to the left, makes more reactants Removal of Heat = Shifts toward heat, pushing it to the left, makes more reactants Removal of PCl3 as it is formed = Shifts toward PCl3, pushing it to the right, makes more products

Equilibrium Constants o All reactions can be classified as a general reaction with the following formula:

+ +

o Equilibrium Constant (Keq) = The ration of product concentrations to reactant concentrations in equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation

[] ? [] = [] ? []

o KEY = A value of Keq greater than 1 means that the products are favored over reactants o KEY = A value of Keq less than 1 means that the reactants are favored over products

There is no unit on equilibrium constants since each reaction has different number of

products/reactants and is balanced using different coefficients, so do not use a unit

Sample Problem 18.1 ? Expressing and Calculating Keq

A liter of a gas mixture at equilibrium at 10?C contains 0.0045 mol of N2O4 and 0.030 mol of NO2.

What is the expression for the equilibrium constant and calculate the equilibrium contant

Expression

for

Keq

=

[2]2 [24]

24() 22()

Calculation

of

Keq

=

0.0302 0.0045

=

0.20

Sample Problem 18.2 ? Finding the Equilibrium Constant

1.00 mol of H2 & I2 react. At equilibrium, 1.56 mol of colorless HI is present. What is Keq of the reaction? 2() + 2() 2()

1.56 mol of HI produced means 0.78 mol of each reactant has been used, leaving 0.22 mol of each (1.00 ? 0.78 = 0.22)

=

[]2 [2]?[2]

=

(1.56)2 (0.22)?(0.22)

=

50

The concentrations of any reactant/product can also be determined if the Keq is known and the remaining concentrations of other reactants/products is known as well

18.3

Solubility Equilibrium

The Solubility Product Constant

o As long as there is still some solid left undissolved for a compound, a Keq can be written using the

dissociation equation for that solid:

() (+) + (-)

[+] ? [-] = []

o Rearranging the Keq equation by putting the [AgCl] with the Keq, allows us to drop the [AgCl] part

because the concentration of AgCl is a constant when there is solid left (saturated solution): = [+] ? [-]

o Solubility Product Constant (Ksp) = The product of the concentrations of the ions each raised to a power equal to the coefficient of the ion in the dissociation equation

o KEY = The smaller the numerical value of the solubility product constant, the lower the solubility

of the compound

Sample Problem 18.3 ? Finding the Ion Concentrations in a Saturated Solution

What is the concentration of lead and chromate ions in a saturated lead chromate solution at 25?C? (Ksp=1.8x10-14) 4() (+2) + 4-(2)

According to the balanced equation, there should be an equal number of lead and chromate ions

= [+2] ? [4-2] 1.8 ? 10-14 = 2

= 1.8 ? 10-15 = 1.3 ? 10-7

The Common Ion Effect o The Ksp is related to the maximum amount of the combined ions in solution, so if you add more of one of them, the product of the two must still be equal to the Ksp, meaning the other ion must decrease in concentration in the solution o Common Ion = An ion that is found in both salts in a solution o Common Ion Effect = Lowering of the solubility of an ionic compound as a result of the addition of a common ion

Sample Problem 18.4 ? Finding Equilibrium Ion Concentrations in the Presence of a Common Ion The Ksp of AgBr is 5.0x10-13. What is the [Br-] of 1 L saturated AgBr if 0.020 mol of AgNO3 is added?

The bromide concentration = x, which means the silver concentration = x+0.20

Because the Ksp is so small, the amount of solid dissolved is very small, so it is insignificant, making the formula:

= [+] ? [-] = [+] ?

=

[+]

=

5.0?10-13 0.20

=

2.5

?

10-11

o KEY = If the product of the concentrations of two ions in the mixture is greater than the Ksq of the compound formed from the ions, a precipitate will form

18.4

Entropy and Free Energy Free Energy and Spontaneous Reactions

o Free Energy = Energy that is available to do work o Spontaneous Reaction = Reaction that occurs naturally and favors the formation of products at the

specified conditions o KEY = Spontaneous reactions produce substantial amounts of products at equilibrium and release

free energy o Nonspontaneous Reaction = Reaction that does not favor the formation of products at the specified

conditions Entropy

o Entropy (S) = A measure of the disorder of a system Gases have more entropy than liquids, and liquids have more entropy than gases S increases when a large object is split into smaller ones S increases if the # of product molecules are greater than the # of reactant molecules S increases when the temperature increases due to more chaotic motion

o Law of Disorder = The natural tendency is for systems to move in the direction of maximum disorder or randomness

o KEY = An increase in entropy favors the spontaneous chemical reaction o KEY = A decrease in entropy favors the nonspontaneous reaction Enthalpy, Entropy, and Free Energy o KEY = The size and direction of enthalpy changes and entropy changes together determine

whether a reaction is spontaneous; whether it favors the products and releases energy Gibbs Free Energy

o Gibbs Free-Energy Change (G) = The maximum amount of energy that can be coupled to another process to do useful work = - H is the change in enthalpy S is the change in entropy T is the temperature measured in Kelvin

o KEY = The numerical value of G is negative in spontaneous processes o KEY = The numerical value of G is positive in nonspontaneous processes

18.5

The Progress of Chemical Reactions Rate Laws

o Rate Law = Expression for the rate of a reaction in terms of the concentration of reactants o Specific Rate Constant (k) = Proportinality constant relating the concentrations of reactants to the

rate of the reaction. It depends on the conditions of the reaction o KEY = The value of the specific rate constant, k, is large if the products form quickly o KEY = The value of the specific rate constant, k, is small if the products form slowly o First-Order Reaction = The rate is directly proportional to the concentration of only one reactant

+ +

= [][]

o All other "orders" are calculated by adding the exponents within the rate equation 2nd order would be 1st in A and 1st in B, or just 2nd in A 3rd order would be 1st in A and 2nd in B, or 2nd in A and 1st in B

Reaction Mechanisms o Elementary Reaction = Products are produced from reactants in a single step o Reaction Mechanism = Products are produced from reactants in multiple steps o KEY = Peaks in a Reaction Progress Curve correspond to the energies of the activated complexes o KEY = Valleys in a Reaction Progress Curve correspond to the energy of an intermediate o Intermediate = The product of the step before it in the reaction mechanism, but the reactants of the next step in the reaction mechanism

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