8. Reaction Rates and Chemical Equilibrium

Question 1

Chemical kinetics

Answer 1

Study of reaction rates

Question 2

Rate of reaction

Answer 2

-Change in concentration of reactant (or product) per unit time
-measured in laboratory
-not constant over a long period of time

Question 3

Initial rate

Answer 3

-at the beginning, in most reactions, the change in concentration is directly proportional to timee

-later, as the reactant is used up, rate decreases

Question 4

Effective collision

Answer 4

-a collision between two molecules or ions that results in a chemical reaction

*typically kase, nagba-bounce apart lng ung molecule A and B without reacting

Question 5

Why are some collisions effective whereas others are not?

Answer 5

1. Activation energy must be achieved
   -energy required to break bonds of reactant molecules
   -energy comes from collision
   -energy depends on relative speed of colliding objects and on their angle of approach
2. A reaction may not take place if the molecules are not oriented properly when they collide
   -HCl(g) and H2O(l)
   -molecules must collide such that H of HCl hits O of water; otherwise no reaction

Question 6

Energy diagrams

Answer 6

-schematic representation of energy changes that yake place as reactants are converted to products

Question 7

Activation energy

Answer 7

-minimum amount of energy needed to break bonds
⬆️Ea -slower reaction rate
⬇️Ea -faster reaction rate

Question 8

Bonds in reaction

Answer 8

-BOND FORMATION- releases energy
-BOND DISSOCIATION- requires energy input

Question 9

Phase diagram
ENDOTHERMIC REACTION

Answer 9

-energy product > energy reactant
(+) ∆H°: more energy needed to break bonds than is released in forming bonds
-“uphill” reactions are endothermic

Question 10

Phase diagram
EXOTHERMIC REACTION

Answer 10

-energy product < energy reactant
(-) ∆H°: more energy is released in forming bonds than needed to break bonds
-“downhill” reaction are exothermic

Question 11

Energy of reaction

Answer 11

-difference between energy of reactant and product

Question 12

Transition state

Answer 12

-top of the hill on an energy diagram
-one or more original bonds are partially broken and one or more new bonds is in the process of formation

Question 13

Speed of reaction and collision

Answer 13

⬆️ speed of rxn ⬆️ probability of effective collision

Question 14

Limiting rate (overall rate)

Answer 14

-slower reaction

Question 15

How to change the rate of chemical reaction
1. Nature of reactant

Answer 15

-ions in aqueous solution are extremely rapid; activation energy is low because usually no covalent bond must be broken

-reactions between covalent molecules is slower

Question 16

How to change the rate of chemical reaction
2. Concentration

Answer 16

-in most cases
⬆️ Concentration of reactant/ product ⬆️Reaction rate

Question 17

Rate constant, k

Answer 17

Proportionality constant between molar concentration of reactants and rate of reaction

Question 18

How to change the rate of chemical reaction
3. Temperature

Answer 18

⬆️ Temperature ⬆️Reaction rate

Question 19

Collision theory: effects of temperature

Answer 19

1. ⬆️ Temperature, molecules move more rapidly, collide more frequently, higher reaction rate
2. ⬆️ Temperature, not only the average speed (kinetic energy) of the molecules is greater, but there is also distribution of speeds.
   Number of very fast molecules increases much more than the number with average speed, more effective collision

Question 20

How to change the rate of chemical reaction
4. Presence of catalyst

Answer 20

-any substance the increases rate of reaction without itself being used up
-allows reaction to take different pathway, one with LOWER activation energy
-catalyst may provide a surface where reactants can meet

Question 21

Heterogeneous catalyst

Answer 21

A catalyst in a separate phase from reactants (i.e. platinum Pt(s) in reaction between formaldehyde CH2O (g) and H2(g)

Question 22

Homogeneous catalyst

Answer 22

-catalyst in same phas as reactants

Question 23

Reversible reaction

Answer 23

-reaction can be made to go in either direction

Question 24

Dynamic equilibrium

Answer 24

-a state in which the rate of the forward reaction equals the rate of the reverse reaction
-no change in concentration

Question 25

Equilibrium constant

Answer 25

-value calculated from the equilibrium expression fo a given reaction indicating in which direction reaction goes
-at constant temperature, equilibrium constants remain the same no matter what concentrations we have
-value of Keq changes when temperature changes
-large K proceeds to completion (to the right)
-small K hardly goes forward at all

Keq = ([C]^c[D]^d) / ([A]^a[B]^b)

*Concentrations at equilibrium

-in dilute solution, omit concentration of solvent because the molarity of solvent is effectively constant
-rate of reaction (how long it takes to reach equilibrium) and Keq have no relationship

Question 26

Le Chatelier’s Principle

Answer 26

-when a stress is applied to a system in chemical equilibrium, the position of the equilibrium shifts in the direction that will relieve the applied stress

-Henri Le Chatelier (1888)

Question 27

Types of stress that can be put on chemical equilibria
1. Addition of a reaction component

Answer 27

-if reactant is added:
equilibrium shifts to formation of more products (para maconsume yung dinagdag na reactant)

-if product is added:
equilibrium shifts to formation of reactants

Question 28

Types of stress that can be put on chemical equilibria
2. Removal of a reaction component (i.e. thru distillation)

Answer 28

-if reactant is removed (or decreased in concentration), equilibrium shifts to reactant (para mapalitan ung nabawas)
-if product is removed, equilibrium shifts to product side

Question 29

Types of stress that can be put on chemical equilibria
3. Change in temperature

Answer 29

•For exothermic reaction
-increase temperature (heat added): equilibrium shifts to reactant (endothermic yung reverse, so maaabsorb ung added heat)
-decrease temperature (heat removed): equilibrium shifts to product (exothermic forward) para magrelease ng heat

•For endothermic reaction
-increase temperature (heat added): equilibrium shifts to product (endothermic forward para maabsorb ung heat added
-decrease temperature (heat removed): equilibrium shifts to reactant (exothermic reverse) para magrelease ng heat

Question 30

Types of stress that can be put on chemical equilibria
4. Change in oressure

Answer 30

-influences equilibrium only if one or more components of reaction mixture are gases
-increase of pressure shifts the reaction toward side with fewer moles of gas
-decrease of pressure shifts the reaction toward side with more moles of gas

Question 31

Types of stress that can be put on chemical equilibria
5. Effects of catalyst

Answer 31

-for reversible reaction, catalyst always increase rates of BOTH FORWARD AND REVERSE reactions

Question 32

Haber process

Answer 32

-artificial fixing of atmospheric nitrogen (instead of using bacteria)
-early problem: conflict between reaction rate and equilibrium
•exothermic so ideal low temp to produce more product
•kaso low temp, mabagal

N2 + 3H2 <–> 2NH3 + 22 kcal

-Fritz Haber (1908): discovered catalyst that permits the reaction to take place at a convenient rate at 500C
-NH3 converted to fertilizer