CHEMISTRY: Reaction Rates & Chemical Equilibrium

Collision Theory
reaction takes place only when the molecules collide with proper orientation and sufficient energy

Activation energy
minimum amount of energy needed to break the bonds b/w atoms of reactants; min energy for reaction to occur

What are the 3 conditions required for a reaction to occur?
Collision of reactants, proper Orientation, and Sufficient energy

How is the rate of a reaction determined?
by measuring the amount of a reactant used up, or the amount of product formed in a certain amount of time

Reaction Rate
Reaction rate=∆ concentration of reactant/product÷∆ time

Factors that affect rate of reaction are?
Temperature, concentration, and addition of catalyst

How does temp affect reaction rate?
@ ↑ temp the ↑ in kinetic energy makes reacting molecules move faster, causing more collisions, and sufficient energy to react

How does concentration affect rate of reaction?
the rate of reaction ↑ with the ↑ in concentration of reactants

How does a catalyst affect the rat of reaction?
catalyst speeds up reactions in both directions equally by decreasing activation energy; doesn’t participate in reaction

Reversible reactions
*proceed in both forward and reverse directions
*2 reaction rates (forward & reverse)
*molecules react, forward reaction is faster than reverse
*products accumulate, forward reaction decreases, reverse

Chemical Equilibrium
when there are no further changes in the concentration of reactants or products

What happens at chemical equilibrium?
reactions occur in opposite direction @ the same rate, which means concentration of the reactants/products remain constant

Explain equilibrium constant expression.
*multiply the concentration of products
*divide by concentration of reactants
*raised to a power (the coefficient in the balanced equation)
*bracket around each substance (to rep mol/L or M)

Define equilibrium constant (Kc).
numerical value obtained by substituting molar concentrations @ equilibrium into expression: Kc=[Product]÷[Reactant]

A reaction in which all the reactants & products are gases
Homogeneous Equilibrium

When reactants are in 2 or more states
Heterogeneous Equilibrium

Are liquids and solids included in equilibrium constant? Why/Why not?
concentration of pure solids & liquids in heterogeneous equilibrium are constant; therefore NOT included in equilibrium constant

A reaction at specific heat has how many values for Kc?
1; one

Which are reversible?
a. breaking glass
b. melting snow
c. heating a pan
b. melting snow

Which are at equilibrium?
a. the rate of forward reaction is 2× faster than rate of reverse
b. concentration of reactants/products doesn’t change
c. rate of reverse reaction doesn’t change
b. concentration of reactants/products doesn’t change
c. rate of reverse reaction doesn’t change

Which is the correct Kc expression for the reaction.
a. Kc=[CS₂][H₂]⁴÷[CH₄][H₂S]²
b. Kc=[CS₂][H₂S]⁴÷[CH]⁴[H]₂⁴
a. Kc=[CS₂][H₂]⁴÷[CH₄][H₂S]²

Size of equilibrium constant depends on what?
whether equilibrium is reached with more reactants than products or vice versa

What effect does the size of Kc have on rate of equilibrium?
none, size of Kc doesn’t effect how fast equilibrium is reached

Large Kc
large amount of product

Small Kc
large amount of reactants; very small Kc produces almost no products

Kc close to 1 (rare)
equal concentration of reactants & products

Le Châtelier’s Principle
a system that is disturbed at equilibrium will shift in the direction that will reduce the stress to reestablish equilibrium

Concentration change, how does reaction shift?
a. add reactant
b. add product
c. remove reactant
d. remove product
a. reaction shifts→products (more products)
b. reaction shifts→reactants (more reactants)
c. reaction shifts→reactants
d. reaction shifts→products

If the volume of a gas mixture changes at equilibrium, what must also change?
Concentration of the gases

An increase/decrease in volume of gases will affect concentration, how?
↑ volume ↓concentration; ↓volume ↑concentration

On which side of the equation is heat written in endothermic and exothermic reactions?
Endothermic ; right (reactant side)
Exothermic ; left (product side)

How does temperature affect endothermic reactions?
↑heat; reaction shifts →products (to remove heat)
↓heat; reaction shifts →reactants (to add heat)

How does temperature affect exothermic reactions?
↑heat; reaction shifts→reactants (to remove heat)
↓heat; reaction shifts→products (to add heat)

Indicate whether the Kc has mostly products, reactants, or both:
a. Kc=3.7×10⁸
b. Kc=4.7
c. 1.7×10⁻⁵⁶
a. mostly reactants
b. both
c.mostly products

Indicate how the system shifts for each change for the given reaction: 3O₂(g)+heat⇌2O₃
a. add more O₂
b. add more O₃
c. increase temp
d. increase volume of container
e. add catalyst
a. reaction shifts→product (remove heat)
b. reaction shifts→reactant (add heat)
c. reaction shifts→product (increase rate)
d. reaction shifts→reactant (more moles)
e. reaction shifts→increases rate of both reactions equally