Unit 8 - Rates of Reaction and Equilibrium

Rate

Measure of changes in a chemical reaction over time(Rate = change in quantity/time)

Rates can be measured by…

1.) Time to complete reaction

2.) Rate of disappearance of reactants

3.) Rate of appearance of products

Collision Theory

Atoms, Ions, and Molecules must collide with enough energy to rearrange and form chemical bonds

What do atoms, ions, and molecules need to chemically react?

Sufficient kinetic energy and molecular orientation

Factors affecting reaction rates

1.) Nature of Reactants

2.) Concentration

3.) Temperature

4.) Catalyst

5.) Inhibitor

What does the nature of reactants depend on?

Shape, Surface Area, Size, Electronegativity, and the ability to gain/lose electrons

How does surface area affect reaction rates?

An increase in surface area increases the exposure of reactants to one another . The greater this exposure, the more collisions there are with reacting particles.

How does the size of molecules affect reaction rates?

Bigger molecules are slow which means they don't have enough kinetic energy. Smaller molecules are faster which means they have a higher kinetic energy.

How does concentration affect reaction rates?

Increased concentration increases the rate. There is increased likelihood that molecules will collide

How does temperature affect reaction rates?

Higher temperatures generally increase reaction rates because the molecules have a higher kinetic energy when colliding. Increase of temperature in 10 degrees causes rates to double.

What is a catalyst?

Substance that speeds up the rate of a chemical reaction

What is an Inhibitor?

Material used to decrease the rate of a reaction

Rate Law Equation

Rate = k [A]ⁿ

What is k in the Rate Law Equation?

k constant that is unique to substance and temperature

Rules to Rate Law Equation

Only reactants in the gaseous or aqueous state are included in the equation

Activation Energy

Minimum amount of energy needed when particles collide in order to react

Boltzmann Curve

area under curve = total number of molecules

y-axis = number of molecules with a particular energy

x-axis = energy

increase temperature = increase proportion of molecules with energies greater than or equal to activation energy leading to increase in frequency of successful collisions

catalysts = provide an alternative reaction pathway with a lower activation energy (Ea)

more molecules have energies in excess of Ea

Activated Complex

an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier

Difference of Reactants energy and products energy

Enthalpy or Heat of Reaction

Endothermic

Absorbs heat

Exothermic

Releases heat

What do catalysts do to activated complex?

Lowers the whole peak

What do inhibitors do to activated complex?

Raises the whole peak

Reaction Order

Defines how the rate is affected by the concentration of that reactant

How do you get reaction order?

The sum of the exponents in a rate law, where each exponent provides the reaction order with respect to its reactant.

Lower reaction order generally means it is…

faster

Equilibrium

a state in which the rate of the forward reaction equals the rate of the reverse reaction

Equilibrium reactions are…

reversible

Both the forward reaction and the reverse reaction of an equilibrium are…

occurring at the same time and rate

Conditions of Equilibrium

1.) Closed system

2.) Opposite reactions at the same rate

3.) Started reaction with either the product or reactant

4.) Temperature, pressure, and concentration are kept constant

Dynamic systems

systems that are dependent on one another and change in relation to one another - reverts back to equilibrium after change

Le Chatelier's Principle

States that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

In a dynamic system, if a stress or stressors are added to a system in equilibrium…

the system counteracts that change and reaches a new state of equilibrium

Favored

Greater concentration

Shift Right

Favors the product; Forward reactions speed up

Shifts Lefts

Favors the reactants; Reverse reactions speed up

Changes in concentration and temperature

Increase in concentration/temp, reaction shifts away from change
Decrease in concentration/temp, reaction shifts towards change

Whichever way arrow is pointing, the molecules increase and others decrease

Changes in pressure and volume

Increases in pressure(decrease in volume) causes a shift towards less gas moles (more favors less)

Decrease in pressure(increase in volume) causes a shift towards more gas moles (less favors more)

Changes in pressure and volume only affect…

gases

What does a catalyst do to equilibrium?

Increases both forward + reverse reaction rates; Does not favor nor create stress

What does an inert gas(noble gas) do to equilibrium?

noble gases are nonreactive; Does not create stress

Equilibrium expression is a ration between…

products and reactants

In chemical equilibrium, the fact that the equilibrium constant (Keq or Kc) remains a constant value for a given reaction at a specific temperature means…

the ratio of product concentrations to reactant concentrations (each raised to their stoichiometric powers) is always the same at equilibrium, regardless of the initial amounts of substances present

Ka

Acids

Kb

Bases

What is the purpose of Keq?

Find which part of the reaction is favored

Ksp

Solubility product constant

Ksp indicates how much of a substance will…

dissolve in water; Product of Keq + conscentration of undissolved solid(usually 1)