Chemistry S2 - Chemical Reaction Rates (Kinetics)

Reaction Rates

how fast/slow reaction takes place

Rate of Reaction

rate of disappearance of reactants/rate of appearance of products depending on what is easier to observe (units of M/s)

The Collision Theory

For a reaction to take place certain conditions must be met

• # of collisions

• orientation of collisions

• sufficient energy of collisions

What is it called when collision theory conditions are met?

effective collisions

Orientation of Particles

for an “effective collision” to occur, reactant moles must be oriented in space correctly to facilitate breaking & forming bonds & rearrangement of atoms result in formation of product moles

What do potential energy diagrams represent?

the energy pathway of a reaction

Activated Complex

formed at top of curve (energy hill); at this pt in reaction process bonds are breaking, new bonds formed

Activation Energy

energy needed to form activated complex, if reaction doesn’t have enough energy reactants won’t react. if sufficient energy available, products are formed.

Reaction Rate Factors

• nature of reactants

• temperature

• surface area/stirring

• presence of catalysts

• concentration of reactants

Nature of Reactants

• rates depend on reactivity of reactant

• ionic reactants occur almost instantly (no electron rearrangement needed)

• covalent reactants occur slower (atoms & electrons need to rearrange)

Temperature

• increased temp increases energy of collisions resulting in more effective collisions

• takes energy to break bonds (endothermic)

• energy released when new bonds form (exothermic)

Surface Area/Stirring

• increased SA (smaller pieces) causes increased collisions

• stirring has same effect by removing reacted particles quickly

Catalyst

substance that increases rate of chemical reaction w/o being permanently changed/used in reaction

Presence of Catalyst

• works by providing alternate pathway that has lower Ea than original reaction

• in our bodies = enzymes

• lowers Ea, reactants/product energy or ΔH don’t change

Inhibitor

• opposite of catalyst, forms hew higher Ea/pathway (preservatives work like this)

• raises Ea, slows down reaction significantly, reactants/product energy or ΔH don’t change

Concentration of Reactants

• increased concentrations of reactants = more collisions

• for gases: increasing pressure causes increase in concentration due to more particles per volume increases collisions

The Rate Law

• mathematical expression relating concentration of reactants to reaction rate

• found experimentally, based on slowest step in reaction mechanism

What is the rate law equation?

• Rate = k[A]^x[B]^y

• products don’t appear in rate law, just reactants

Zero Order

concentration of reactant doesn’t affect rate of production of product. concentration of reactant doubled, production of product not affected

First Order

concentration of reactant does affect rate of production of product. concentration of reactant doubled, production of product double.

Second Order

concentration of reactant does affect rate of production of product. concentration of reactant doubled, production of product quadruples.

Third Order

concentration of reactant does affect rate of production of product. concentration of reactant doubled, production of product increases by a factor of 8

Overall Order

sum of all orders

Elementary Reaction

reaction in which reactants converted to products in single step

Can a reaction ever proceed faster than its lowest step?

No

Intermediates

substances that are produced by 1 reaction later consumed by another reaction. start as product, end as reactant. not included in rate law

Catalysts

substances that start reaction later produced by another reaction. start as reactant, end as product. not included in rate law

What does each peak represent in an energy diagram?

an elementary reaction/step in the reaction

What does each valley represent in an energy diagram?

an intermediate

What is the highest peak in an energy diagram?

the activated complex - represents slow step rate law is based off

Gibbs Free Energy (G)

spontaneous, reaction goes on own if given necessary energy to get over energy activation (Ea) “barrier”, tend to favor states that have low energy & high disorder (chaos)

-ΔG

reaction spontaneous (energy provided)

+ΔG

not spontaneous (energy absorbed)

Entropy (ΔS)

measure of disorder in system

2nd Law of Thermodynamics

entropy of universe must always increase

What things increase entropy of universe?

• system going from ordered —> disordered

• heat being released into surroundings

Entropy (+ΔS) Examples

• room getting messy

• wall crumbling

• going from solid-liquid-gas

• breaking into pieces

• any reaction more particles as products than as reactants

• increasing temperature

Gibbs Free Energy Equation

ΔG = ΔH - TΔS