Reaction rate is
a measure of how quickly or slowly reactants r consumed, or products r formed, in a chem reaction
reaction rate= (change in mass, mole,conc)/change in time
- by convention, reaction rates r always +ve cuz they measure how fast the reaction is proceeding or moving forward. A -ve sign is ignored (just means consumed)
Why is the reaction rate fast in the beginning?
- the conc of reactants is high & thus rate of collisions will be higher. The reactants r consumed to form the products very quickly. –> slope is very steep
- as reaction proceeds the conc of reactants decreases & slope is less steep
- when no reactants are left, slope=0
average reaction rate
- overal change in conc of reactant or product over given time period
- slope between 2 points. line=secant
Diff methods of measuring reaction rates
1) reactions involving gases–> r=ΔV/Δt or r=ΔP/Δt
2) reactions involving temp–> r=ΔT/Δt
3) reactions involving colours–> r=Δobservance/Δt
4) reactions involving ions
–> if ions r produced or consumed–> electricity is generated–> r=Δconductivity/Δt
–> if H+ or OH- ions r produced or consumed–> r= ΔpH/Δt
The Collision Theory
- For a reaction to occur, reactants must collide with each other effectively
1) the collision orientation of the reactants (collision geometry) must be favourable
2) The collision must occur with enough activation E so bonds in the reactants can b broken & new bonds formed in the products
the 2 conditons= effective collisions & only effective collisions make products
–> rate of reaction depends on # of effective collisions not total # of collisions
Activation Energy is
the min collision E needed for a reaction to occur –> this E is used to break the reactant bonds
The activation complex is
the fleeting molecular configuration that exists @ the of the E barrier that the reactants must surmount to become the products
-this complex is not a fully formed structure–> @ this point, the bonds in reactants=partially broken, bonds in products=partially formed
–> it can’t be isolated like the reactants or products
- cuz the bonds rearrange, the activated complex is high in E & unstable. From this point, the activated complex can go forward to form products or go backward & re-form the reactants
Enthalpy Change (ΔH)
- diff between the PE of the reactants & that of the products. This E is determined after the reaction has occured (a product has been formed)
6 factors affecting reaction rates: List Them
- concentration
- increased pressure
- surface area
- temp
- catalyst
- nature of reactant
factors affecting reaction rates: Conc
- increased conc = more collisions = rate increases
factors affecting reaction rates: Increased Pressure
- only applicable for gas state
- we increase the pressure of the system by reducing the volume of the container–> molecules r closer to each other= increased # of collisions
factors affecting reaction rates: Surface Area
- increased surface area, more exposure
- in heterogenous systems (reactants in dif states g/s, l/s …) increases# of effective collisions & increases rate –> greater SA of reactant solids, faster the rate
factors affecting reaction rates: temp
- maxwell boltzmann distribution of molecules
- temp increase= amount of fast moving molecules increase= collide MORE OFTEN & with MORE E, making it easier to overcome the Ea to break their bonds= rate increases
- vice versa
- temp does not change Ea
factors affecting reaction rates: Catalyst
- a substance that increases the rate of a chem reaction by providing an alternative pathway with lower Ea
- enzymes= protein-based catalysts. All enzyme= catalysts, not all catalysts r enzymes
- are neither reactant or product. –> it’s regenerated at the end of a reaction & can be used again
- unlike temp, the presence of a catalyst doesn’t change the shape of the curve. –> it makes the reaction go faster by shifting (lower) the Ea more to the left, allowing more molecules to overcome the Ea
- only lowers Ea, ΔH doesn’t change
factors affecting reaction rates: nature of reactant
1) similar elements react similarly but @ diff rates
2) reactants in gas or aqueous system react more quickly than solids–> more KE, more frequent collisions & collide with more E
3) Small simple ions & molecules react more quickly than larger ones–> reactions r typically done in a medium (a solvent that can either be a liquid/gas). Small molecules have less drag/friction / resistance due 2 having smaller SA. They move more quickly, colliding with each other more often
4) Charged particles in solution react more quickly than neutral molecules –> neutral reactant molecules r moving randomly & colliding with each other by chance, whereas charged ones will attract each other & have a higher chance of collision
5) weak bonds r easier to break (smaller Ea), strong bonds r harder (higher Ea)
Reaction Mechanism
- rather than occuring as 1 massive collision involving all reactants molecules, most reactions proceed as a series of steps= elementary steps
- the series of elementary steps that take place as a reaction proceeds from reactants to products is called the “reaction mechanism”
Reaction Intermediate
- compounds that r formed in 1 elementary step & used up in a subsequent elementary step, so they do not appear in overall equation
Rate Law
- the relationship between the conc of the reaction and the rate of the reaction
- the rate will always b proportional to the producer of the initial conc of reactants where these conc are raised to some exponential values
- the rate law depends on the RDS of a reaction mech so it must be determined experimentally
