Reaction Kinetics Flashcards
Rate of reaction
Change in concentration of reactant consumed or product produced per unit time.
Half-life of a reaction, t(1/2)
The amount of time needed for concentration of reactant to fall to half its initial value.
What is rate constant, k, dependent on?
1) Temperature 2) Activation Energy
Zero-ordered Rxn:
Eqn: Rate = K[a]^0
Rate-[A] Graph: Straight horizontal line
[A]-T graph: Straight decreasing graph
K unit = mol dm-3 s-1
First-Order Reaction
Eqn: Rate = K[a] Rate-[A] Graph: Straight increasing graph passing through origin [A]-T graph: Curve that is decreasing K unit = s-1 Unique: Has half-life
Equation of half-life
T(1/2) = ln2/k
Number of half life elapsed
[A1]/[A2] = (0.5)^n
Second-Order rxn
Eqn: Rate = K[a]^2
Rate-[A] Graph: half of x^2 graph
Rate-[A]^2 graph: Straight increasing graph
[A]-T graph: Straight decreasing graph but t(1/2) not constant
K unit = mol-1 dm3 s-1
When can Pseudo First-order Rxn occur
1) When one of Rxt is in large excess
2) When one of Rxt is also the solvent
How to use inspection method
“Comparing Expt _ and _ when [] and [] are constant
When [the one uw find] is increased/decreased by __ times, initial rate is increased/decreased by __ times.
Hence, rxt is __-order wrt [~]”
How to answer half life thingy
“Since half-life is (approximately) constant at ___ mins, the reaction is first-order wrt [__]”
Collision Theory
A) Chemical reaction occur as a result of particles colliding with one another
B) Only particles which collide with energy greater or equal to Activation Energy, Ea, will be able to react. These Collisions are called effective collisions.
Activation Energy
The minimum energy molecules need to possess for a reaction to occur
Factors affecting rate of reaction
1) Size of Particle
2) Concentration / Pressure of a gas
3) Temperature
4) ) Catalyst
How does particle size affect rate?
^ Surface area of a solid reactant -> ^ Frequency of effective collisions between X and Y -> ^Rate
How does concentration/pressure affect rate?
^Pa/[_] -> Particles are closer -> ^Number of collision -> ^Frequency of effective collision -> ^ Rate
How does temp affect rate?
^Temp -> ^ KE and move about faster -> ^Number of particles having energy greater than or equal to Ea -> ^Frequency of effective collisions-> ^ Rate
On Boltzmann Distribution Curve, when Temp increases what changes
Peak go rightward + lower
Amt of particle at Ea is increased
How does catalyst affect rate?
Catalyst provides an alternative reaction pathway with lower activation Energy, Ea
Number of particles having energy greater than or equal to the Ea increases
Frequency of effective Collision increases
Rate increase
Catalyst
A substance that increases the rate of chemical reaction with itself being regenerated and chemically unchanged at the end of the reaction.
Type of Catalyst
1) Heterogenous Catalyst
2) Homogenous Catalyst
3) Autocatalyst
4) Enzymes
How does heterogeneous catalyst work
1) Reactants are adsorbed onto catalyst surface
2) Due to availability of 3d and 4s electron, catalyst will form weak bond with the reactants. This weakens the bond in the reactant molecules. This a) lower Ea and 2) Increases surface concentration
3) When product is formed, they are desorbed from the catalyst surface
How to quench
1) Use large amt of cold water to lower [reactant] and slow down the reaction
2) Quenching agent to react immediately with one of reactant and stop rxn
3) Inhibitor/Negative Catalyst to slow down rxn
