6 REACTION KINETICS Flashcards
rate of reaction
the change in concentration of a reactant or product per unit time
unit for rate
moldm-3s-1
what can be determined through experiments
rate equation, order of reaction and rate constant k
at a given temperature, rate constant k is
constant
t1/2 =
ln2/k
conc-time graph: straight line
rate is constant and is independent of [R]. The reaction is zero order w.r.t. [R]
conc-time graph: curve with constant t1/2
the reaction is 1st order w.r.t. [R]
conc-time graph: curve with varying t1/2
find rate
order of reaction wrt [R] can be determined from the gradient. Plot rate-conc to find.
t1/2 of [P]-time: take t1/2 at
[P]= 1/2 & [P]=3/4
rate-conc graph: 0 order
The graph is a horizontal line. Hence, rate is independent of [R], The reaction is zero order wrt [R]
rate-conc graph: 1st order
The graph is a straight line passing through the origin. Hence, rate is directly proportional to [R], The reaction is first order wrt [R]
rate-conc graph: >1st order
then how to find order of reaction?
The graph is a curve. The reaction is not first order wrt [R]
eg rate=k[R]^2 plot rate against [R]^2
OR lg rate= lg k+ 2lg[R]
rate-time graph: 0 order
horizontal line
rate-time graph: 1st order
decreases exponentially with time
rate-time graph: >1st order
decreases exponentially with time with a steeper gradient
initial rate
[I2]/t when [R] is small
rate ∝1/t when the total volume of the mixture is kept constant
activation energy Ea
the energy barrier the reactants must overcome in order to become the products
energy profile diagram
y-axis: energy. x-axis: reaction progress
2 factors affecting rates of reaction
1 frequency of effective collision
2 proportion of particles with KE≥Ea
factors affecting rates of reaction: conc or pressure
as conc↑, ↑particles per unit volume, particles are closer, frequency of effective collision increases, so rate increases
factors affecting rates of reaction: temperature
as T↑, ↑particles with average KE≥Ea↑, frequency of effective collision increases, so rate increases
catalyst
a substance that speed up a chemical reaction by providing an alternative reaction pathway of lower activation energy, while remaining chemically unchanged at the end of the reaction
heterogeneous catalyst: Haber process: adsorption
N2+ 3H2⇌ 2NH3
1 The gaseous molecules are adsorbed onto the active sites by the formation of weak attraction forces.
2 This process brings reactant molecules closer together, weaken the covalent bonds in the reactant molecules,
thus lowering the Ea and
4 orientates reactant molecules correctly.
heterogeneous catalyst: Haber process: desorption
N2+ 3H2⇌ 2NH3
product NH3 desorbs from the catalyst surface so that active sites are exposed for further reaction
heterogeneous catalyst: Haber process: order of reaction wrt [R] is zero when
the catalyst is saturated (active sites are fully occupied)
autocatalysis
reaction is catalysed by one of its products
enzymes as catalyst, they are h___geneous catalyst
homogeneous
2 characteristics of enzymes as catalysts
1 highly specific
2 highly efficient
relationship of [substrate] and rate in an enzyme-catalysed reaction
at low [substrate]: reaction is 1st order wrt [substrate]
at high [substrate]: reaction is zero order wrt [substrate], [enzyme] then matters
2 factors affecting enzyme activities
state & draw
need optimal T & pH
graph of T is skewed
At high pressure, the rate of decomposition of ammonia is independent on the partial pressure of ammonia. But at low pressure, the rate is ∝ partial pressure of ammonia. why?
when [NH3] is low,
1 the active sites on the surface of the catalyst are not saturated,
2 more NH3 can be adsorbed onto the site as P NH3↑,
3 so rate∝ partial pressure of NH3
when [NH3] is high,
1 the active sites on the catalyst are saturated,
2 H2 & N2 must desorb from the active site b4 any NH3 molecules are adsorbed,
3 so ↑ P NH3 has no effect on the rate
