Kinetics Flashcards
Equation for the rate of a chemical reaction
rate = change in [x]
change in time
units = moldm-3s-1
The rate can be determined from the gradient of a concentration v time graph.
A plot of [x] against time gives rate as a gradient.
Rate Expression
k[A]m[B]n
Units of k
First order - k = s-1
Second order - k = mol-1dm3s-1
Third order - k = mol-2dm6s-1
Zero order reaction graphs
Conc against time - no change in rate so no change in gradient, decreasing negative line.
Rate against conc - Straight line horizontally.
First order reactions graphs
Conc against time - decreasing exponential.
Rate against conc - increasing line.
Second order reaction graphs
Conc against time - negative exponential curve. ( +ve gradient for products, -ve gradient for reactants).
Rate against conc - positive exponential curve.
Changes in rate - zero order
[] x 2 = rate has no change
[] x 3 = rate has no change
Changes in rate - first order
[] x 2 = rate x 2 (2^1)
[] x 3 = rate x 3 (3^1)
Changes in rate - second order
[] x 2 = rate x 4 (2^2)
[] x 3 = rate x 9 (3^2)
Changes in rate - third order
[] x 2 = rate x 8 (2^3)
[] x 3 = rate x 27 (3^3)
The effects of temperature change on rate
When temperature is increased, rate increases.
There are more collision with Ea > E, so more frequent successful collisions.
How the rate constant varies with temperature
It increases exponentially.
Increasing the temperature means that MANY more collisions have > Ea.
Activation Energy equation
k = A (e^-Ea/RT)
What is the rate determining step
The species in the rare equation are the reactants involved in the rate determining step or before the rate determining step
A +2B —> AB^2. slow
AB2 + B —> AB3 fast
which the the rate determining step and rate equation
the slow one is RDS
rate = k[A] [B^2]
