Rates Flashcards
Rate of reaction
Change of concentration over time
Order
The effect the concentration of the reactant has on the rate
Overall order
The sum (addition) of the orders with respect to each reactant.
Rate constant
A proportional constant, which multiplies with the concentration and orders to equal the rate.
Half-life
The time for half of the reactant to be used up.
Rate-determining step
The slowest step in the series of steps which make a reaction
Example of a rate equation
Rate = k [A]^m {B]^n
Zero order
When the concentration of a reactant has no effect on the rate {A]^0
First order
The rate depends on the concentration raised to the power of one, so if the concentration is tripled the rate is also tripled. [A]^1
Second order
The rate depends on the concentration raised to the power of two, so if the concentration is tripled the rate increases by a factor of 9. [A]^2
Determining the units of rate constant K
Put the units of the rate and divide it by the units of the concentration, if it is second order you divide it by the units squared
How to find out K
Divide the rate by the concentration, if it is a second order reaction you square the concentration
How to find out orders
You compare the initial rate at various concentrations and see how much the rate changes, i.e. does it double or is it squared.
How can you monitor rate
Colorimeter, mass loss, gas collected
Shape of a zero order concentration-time graph
Straight light with a negative gradient
Shape of a first order concentration-time graph
Curve down with decreasing gradient over time, less steep then second
Shape of a second order concentration-time graph
Curve, gradient decreases over time, more steep then first
Rates and half life
First order reactions have a constant half life
How to calculate rate from a graph
Draw a tangent at a particular concentration, the gradient of the tangent is equal to the rate.
Calculating rate constant from half life
K = ln2 / half life
Zero order rate-concentration graph
Horizontal straight line with no gradient, y-intercept is K. Reaction rate does not change with concentration
First order rate-concentration graph
Straight line that goes through the origin. Rate constant an be determined by measuring the gradient
Second order rate-concentration graph
An upward curve with an increasing gradient. Rate constant can be determined by drawing a second graph of the rate against concentration squared. This results in a straight line going through the origins, the gradient equals the rate constant
How do you if a rate mechanism and rate determining step is correct?
The rate equation only includes reacting species involved in the rate determining step. The orders in the rate equation match the number of species involved in the rate determining step.
The effect of temperature on rate constant
As temperature increases the rate and rate constant increase in value.
Why does increased temperature increase the rate constant
The Boltzman distribution is shifted to the right so more particles have an energy above the activation energy. Particles move faster and increase more frequently
The Arrhenius equation
K = Ae^-Ea/Rt R= gas constant T=temperature Ea= activation energy A = frequency factor
Logarithmic form of the Arrhenius equation
ln(k) = (-Ea / RT) + ln A
How to get values for Arrhenius equation from a graph
Gradient is -Ea/R
Y intercept is A
How does increasing the concentration affect the rate of reaction
An increase in concentration increases the rate of reaction. An increase in concentration increases the number of particles in the same volume. The particles are closer together and collide more frequently. In a given period of time there will be more effective collisions and an increased rate of reactions.
Effect of pressure on the rate of reaction
As the pressure increases the concentration of gas molecules increases as the same number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently leading to more effective collisions in the same amount of time.
How to measure reaction rate
You draw a tangent and calculate the gradient. The gradient is equal to the rate
Catalyst
Increases the arte of reaction without being used up itself
