3.5 - Chemical Kinetics Flashcards
What factors effect the rate of a chemical process?
- temperature
- concentration
- surface area of solid
- presence of a catalyst
What is the reaction rate?
Change in concentration of a reactant or a product per unit time
- how quickly reactants are used up
- how quickly products are formed
What is the average rate?
Total change in the concentration of reactants/products divided by the time taken for the reaction to take place
What is the instantaneous rate?
Rate of change occuring at a specific time
(find the gradient of a tangent y/x)
What is the rate equation?
Mathmatical expression in which the rate is given in terms of concentrations of the reactants and the rate constant, k
- each term is raised to a power which relates to the effect the reactant has on the rate of reaction
rate = k(A)^x(B)^y
The sum of the powers gives overall order, can be anything but usually 0, 1 or 2
What is zero order?
The concentration of the reactant does not affect the rate of the reaction
What is first order?
If the concentration doubles, the rate will double
What is second order?
If the concentration of the reactants doubles, the rate of the reaction will be 4x faster
What are rate graphs?
- rate vs concentration/ concentration vs time
- the shape of these graphs can tell us the order of the reaction with respect to the reactant
- the gradient of these graphs tells us the value of the rate constant or the rate respectively
What does 1st order graph look like for RVC
Straight diagonal line
The rate is proportional to the conc of A
If the concentration of A doubles, so does the rate
What does zero order graph look like for RVC?
Straight line horizontal
The rate is independent of the concentration of A in a reaction which is zero order in respect to A
If A changes, the rate stays the same
What does 2nd order graph look like for RVC?
Curved upwards
The rate is proportional to the second power of A
If conc of A triples, the rate will be 9x faster
If plotted (A)2, line will look like 1st order
What does zero order graph look like for CVT?
As the reaction proceeds A is used up and the concentration drops
A does not affect the rate
Diagonal line downwards
What does a 1st order graph look like for CVT?
A is used up and the concentration drops
A does not effect the rate and so its dropping concentration causes rate to decrease and hence the speed in which its being consumed reduces
Curved line downwards
What does a 2nd order graph look like for CVT?
A is used up and conc drops
A affects rate more significantly and causes rate to decrease sharply, steeper than first order
Steep curved line downwards
What is half life and what is the equation for it?
The half life of a reaction, t1/2, is the time taken for the concentration of the reactant to reach half its initial value
(A)t1/2 = 1/2(A)t0
How can we tell apart 1st and 2nd order for CVT graphs?
The half life of a first order reaction is independant of concentration, so the half life stays the same.
What is the relationship/equation for the rate constant (k) and the half life of a reaction?
t1/2 = 0.693/k
What is a rate determining step?
The slowest elementary step and as such limits the overall rate of a multistep mechanism
As a general rule, if a reactant does not appear in the rate equation, it must come after the rate determining step
What is the link between rate and temperature?
As you increase the temperature, a larger fraction of particles have sufficient energy to react
They react to form intermediates of the products having overcome the activation energy and collide in the correct orientation
What is the Arrhenius equation?
k = Ae ^ -Ea/RT
k = rate constant
T = temperature (in kelvin)
A = pre-exponential factor, a constant for each chemical reaction
Ea = activation energy
R = gas constant
What does the Arrhenius equation tell us about the factors affecting rate of reaction?
If we raise the value of T, the power to which Ae is raised would get smaller
As the power is negative, the smaller the power the larger Ae becomes (and thus k larger too)
Decreasing the activation energy also has the same effect and increases k (seen in adding a catalyst)
Raising the temperature increases the value of the rate constant and hence the rate
