Rate equations (3.1.9) Flashcards
what does […] mean?
concentration of … measured in mol dm^-3 eg. [HCl] = 0.1
how do reactions occur and why?
via a series of bimolecular steps (each step involves 2 particles) because the chance of more than 2 particles colliding at the same time, with the right amount of energy and in the correct orientation is essentially 0
what rate equation relates the rate of a chemical reaction to the concentration of reactants?
Rate = k [A]^m [B]^n
rate in mol dm^-3 s^-1
concentration of A and B = mol dm^-3
m and n = orders of reaction with respect to reactants A and B
k = rate constant
what are the orders m and n restricted to?
0, 1 and 2
what is the power to which the concentration is raised called?
the order of reaction with respect to… (if the power is 1 and the concentration is of A, the order of reaction with respect to A is 1)
what is the relationship between the rate factor and concentration factor?
rate factor = (concentration factor)^order
how can you work out the total order for a reaction?
add the individual orders together (m + n)
what is the relationship between concentration and rate for zero order?
the concentration of A has no effect on the rate of reaction ie. if concentration doubles there is no change in rate
r = k[A]^0 = k
what is the relationship between concentration and rate for first order?
the rate of reaction is directly proportional to the concentration of A ie. if concentration doubles, rate doubles
r = k[A]^1
what is the relationship between concentration and rate for second order?
the rate of reaction is proportional to the concentration of A squared ie. if concentration doubles, rate increases by 4 times (2 squared)
r = k[A]^2
what are the key features of the rate constant (k)?
- the unit depends on the overall order of reaction (work out from rate of reaction)
- the value is independent of concentration and time - it is constant at a fixed temperature
- the value refers to a specific temperature and increases if we increase temperature
what is the unit of k for a 1st order overall reaction?
s^-1
what is the unit of k for a 2nd order overall reaction?
mol^-1 dm^3 s^-1
what is the unit of k for a 3rd order overall reaction?
mol^-2 dm^6 s^-1
how do you investigate rate of reaction to determine rate equations?
- reaction is carried out using known concentrations of each substance
- rate of reaction is measured and recorded
- the concentration of one substance is changed by a known amount eg. doubled while the others are kept constant
- the new rate is measured and recorded
- the concentration of each substance is changed in turn, using the original concentrations of the others, and the new rate is measured and recorded each time
what is the definition of order of reaction?
the power to which the concentration of that reactant is raised in the rate equation
what is the definition of rate constant?
a measure of the speed of a chemical reaction
for the equation P + Q -> R with rate equation rate = k [P]^2[T], what is the effect on rate and new initial rate when:
a. initial rate is 6.0 and change in concentration of reactants is [P] x 2
b. initial rate is 60 and change in concentration of reactants is [R] ÷ 5
c. initial rate is 20 and change in concentration of reactants is [P] x 2, [Q] x 10, [T] ÷ 1.5
a. x 4, 24
b. none, 60
c. x 2.67, 53.3
how can you calculate the order with respect to a particular reactant using data provided?
- select which experiments you are using (ideally where concentration of reactant you are looking at stays the same)
- identify what happens to the concentration of each reactant and the rate between the experiments
- use rate factor=concentration factor^order to work out the order for the reactant
what does the Arrhenius equation show?
how the rate constant (k) varies with temperature
what are the components and their units of the Arrhenius equation?
k = rate constant
Ea = activation energy in J mol^-1
R = gas constant
T = temperature in Kelvin
A = Arrhenius constant
e = mathematical constant (use e button on calculator)
how can the Arrhenius equation be rearranged and what can this be used for?
ln k = – E a / R T + ln A (ln = log)
can use this with experimental data to plot a graph of ln k on the y-axis and 1/T on the x-axis
the gradient of the graph will be - E a / R, which can be used to calculate E a (E a = -gradient/R)
what is the rate equation?
an experimentally determined relationship
what can the orders with respect to reactants do?
provide information about the mechanism of a reaction
how can concentration-time graphs be used to deduce the rate of a reaction?
by drawing a tangent and calculating its gradient
what do the concentration of reactant-time graphs look like for each order of reaction with respect to that reactant?
0 order - grad is constant so straight line going downwards from top of y to end of x
1st order - grad decreases so curved line starting at top of y and going down to end of x
2nd order - grad decreases very quickly so same as 1st order but more steep
how can concentration-time graphs be used to deduce the initial rate of a reaction?
draw a tangent and calculate its gradient at time=0
how can rate-concentration graphs be used to deduce the order with resect to a reactant?
by looking at their shape
what do the rate-concentration graphs look like for each order of reaction?
0 order - straight horizontal line (grad is 0)
1st order - straight line through the origin (grad is constant)
2nd order - line starting from origin and curving upwards (grad increases)
rate-concentration^2 graph for 2nd order - straight line through the origin
what is the rate determining step (RDS)?
the slowest step of a chemical reaction and the one that determines the rate at which the overall reaction proceeds
what must be the case for any species which appears in the rate equation?
it must affect the rate
if a species affects the rate, which part of the reaction must it take part in?
the rate determining step or any step that occurs before the rate determining step
which species in a reaction won’t appear in the rate equation?
intermediate species (those formed in one step and used in a subsequent step)
what does the order with respect to a reactant show?
the number of particles of that reactant that are involved in the rate determining step or any step before it
how do you identify the rate determining step in a reaction?
- look at the rate equation or work it out
- compare the rate equation to the equations for each step in the reaction
- any species in the rate equation must appear in the rate determining step or a prior step
- the step in which the last substance in the rate equation is ‘ticked off’ is the rate determining step
how can you use the rate determining step to find the rate equation?
ignore any steps after the rate determining step - write the rate equation using all the reactants with the order with respect to them being the number of times they appear (don’t include any intermediate species)
