Rates Flashcards
Dr Martin
What is the rate of a reaction?
the speed at which reactants are used up or products are formed.
rate =
change in concentration/time
What is the rate of reaction considered in terms of?
decrease in concentration of reactants per unit time/increase in concentration of product per unit time
What does d[A] / dt mean?
the rate of change of the concentration of A
What does [A] mean? What are the units?
concentration of A
usually mol dm⁻³ s⁻¹
What is the link between the rate at which the reactants are used up and the rate at which the products are formed?
they are equal
Draw [A] against time
see page 1
How would the rate of reaction be determined from the graph of [A] against time?
draw a tangent at any point and calculate gradient.
What experiments could be used to measure the rate of reaction? Draw the graphs.
volume of gas collected (or water-filled, upturned measuring cylinder for gases that don’t dissolve in water (CO₂ dissolves in water))
decrease of mass
see diagrams and graphs on page 2
How can colour changes be followed in a reaction?
by using a colorimeter
How is a colorimeter used?
light source –(high intensity)–> sample –(lower intensity)–> detector (some light is absorbed by sample)
see diagram on page 3
What conditions must be fulfilled for a collision to result in a reaction?
collision involves more than a certain minimum amount of energy – above activation energy
molecules must collide with the correct orientations
What is the activation energy?
the minimum amount of energy that is required before a collision to result in a reaction
Why is activation energy needed to result in a reaction?
to overcome repulsions (internuclear and between electrons), to start breaking bonds, to deform molecules, to allow rearrangement of atoms.
When does a successful collision occur?
when E ≥ Ea
What factors affect the rate of a chemical reaction?
concentration of reactants
surface area of solid reactants
temperature
catalysis
light – photocatalysis
What is the effect of concentration on the rate of reaction?
particles collide more often with more particles in a certain volume and so there is a greater chance of a successful collision occurring in a certain time.
What is the effect of surface area of solid reactants on the rate of reaction?
there are more particles exposed at the surface, so there is greater chance of another reactant particle colliding with a particle on the surface and a reaction occurring.
Write the rate equation.
Rate = k[A]^m[B]^n
m and n can only be 0, 1, 2. They are the orders of the reaction. The overall order of the reaction is m+n
How do we find the rate equation? What does it do?
it is experimentally determined
related the rate of reaction to the concentrations of substances in the reaction mixture.
what is k called? what is it?
the rate constant
a constant of proportionality relating the concentrations in the experimentally determined rate equation to the rate of a chemical reaction.
does the rate constant change with temperature?
yes (doubles with every 10 degrees it increases)
What is the order of a reaction?
the exponent to which the reactant’s concentration is raised in the experimentally determined rate equation.
How would the overall order of the reaction be found?
the sum of the exponents of the concentration terms in the experimentally determined rate equation.
How to know if the order is 0, 1 or 2?
no change in concentration = 0
concentration and rate increase by same amount = 1
rate goes up by how much concentration goes up by squared = 2
What are the units of the rate constant?
you have to find them out every time by using the rate equation, but with the units.
What do zero order reactions show?
the rate is independent of the concentration
see page 9 for graphs
What is the half life like in first order reactions? What is the half-life?
constant
time taken for the concentration of a reagent to fall to half its original value
t½ =
In2/k = 0.693/k
what do first order reactions show?
rate is directly proportional to the concentration
gradient = k
see page 9 for graphs
What do second order reactions show?
the rate of reaction is proportional to concentration squared.
gradient is y = ax², which is half a parabola
What are the rate equations for 1st, 2nd, and 0 order reactions?
0: Rate = k
1: Rate = k[?]
2: Rate = k[?]²
How to determine the rate equation, rate constant, and order by experiment.
A + B –> C
experiments using fixed amount of B and change concentration of A.
each experiment yields a graph of [A] against time.
initial rates worked out from the graphs by drawing tangents
find order of reaction with respect to A by plotting a graph of initial rate against [A] (this is the only point we know the [A])
repeat using fixed amount of A and change [B]
rate equation and rate constant may be worked out using the orders.
What does a reaction mechanism show?
all the simple steps which make up a more complex reaction.
Consider: A + B + C –> D Rate k[A][B]
Find the reaction mechanism. What is the rate determining step?
step 1: rate = k₁[A][B] (rate determining step) (slow step)
step 2: rate = k₂[C][Q]
Why is the slowest step in a reaction important?
it is the rate determining step, and only the steps up to the rate determining step are including in the overall rate expression.
What is molecularity? What are the types?
the number of reacting particles that take part in a particular elementary step in a mechanism.
unimolecular – single particle involved in step
bimolecular: two particles involved in step
termolecular: three particles involved in step.
CH₃COCH₃ (aq) + I₂ (aq)–> CH₃COCH₂I (aq) + HI (aq)
The reaction is acid catalysed.
Rate expression: Rate = k[CH₃COCH₃][H⁺]
The rate expression does not include I₂, so it can be deduced that…
propose the mechanism
the order with respect to I₂ is 0, so I₂ is involved in a fast step that takes place after the rate determining step.
CH₃COCH₃ + H⁺ –> CH₃COCH₃⁺ SLOW
CH₃COCH₃⁺ + I₂ –> CH₂ICOCH₃ + HI + H⁺ FAST
must say whether fast or slow.
What is the relationship between temperature and kinetic energy?
average kinetic energy of the molecules in a gas is proportional to the absolute temperature (K)
T (K) ⋉ ½mv²
What is effect of temperature on the rate of reaction?
as temperature increased, molecules move fast and the collision rate increases. This is only a small effect though, and rate only increased by a very small amount.
Draw the graph of Maxwell-Boltzmann Energy Distribution.
See page 15
skewed
y-axis: number of particles
x-axis: kinetic energy
shaded section on the right at the Ea – proportion of particles that can react
asymptote eventually
How does the Maxwell-Boltzmann Energy Distribution graph change as temperature is increased?
see page 15
skewed part is moved over to the right and squished down.
shaded area is at same point, but there is more area in it as more particles can react.
proportion of particles with energy greater than the activation energy has increased significantly, so the rate increases significantly
What is the reason for the increase in rate of reaction with an increase in temperature?
increase in the proportion of particles with energy greater than or equal to the activation energy.
In the expression Rate = k[A][B], how is the effect of temperature variation shown?
value of the rate constant changes
What happens to the rate constant as the temperature increases?
the rate constant increases exponentially
What does the Arrhenius factor mean?
contains information related to the number and frequency of collisions, and the orientation of the collisions
(A is essentially a constant)
What does e^(-Ea)/(RT) represent in the equation: k = Ae^(-Ea)/(RT)?
the fraction of collisions that have E ≥ Ea (but not all collisions with E ≥ Ea result in reaction. Must collide with correct orientation and A contains a factor that allows for this)
What are the units in the equation: In k = (-Ea)/(RT) + In A
-Ea mut be in J mol⁻¹
R must be in J K⁻¹ mol⁻¹
What happens if the temperature increases by 10K?
rate will double
How to find the activation energy by a graphical method?
conduct a series of experiments at a range of temperatures
calculate rate constant for each temperature
plot a graph of In k (y-axis) against 1/T (x-axis), where T is absolute temperature (K) (straight line)
determine gradient (- value with units K)
put gradient = to -Ea/R (gas constant)
cancel negative signs and multiply gradient by R to get value of Ea in J mol⁻¹
divide value obtained by 1000 to get activation energy in kJ mol⁻¹
determine the intercept of the line on the y-axis (In K) – equal to In A
use buttons SHIFT/2nd In on calculator to convert In A to A.
units of A are same as those of k
What is a catalyst?
a substance which increases the rate of a chemical reaction without being used up in the reaction.
How does a catalyst work?
allowing the reaction to proceed by an alternative pathway of lower activation energy
draw the graph of when the mechanism changes with a catalyst
See page 18
Show the effects of a catalyst in the Maxwell-Boltzmann distribution.
See page 18
How does a catalyst affect the rate equation, Rate = k[A][B]
increases the rate constant
