5.1.1: How fast? Flashcards
Define rate of reaction
Change in concentration of a reactant or
a product per unit time
At a given instant, how could you calculate the rate of reaction?
Rate of reaction = change in concentration of reactants or products / time
If the order is zero with respect to a reactant, what does that mean?
Changing the concentration of the reactant has no effect on the rate
What is the effect on rate in a first order reaction?
Rate is directly proportional to the concentration
What is the effect on rate in a second order reaction?
Change in rate = change in concentration squared
Write a generic rate equation and state what each term means
Rate = k [X]x [Y]y;
k = rate constant for the reaction
[X] and [Y] are concentrations of species X and Y respectively
x and y are the orders of reaction with respect to X and Y
Do zero order reactants appear on rate equation? Why?
No, because they don’t have an effect on
the rate
How is overall order of a reaction calculated?
Sum of individual orders
How would you calculate the units of the rate constant?
Units of rate are mol dm-3s-1 and units of concentration are mol dm-3
Rearrange rate equation to get k=
Sub in units and cancel them out
When the overall order is 3, what would be the units for rate constant?
dm6 mol-2 s-1
How could you measure the rate of reaction experimentally (different methods)?
Use a colorimeter at suitable intervals if there is a
colour change.
If gas is evolved, use a gas syringe to collect
volume of gas evolved, or measure the change in
mass of the reaction mixture.
What does half life mean?
The time taken for concentration of a reactant to decrease by half
What is the symbol for half life?
t1/2
What is the relationship between first order reactions and half life?
First order reactants have constant half lives
What is the equation that is used to determine rate constant using half life in a first order reaction?
k = ln2 / t1/2
How would you draw a rate concentration graph?
Plot [A] against time, draw tangents at different values → draw a secondary graph of rate against [A]
What is the relationship between rate and time?
Rate ∝ 1/t
How to determine the rate constant from a rate concentration graph of first order?
k = rate / concentration
What is rate determining step?
The slowest step in a reaction with multiple steps
How does the rate determining step link to the species involved in the rate equation?
Any species involved in the rate determining step
appear in the rate equation. Species only involved
after the rate determining step do not appear in the
rate equation
For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?
The order of the reactant
The rate equation of a reaction is rate = k[NO]2
How many molecules of NO will be present in the rate determining equation?
2
What affects the value of the rate constant for a given reaction?
Temperature, nothing else
What is the effect of a 10℃ temperature increase on the rate of reaction, roughly?
Doubles rate of reaction
What is the Arrhenius equation? What does each term mean?
k = Ae(to the power of(−E/RT)) k= rate constant for reaction A= pre exponential factor (number of collisions between reactant molecules) e= mathematical quantity R= gas constant T= temperature in kelvin E= activation energy for reaction in joules
How can you convert the Arrhenius equation into a useful form for plotting a graph?
ln k= -Ea/RT + ln A
Graph of lnk against 1/T is a straight line:
gradient = -EA/R and y intercept is lnA
