How fast? Flashcards
What is the rate of chemical reaction defined as?
the change in concentration of reactant or product per unit time
What does changing the concentration of substances taking part in a reaction do?
usually (but not always) change the rate of reaction (rate equation shows this mathematically)
What is the general rate equation?
Rate = k[A]m [B]n
k= rate constant
A] = concentration of reactant A (moldm-3)
[B] = concentration of reactant B (moldm-3)
m = order of reaction with respect to A
n = order of reaction with respect to B
What is the order of reaction?
a measure of how a reactant affects the rate of reaction
what can the orders only be?
zero[A]0, first[A]1 or second[A]2
what are rate constants dependent on?
temp
How is order with respect calculated?
1st order- doubles, rate doubles
2nd order- doubles, rate quadruples
0 order- no effect on rate
Why does increasing temp without changing conc result in a fast rate of reaction?
At a higher temperature more particles have Ea or above. Hence there are more successfül collisions per unit time.
What happens if the 2 elements that are 1st order are doubled?
Rate quadruples
What does decreasing by a factor of 4 mean?
1/4
How does 2nd order affect rate?
If [C]× 2
rate × 4
If [C]× 3
rate × 9
If [C]× ½
rate × ¼
What are the units for rate?
moldm-3s-1
How can the rate of a chemical reaction be followed?
by measuring either the rate of increase
in the concentration of a product or the rate of decrease in the concentration of a reactant
What is the rate of reaction defined as?
the change in concentration of a reactant or a product
per unit time.
Rate = Change in concentration/time
Units = (moldm-3)/s= moldm-3s-1
What is half life?
the time taken for the concentration of a reactant to fall to half the original value.
What is the half life of a first order reaction?
A first order reaction has a constant half-life. The half-life of a first order reaction is independent of the concentration of the reactants.
Uranium-238 decays by a first order process with a constant half-life of 4.5x10 9 years.
The rate equation can be written as rate=k [238U].
(i) How long will it take a 200g sample of 238U to decay to 25g?
half life x 3
(c) Archaeologists can determine the age of organic matter by measuring the amount of 14C in a sample. The decay of 14C is first order (t1/2 of 14C = 5600 years).
Estimate the age of a piece of wood found to contain 1/16th as much 14C as living material.
1/16=4 half lives
4x5600
what does t1/2 stand for?
half life
How can you calculate the rate constant from its half life?
K=ln2/t1/2
What does a constant half life mean?
1st order
What is half life independent of?
initial conc
How is half life in a graph worked out?
the time at which the initial conc half’s
How do we produce a rate-concentration graph and what does it
show us?
We can draw a concentration-time graph and calculate the gradient (rate) at
different concentrations; hence we can plot the rate against concentration. Using two tangents on a concentration-time graph can therefore also be used to prove that a reaction is first order.
(b) We can record the overall time (t) taken for completion of a reaction at different concentrations of reactant and then use 1/t to represent rate. e.g. disappearing cross
What happens in the reaction between sodium thiosulfate and HCL?
This is a useful reaction to study
because two colourless solutions react together and a yellow precipitate is observed. Each time the experiment is carried out, the concentration of one of the reactants is varied whilst keeping the concentration of the other reactant constant. The time taken for the precipitate to form is measured.
How do you work out rate for the disappearing cross experiment?
Since the appearance of a product is being monitored in this case, the rate is inversely proportional to time. By calculating the value of 1/t at the time the precipitate appears, an approximation of the rate of each individual reaction can be found
To determine the time taken for reaction to reach completion (colour change) what is used?
coloromiter
What is a fist order reactant directly proportional to when calculating rate using time taken?
change in conc
what axis is rate on on a rate conc graph?
y axis
what are the two types of graphs?
conc (y) time (x)
rate (y) conc (x)
If the rate determining step (slow step) is the first step then what happens?
then the species on the left hand side of that step will appear in the rate equation, and their balancing numbers (stoichiometry) become the powers of the concentrations.
What is the definition of the rds?
The slowest step in the reaction mechanism of a multi-step reaction
which determines the overall rate
How do you identify the rds?
the stoichiometry of the second step and preceding steps match the rate equation.
What is an intermediate?
doesn’t appear in main equation
What are rate constants dependent on?
changes in temperature. An increase in temperature by 10OC approximately doubles the rate of the
reaction.
What do the letter stand for in the arhenius equation?
A = constant of proportionality known as the pre-exponential factor.
e = mathematical constant with the value of 2.71828. This is the inverse natural log.
Ea = activation energy of the reaction – units J (not kJ)
R = gas constant (J K−1 mol−1)
T = temperature – units K (not oC).
Which parts of LnK = LnA - Ea/RT equate to y=mx + c?
Y= LnK
m(gradient)= -Ea/R
x= 1/T
c= LnA
What can the arrhenius plot be used to identify?
An Arrhenius plot can be used to identify:
• A: the intercept on the graph is equal to In A
• The activation energy, Ea: the gradient is equal to –Ea/R, where R is the known gas constant, so this can be rearranged easily to find Ea
what is the inverse of Ln?
e ln
How do you calculate activation energy from the arrhenius equation?
From Arrhenius equation: Rearrange: Therefore Gradient (m) = −Ea / R
−Ea = Gradient × R
Ea = −Gradient x R
