Chapter 18 - Rates of Reactions Flashcards
Define rate
a change in concentration divided by a change in time
What are the units for rate?
moldm-3s-1
List 5 ways of determining reaction rates experimentally
measuring pH changes measuring mass loss (with gases) measuring formation of a precipitate measuring changes in colour measuring changes in pressure or volume
How can you calculate rate from a graph?
Rate can be calculated by drawing a tangent to a curve and working out the gradient of the tangent
How is zero order rate and concentration linked?
rate is independent of [A]
How is first order rate and concentration linked?
rate will change by the same amount as the [A]
How is second order rate and concentration linked?
rate will change by the square of the change of [A]
what is the rate equation?
rate = k [a]n [b]m
How can orders be calculated?
can only be calculated experimentally and not by looking at the overall equation.
what is the units for rate constant?
they are dependent on the number of concentration terms in the rate equation.
What is the symbol for rate constant?
k
What does a large value for k indicate?
a fast rate of reaction
What does a small value for k indicate?
a slow rate of reaction
When does k change?
When concentration changes or when temperature changes
Define half life
the time taken for the concentration of a reactant to fall to half of its original value.
What are the 2 main experimental methods to obtain information about order?
continuous monitoring - set up 1 reaction mixture only and follow how the concentration varies with time (continuous)
initial rates method - set up several reaction mixtures and vary the concentration of 1 reactant per iteration of the experiment allowing us to find initial rate for each reactant by measuring how long an overall colour change takes
What assumptions can we make to show that initial rate remains constant over the course of the clock method experiment?
the temperature remains constant
the concentration of the reactants doesnt change
the reaction has not proceeded to far when the end point is reached
rate remains constant during the time that we are measuring
What are the 2 ways of determining rate constant from experiment?
From the rate equation
From the half life
What is the equation for calculating rate constant from half life?
k = (ln)2 / t1/2
What is used as an indicator in the iodine clock experiment? When does the blue-black colour form?
sodium thiosulfate and starch.
when the sodium thiosulfate is used up I- begins to react with starch solution to form the blue-black colour
What is the half equation for the iodine clock experiment?
H2O2 + 2H+ +2I- –> 2H2O + I2
How can you work out the order of reaction in the iodine clock experiment?
Vary the concentration of H2O2 and keep all other reactants the same leading to a change in rate, and we can use this to calculate orders
describe a concentration-time graph for zero order
proportional showing negative correlation
describe a concentration-time graph for first order
Negative correlation exponential
describe a concentration-time graph for second order
Negative correlation exponential but steeper graph and levels out sooner
describe a concentration rate graph for zero order
Straight line horizontal
describe a concentration rate graph for first order
proportional showing positive correlation
describe a concentration rate graph for second order
Positive correlation exponential
What is the rate determining step?
the slowest rate in a multistep reaction
Why can changing the concentration of a particular reactant not necessarily have an impact on rate?
Because collisions between 2 or more particles are unlikely to occur simultaneously so only 1 step will affect the rate so the change in concentration will only cause a change in the reactants involved in the RDS
How can you work out the 2 step reaction mechanism when given the rate equation?
decide which of the species is involved in the slow step (these are all of those which are in the rate equation)
decide which possible products can be formed from the reactants that you used.
Look at the overall equation and decide what still needs to be accounted for.
Use the products of the RDS and the reactants which still need to be used up to make the final product.
What happens to k when temperature increases?
It increases
What happens to k when rate of reaction increases?
It increases
As temperature rises by 10 degrees celcius what happens to rate and rate constant?
They double
Which 2 factors cause rate to increase with increasing temperature?
An increased proportion of particles exceed Ea
More frequent successful collisions
Which factor has the biggest impact on rate?
An increased proportion of particles exceed Ea
How can you calculate the pre- exponential factor from lnA?
e to the power of lnA (e cancels out ln)
How can you calculate lnA from a 1/T lnK graph?
Find the y intercept
How can you calculate -Ea/R from a 1/T lnK graph?
Find the gradient of the line
Give the rearranged arrhenius equation
lnK = (-Ea/R) x (1/T) +lnA
Give the standard arrhenius equation
k = Ae to the power of`(-Ea/RT)
What unit is temperature in for the arrhenius equation?
Kelvin
What is R equal to?
the gas constant (8.314 Jmol-1/k-1)
what is A in the arrhenius equation?
The frequency factor
What does the arrhenius equation calculate?
k - rate constant
