Topic A6 - Kinetics II
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How do you know if a reactant is first order?
rate is proportional to concentration
What does a concentration against time graph look like for a first order reactant?
-decreasing curve -with constant half-life
What does a rate against concentration graph look like for a first order reactant?
-straight line through origin (directly proportional)
What does a concentration against time graph look like for a second order reactant?
-decreasing curve -not a constant half-life
What does a rate against concentration graph look like for a second order reactant?
-increasing curve (y=x^2 shape)
How do you determine the rate constant (k) from the constant half-life of a first order reactant?
k = ln2/ half life
What is continuous monitoring?
taking continuous measurements throughout the course of a reaction
What data can you collect as continuous monitoring?
-volume of gas -mass change -absorbance (using a colorimeter)
What does the Arrhenius equation show?
exponential relationship between the rate constant and temperature
What does the exponential factor of the Arrhenius equation show?
proportion of molecules that exceed the activation energy and hence have sufficient energy for reaction to take place
What is the logarithmic form of the Arrhenius equation?
ln(k) = -Ea/ RT + ln(A)
What are the rules for determining the rate-determining step?
• 0th order reactants can only be involved in steps after the RDS • 1st order reactants are usually in the RDS (but can rarely be before) • 2nd order reactants are either -2 species in RDS -1 species in RDS and 1 before -(rarely) 2 species before RDS
Steps before and including RDS must incldue what?
must add together to give species in same ratio as they are in the rate equation (i.e. 2 of 2nd order species, 1 of first order)