Chapter 18 - Rates of equations Flashcards
For reactant [A] and power n , give the equation for rate
rate proportional to [A]n
What is a zero order reaction?
When concentration of a reactant has no effect on the rate
rate proportional to [A]0
What is a first order reaction?
- Any change in concentration gives the same change to the rate
- rate proportional to [A]1
e. g. if [A] x 2, reaction rate increases by x 2
What is a second order reaction?
- For any change in concentration, the rate changes by the square of the conc change
- rate proportional to [A]2
e. g. if [A] x3, rate x9
Give the rate equation for A and B - two reactants
rate = k [A]m[B]n
where k = rate constant, m = order of reaction with respect to A and n = order of reaction with respect to B
What is the rate constant?
- Proportionality constant
- Number that mathematically converts between the rate of reaction and concentration and orders
How do you work out overall order of reaction?
Overall order = sum of orders with respect to each reactant
How do you work out rate constant units?
- ) Rearrange equation to make k the subject
- ) Substitute units into expression for k
- ) Cancel common units and show the final units on a single line
What is initial rate?
Instantaneous rate at the beginning of an experiment when t=0
How are concentration-time graph’s data collected?
Continuous monitoring
Give the 3 methods of continuous monitoring of reactions?
- Monitoring by gas collection
- Monitoring by mass loss
- Monitoring by calorimetry
How does a calorimeter work?
Wavelength of the light passing through a coloured solution is controlled using a filter. The amount of light absorbed by a solution is measured, which is directly linked to concentration of solution
What is the gradient of a concentration-time graph?
rate of reaction
Draw the concentration-time graph of a zero order reaction
Draw the concentration-time graph of a first order reaction?
What is half life?
Time taken for half of a reactant to be used up. Exponential decay pattern
What is special about the half life in first order reactions?
Half life is constant
How do you work out rate constant of first order reaction?
Draw tangent on concentration-time graph and calculate gradient, giving rate. Divide this by point of concentartion the tangent was drawn from
What is the equation for rate constant from the half life?
k = ln2/ t(1/2)
Draw a rate-concentartion graph for a zero order reaction
Draw the rate-concentration graph for a first order reaction
Draw a rate-concentration graph of a second order reaction
What is the gradient of a rate-concentartion graph?
rate constant
What is the rate-determining step?
slowest step in a multi-step reaction
How can we predict reaction mechanisms? (using rate determining step)
- rate equation only includes reacting species involved in the rate-determining step.
- orders of the rate equation match the number of species involved in the rate-determining step
What is the effect of increasing temperature by 10ºC on the rate?
Doubles the rate of reaction and rate constant
What are the 2 factors that contribute to increased rate when temperature increases?
- Increasing the temperature shifts the boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy
- Particles move faster and collide more frequently
What is the rate mainly determined on?
Ea
Give the arrhenius equation
k = Ae-Ea/RT
What does the exponential factor (e) in arrhenius equation represent?
proportion of molecules that exceed Ea and have sufficient energy for a reaction to take place
What does the pre-exponential factor (A) in the arrhenius equation represent?
Frequency of collisions with correct orientation
Give the arrhenius equation in linear form
lnk = -Ea/RT + lnA
Compare y=mx+c to linear arrhenius equation
y= mx+c
lnk = -Ea/RT 1/T + lnA
y-int - lnA
Gradient - -Ea/R
