How fast? Flashcards
What is the rate of reaction?
Change in concentration/ change in time
What is rate of reaction proportional to?
The concentration of a particular reactant raised to a power
Rate ∝ [A]^n
If the concentration of a reactant is raised to the power n, what is n?
The order of the reaction
What is zero order?
When the concentration of a reactant has no effect on the rate. Rate ∝ [A]^0.
Concentration has no effect on the rate for zero order because any number raised to 0 equal 1
What is first order?
when the rate depends on its concentration raised to the power 1 - rate ∝ [A]^1
- If the concentration of A is doubled, the reaction rate increases by a factor of 2^1
- If the concentration of A is tripled, the reaction rate increases by a factor of 3^1 = 3
What is second order?
When the rate depends on its concentrations raised to the power 2 (squared) - rate ∝ [A]^2
- If the concentration of A is doubled, the reaction rate increases by a factor of 2^2 = 4
- If the concentration of A is tripled, the reaction rate increases by a factor of 3^2 = 9
What is the overall order?
The sum of the orders with respect to each reactant - it gives the overall effect of the concentrations of all the reactants on the rate of reaction
What is the rate constant?
The proportionality constant; its a constant value that converts between the rate of reaction and concentration of orders
What is half-life?
The time taken for half of a reactant to be used up
What is the rate-determining step?
The slowest data in a multi-step reaction
How do you find the orders of reactants when given the rates?
Compare 2 experiments, with one of the reactants concentrations staying the same and the others changing - analyse what effect this have on the rate
What is the rate equation?
rate = k[A]^m[B]^n
k = rate constant
[A] = concentration of A
m = order of A
How can you calculate the rate constant?
k = rate/ [A]^m[B]^n
Whats the units of the rate constant if the overall order is 0?
mol dm^-3 s^-1
k = rate
Whats the unit of rate constant if the overall order is 1?
s^-1
What is the unit of the rate constant if the overall order is 2?
mol^-1 dm^3 s^-1
What is the concentration time graph for a zero order reaction?
What is the concentration-time graph for a first order reaction?
What is the concentration-time graph for a second order reaction?
What is the gradient of a concentration-time graph equal to?
The rate of reaction
How can you calculate the rate constant from a concentration-time graph?
- Pick a point on the graph of a specific reaction
- Draw a tangent to the line at this point
- Calculate the gradient of the tangent
- k = rate/ [A] : rate = gradient and [A] is the concentration at the chosen point
How can a first order relationship be confirmed from a conecentration-time graph?
Measuring successive half-lives - if they are the same the reaction is first order with respect to the reactant. Measure at-least 3 to check!
What is the rate constant equation for a first order reaction?
k = ln 2 / t 1/2
k = rate constant
t 1/2 = half life
What is the rate-concentration graph for a zero order reaction?
What is the rate-concentration graph for a for a first order reaction?
What is the rate-concentration graph for a second order reaction?
How do you investigate initial rates using iodine clocks?
How do you investigate reaction rates using continuous monitoring of Mg and HCl?
- For each HCl concentration, plot a graph of time on the x-axis against volume of H2 gas produced on the x-axis
- Draw a line of best fit
- Draw a tangent at time = 0s. Use the gradient to the initial rate for each concentration HCl
How do you investigate rates by analysing colorimetry?
What does the rate equation include?
It only includes repeating species involved in the rate-determining step
The orders in the rate equation match the number of species involved
If (CH3)3 CBr + OH- —> (CH3)3COH + Br-
and the rate equation is rate = k[(CH3)3CBr]
what does this show?
The slow rate-determining step involves only one molecule of (CH3)3CBr
OH- has no effect on the reaction rate and must be involved in the fast step
What happens to the rate of reaction and rate constant as the temperature increases?
The rate of reaction and rate constant will also increase
What 2 factors contribute to the increase of rate of reaction and rate constant with temperature?
- Increasing temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy
- As temperature increases, particles move faster and collide more frequently
What is Arrhenius’ equation?
k = Ae^- Ea/ RT
k = rate constant
A = Arrhenius constant
Ea = activation energy
R = gas constant
T = temperature K
How can Arrhenius’ equation be expressed as a logarithmic relationship?
ln k = - Ea/ RT + ln A
If you plotted a graph of ln k against 1/T, what is the gradient and y-intercept?
gradient = -Ea/ R
y - intercept = ln A
