A - Rate Equations Flashcards
What does the reaction rate tell you?
The reaction rate is the change in amount of reactant or product per unit time.
(How fast reactants are converted into products).
How do you work out reaction rate from a graph?
It’s the gradient which is the change in y divided by the change in x.
How do you work out the rate of reaction from a curved graph?
Draw a gradient at a particular point in the reaction and work out the gradient of the tangent. (It has the same gradient as the curve does at that point).
What is the equation to find the rate of a reaction?
Change in concentration divided by time.
The rate equation links the reaction rate to what?
The reactant concentrations.
What does a rate equation tell you?
How the rate of reaction is affected by the concentrations of the reactants.
What is the rate equation for the general reaction A + B —> C + D?
What are the units of rate?
Rate = k[A]^m[B]^n
Where m and n are the orders of reaction and k is the rate constant.
Units of rate are mol dm-3 s-1
What is the order of reaction?
The power to which the concentration of a reactant is raised in the rate equation. It tells you how the concentration of the reactant in question affects the rate of reaction.
When is the order of reaction with respect to A 0, 1 or 2?
If [A] changes and the rate stays the same, the order of reaction with respect to A is 0.
If [A] is proportional to rate, the order of reaction with respect to A is 1.
If the rate is proportional to [A]^2, the order of reaction with respect to A is 2. (So if [A] doubles, the rate will be 2^2=4 times faster.
Using the general rate equation, what is the overall order of reaction?
m + n
How do you work out the orders of reaction?
You can only find them from experiments, you can’t work them out from chemical equations.
What is the rate constant, k?
A number that links the rate of reaction to the concentration of the reactants. The bigger the value of k, the faster the reaction.
What happens to the rate constant if the temperature is increased?
The rate constant is always the same for a certain reaction at a particular temperature but if you increase the temperature, the rate constant will increase too.
Explain why the rate constant must increase when temperature increases.
When you increase the temperature of a reaction, the rate of reaction increases. You’re increasing the number of collisions between reactant molecules, and also the energy of each collision.
But the concentrations of the reactants and the orders of reaction stay the same. So the value of k must increase for the rate equation to balance.
What is the initial rate of reaction?
The rate at the start of the reaction.
How do you find the initial rate of a reaction from a concentration-time graph?
By calculating the gradient of the tangent at time = 0.
If a species has zero order, how is it included in the rate expression?
It’s not included.
Briefly outline how to use the initial rates method to work out the rate equation.
- Repeat an experiment several times using different initial concentrations of the reactants. But you should only change one concentration at a time, keeping the rest constant.
- Calculate the initial rate for each experiment by finding the gradient of the tangent of the graph at time = 0.
- See how the initial concentrations affect the initial rates of reaction to find the order for each reactant.
You can measure the rate of reaction by continuous monitoring. What does this involve?
Following a reaction all the way through to its end by recording the amount of product or reactant you have at regular time intervals. The results can be used to work out how the rate changes over time.
How can you find the order of reaction with respect to a certain reactant from a concentration-time graph?
- Find the gradient at certain points on the graph to give you the rate at that particular concentration.
- Plot each of these points on a new graph with the axes rate and concentration. Then draw a smooth line through points. The shape of the line tells you about the order of reaction with respect to that reactant.
What will the rate-concentration graphs look like for zero, first and second orders?
Zero order - the line will be horizontal, meaning the changing concentration does not affect rate.
First order - a straight line through the origin, meaning the rate is proportional to the concentration of the reaction.
Second order - there will be an exponential curve, meaning the rate is proportional to the concentration squared.
What does the Arrhenius equation show?
How the rate constant, k, varies with temperature.
What is the formula for the Arrhenius equation? Give the meanings of each letter.
-Ea
———
RT
k = Ae
Where: k = rate constant Ea = activation energy (J) T = temperature (K) R = gas constant (8.31 J K-1 mol-1) A = Arrhenius constant
What happens to the rate constant as Ea gets bigger?
It gets smaller.
What does a large activation energy mean for the rate of reaction?
It will be slow.
What happens to the rate constant as temperature increases?
K also increases.
What do higher temperatures mean for a reaction rate?
The rate will increase.
How do you get rid of the exponential in the Arrhenius equation?
By taking the natural log (ln) of everything in the equation.
What are the axes of an Arrhenius plot?
What would the gradient be for this graph?
Y axis is ln k and the x axis is 1/T
The gradient is -Ea/R
What is the rate-determining step?
The slowest step in a multi-step reaction.
What is the overall rate of a multi-step reaction determined by?
The rate-determining step.
What’s another name for the rate-determining step?
Rate-limiting step.
How can the rate equation give information about the reactants in the rate-determining step?
If a reactant appears in the rate equation, it must affect the rate. So this reactant, or something derived from it, must be in the rate-determining step.
If a reactant doesn’t appear in the rate equation, it won’t be involved in the rate determining step (and neither will anything that’s derived from it).
Does the rate determining step have to be the first step in a mechanism?
No.
What information do orders of reaction give about the rate determining step?
The order of a reaction with respect to a reactant shows the number of molecules of that reactant that are involved in the rate determining step.
