Rate Equations Flashcards
What is the definition of the rate of a reaction?
The change in molar concentration of a substance in a set time, usually one second
What are the units of rate of reaction if a reaction takes place in a solution or is a homogeneous gas reaction?
Mol dm-3 s-1
What is the equation for working out rate?
Rate = change in concentration/ change in time
What is the concentration of a substance represented by?
Writing the formula of the substance inside square brackets
How can we measure the rate of change of concentration of substances?
By measuring the concentration of substances at time intervals at a fixed temperature
What is the order of reaction?
The proportionality of the rate to the concentration of reactants where rate = k[A]’x (to the power of x) and x is the order
What is a first order reaction and how would you write it?
It is a reaction whose rate is proportional to the concentration of a reactant and the reaction is said to be first order with respect to reactant A
Rate = k[A]’
What is a reaction that is second order with respect to a reactant and how would you write it?
Where the rate of the reaction is proportional to the concentration of the reactant squared
Rate = k[B]’2
What is a reaction that is zero order with respect to a reactant and how is it written?
One where the rate of the reaction does not change when the concentration of rhe reactant changes
Rate = k[C]’0
How is the overall rate equation formed?
The individual effects of different reactants in rate are combined to form the overall rate equation
How do you find the overall order of a reaction?
By adding together the orders of the reaction with respect to each reactant
What is a general rate equation for two reactants?
Rate = k[A]’m[B]n
What is K in the rate equation?
The rate constant
Why would a reactant not appear in the rate equation?
In the rate is zero order with respect to that particular reactant
What is the rate constant affected by and how?
The temperature. As the temperature increases both the rate of reaction and rate constant increase
How do you calculate the units of the rate constant?
By using the rate equation. The units of k can be calculated by replacing each of the terms in the rate equation by their units and then cancelling out duplicated units
What do the graphs look like for zero order, 1st order and 2nd order reactions with concentration against time?
- Zero order = straight line down
- first order = very curved line
- second order = slightly less curved line
Check snap camera roll
What happens as the reactant is used up in first, second and zero order reactions?
- zero order the reaction continues at a constant rate as the reactant doesn’t affect the rate
- first and second order the rate of reaction becomes slower and the gradient decreases
How can the rate constant be determined from a zero order graph?
By the gradient of a graph of the concentration of that reactant against time
How can you determine whether a reaction is first or second order?
The rate must be determined at different concentrations or reactants and and a graph of the rate must be plotted against concentration of one reactant. The shapes of these graphs are very distinctive
What do the graphs look like for zero, first and second order reactions for rate against concentration?
- zero order = straight line with a zero gradient
- first order = diagonal line with a positive gradient
- second order = curved line inwards
Check snap camera roll
How do you work out the rate constant from graphs for first and second order reactions?
Use a rate against concentration graph and work out the gradient
What are the two ways of plotting a graph of concentration against rate?
- the continuous monitoring method
* the initial rate method
What is the continuous monitoring method?
- If the concentration of a reactant remaining in a solution can be measured at set time intervals then a single reaction can be monitored.
- we plot a veaph of concentration against time and draw the line of best fit
- then choose a number of points on the line of best fit and draw tangents at each of them. The gradient of each tangent is used to determine the rate ar that point
- for each of our chosen points we now have the rate (tangent) and concentration
- the rate and the concentration of the reactant at each point are then drawn in another graph which can be used to determine the order of reaction
What is the initial rate method
- Used when the concentration of a reactant remaining in a solution cannot be measured at set time intervals
- the reaction is carried out multiple times in seperate experiments each with a different starting concentration of a reactant.
- the rate of reaction is followed by measuring the amount or concentration of product (not reactant). A graph of the amount of product against time is plotted for each experiment
- as we know the initial concentration of the reactant for each experiment we measure the rate for the first few seconds (the gradient of a tangent drawn to the line). This determines the initial rate of reaction for each experiment at the start when the concentration of the reactant is known
How else can the order of a reaction be determined without a graph?
From sets of data involving the initial rate of a reaction
What are the steps for working out the order of a reaction from sets of data involving the initial rate of reaction?
• step 1: identify a pair of experiments where the concentration of the reactant you are looking at changes
• step 2: work out what factor the concentration has increased or decreased by
• step 3: work 9ut what factor the rate has increased or decreased by
• step 4: compare the factors that tbe concentration and the rate have changed by:
- zero order reactions: the factor that the rate changes bt is zero
- first order reactions: both the concentrarion and the rate change by the same factor
- second order reactions: the favtor that the rate changes by is the factor that the concentration changes by squared
How do you work out the order if there’s not suitable pairs of experiments in the data?
If you know the order of one reactant and the rate you can rearrange the rate equation to find it
E.g. y is first order
Rate = [y]1 [x]?
Rate/ [y]1 = [x]?
What is the rate determining step?
The slowest step in a series of intermediate stages that determines the overall rate of the chemical reaction
What are the rules with the intermediate steps and the species involved in the rate equation?
- species that are only involved in steps after the rate determining step do not appear in the rate equation
- species that appear in the steps up to and including the rate determining step are in the rate equation
- a substance that acts as a catalyst will appear in the rate equation but not in the overall chemical equation for the reaction
How do you find the rate equation and what can it be used to deduce?
We find the rate equation by experiment and then can use this to deduce possible mechanisms
What is the arrhenius equation and what do the different symbols mean?
K = Ae ^ - EA/ RT
K= rate constant A = arrhenius constant e = mathematical constant with a value of approximately 2.718 EA = activation energy in J mol -1 T = absolute temperature in K R = gas constant with a value of approximately 8.314 J K-1 mol -1
What is the logarithmic form for the arrhenius equation?
ln k = lnA - EA/RT
What does plotting a graph of ln k against 1/T do?
Gives a line of best fit where the gradient is equal to -EA/R and the intercept is equal to lnA
What can values of J and T be used for (arrhenius equation)
To calculate the activation energy for a reaction
