Chapter 18 - Rates of reactions Flashcards
How do you calculate the rate of a reaction?
Change in concentration / time
What are the usual units of rate?
mol dm^-3 s^-1
What are the usual units for concentration?
mol dm^-3
How do you write concentration using shorthand?
[A]
What is the rate of reaction proportional to?
The concentration of a particular reactant raised to the order it has
What are the three main orders a reaction can have?
Zero, first and second
When will a reactant have zero order?
When changing its concentration has no impact on the rate
What is important to remember about a zero order reaction?
Anything raised to the power of zero is 1
When will a reaction have first order?
When the rate of reaction depends on the concentration of a reactant raised to the power 1
When will a reaction have second order?
When the rate of reaction depends on the concentration of a reactant raised to the power 2
What does the rate equation tell you?
The mathematical relationship between the concentrations of reactants and the rate
What is the rate equation?
Rate = k[A]^m x [B]^n
What is k?
The rate constant
What is the overall order of a reaction?
The sum of the orders of all the reactants
Can you determine the rate of a reaction using chemical equations?
No, it must be determined experimentally
What is the initial rate of a reaction?
The instantaneous rate at the beginning of a reaction when t=0
How would you measure the order of a reactant?
Conduct an experiment using a set concentration of the reactant
Record the rate at regular intervals
Conduct the experiment identically but using a different concentration of the reactant
Compare your measurements for the rate of reaction
How do you work out the units for k?
Rearrange the equation to make k the subject
Substitute units in for your readings
Cancel common units and display the final unit on one line
What are the two main methods of continuous monitoring?
Gas collection and mass lost
How do you measure the rate of reaction using a colour change?
Set the colourimeter to the wavelength of light you’re investigating
Calibrate the colourimeter using distilled water and set the absorbance to zero
Take samples from your reaction mixture at regular intervals and measure the absorbance of each one using the colourimeter
What is the shape of the concentration-time graph for a zero order reaction?
A straight line with a constant, negative gradient
What is the shape of the concentration-time graph for a first order reaction?
A downward curve with a decreasing gradient
How do you identify k for a zero order reaction?
It is the gradient of the line
How do you identify k for a first order reaction?
Use the half-life, which should be constant
What is the half-life?
The time taken for half of a reactant to be used up
How can you identify a first-order reaction using half lives?
The half-life will always be constant
What pattern does half life follow?
Exponential decay
What is the equation for calculating k using half-life?
k = ln(2) / half life
How do you calculate k using a graph?
Draw a tangent at a point
Record the concentration and rate
Use the equation k = rate / concentration
What is the shape of the rate-concentration graph for a zero order reaction?
A horizontal line with the gradient 0
The y-intercept gives the value of k
What is the shape of the rate-concentration graph for a first order reaction and why?
A single straight line through the origin
The rate is proportional to the concentration
K is the gradient
What is the shape of the rate-concentration graph for a second order reaction?
An upward curve with increasing gradient
How do you determine k from the rate-concentration graph for a second order reaction?
Plot the second graph of rate against concentration squared
The gradient of this straight line is k
How can a clock reaction be used to measure the rate?
The time is measured from the beginning of the reaction for a precipitate to form
How do you calculate the initial rate using a clock reaction?
1 / time taken for visual change to occur
How does an iodine clock work?
The time taken for the iodine to form (a red-brown colour) is recorded
What assumptions do we make when using clock reactions?
The concentration of the reactant doesn’t change significantly over the time period of the reaction
The temperature stays constant
When the endpoint is seen, the reaction has not proceeded too far
Reasons why it is unlikely that a reaction will occur in a single step
Too many atoms of reactants need to collide
The atoms are unlikely to collide with the correct orientation
What is the reaction mechanism?
The series of steps that make up the overall reaction
What is the slowest step in a reaction called?
The rate-determining step
How are haloalkanes hydrolysed and how can this help us?
They are hydrolysed with hot, aqueous alkali and they can help us t identify the rate-determining step
How is the order of a reactant used in the rate determining step?
The order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in the rate-determining step
How can you predict the rate-determining step?
The rate equation only involves species included in the rate-determining step
The orders in the rate equation match the numbers of species in the rate-determining step
What impact will increasing the temperature by 10c have on the rate constant and the rate of reaction?
In general, it doubles them
Why does the rate constant increase with temperature?
There are more particles with sufficient energy to react
Particles collide faster and more frequently
What is the Arrhenius equation?
k = A x ^-Ea/RT
What does A stand for in the Arrhenius equation?
A pre-exponential factor
What is T measured in for the Arrhenius equation?
K
What does e^-Ea/RT represent?
The proportion of molecules that have sufficient energy to react
What is the logarithmic form of the Arrhenius equation?
ln(k) = -Ea/RT + ln(A)
How can the logarithmic form of the Arrhenius equation be turned into y=mx+c?
y = ln(k) m = -Ea/RT x = 1/T c = ln(A)
