Chapter 18 - Rates Of Reaction 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?
Downward curve with a decreasing gradient over time
How do you identify k for a zero order reaction?
The value of the gradient is equal to the rate constant K
How do you identify k for a first order reaction?
The rate constant (K) can be identified by the half life
What is the half-life?
Is the time taken for half of the reactants to be used up
How can you identify a first-order reaction using half lives?
Using a concentration – time graph by measuring successive half lives. If they are the same, the reaction is first order to respect the reactants
What pattern does half life follow?
Exponential decay
What is the equation for calculating k using half-life?
K= ln2/half life
How do you calculate k using a graph?
Draw a tangent then use K = rate(ie gradient)/[A]
What is the shape of the rate-concentration graph for a zero order reaction?
Horizontal straight lines with zero gradient
What is the shape of the rate-concentration graph for a first order reaction and why?
Straight line graph to the origin
The rate is directly proportional to the concentration for the first order relationship
What is the shape of the rate-concentration graph for a second order reaction?
Upward curve with increasing gradient
How do you determine k from the rate-concentration graph for a second order reaction?
Plotting a second graph of the rate against the concentration squared, result is a straight line through the origin. The gradient of the straight line graph is equal to the rate constant
How can a clock reaction be used to measure the rate?
More convenient way of obtaining the initial rate of reaction by taking a single measurement. Measurements are taken by visual change measured in time.
How do you calculate the initial rate using a clock reaction?
1/t
How does an iodine clock work?
A common type of clock reaction relying on the formation of iodine. Iodine has orange-brown colour.
What assumptions do we make when using clock reactions?
I don’t know 🤔 🤷♀️
What is the reaction mechanism?
Series of steps that make up an overall reaction
What is the slowest step in a reaction called?
Rate – determining step
How are haloalkanes hydrolysed and how can this help us?
By hot aqueous alkali. Hydrolysis of haloalkanes can be investigated experimentally to identify the overall reaction.
How is the order of a reactant used in the rate determining step?
The orders in the rate equation match the number of species involved in the rate – determining step
What impact will increasing the temperature by 10c have on the rate constant and the rate of reaction?
Doubles the rate constant and doubles the rate of the reaction.
Why does the rate constant increase with 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 Arrhenius equation?
k=A e^-Ea/RT
What does A stand for in the Arrhenius equation?
Pre-exponential factor
What is T measured in for the Arrhenius equation?
Temperature in Kelvin
What does e^-Ea/RT represent?
Rate constant
What is the logarithmic form of the Arrhenius equation?
Ln k =(-Ea/R)(1/t)+lnA
Y=mx+c
