1: Reaction Kinetics Flashcards
What does kinetics mean
Things related to the rate of a reaction
What is rate of reaction
The rate of change of concentration of a reaction or product with time
What are the units for rate for reaction
moldm¯³s¯¹ (or per min etc)
How can you find the rate of reaction from a concentration against time graph
Draw a tangent to the curve and calculate the gradient
Examples of continuous monitoring
- Mass may be monitored, where the change is due to loss of a gas to the atmosphere.
- A gas evolved in a reaction may be collected.
- pH can be monitored using a pH meter
- If there is a colour change in a reaction, colorimetry can be used.
How does a colourimeter work
A colorimeter uses a filter to select light in a wavelength range where there is a big difference in
the strength of absorption by the products compared to the reactants (eg strong absorption by
one and zero by the other). This light is passed through the sample and its intensity detected on
the other side
What is the rate equation
The rate equation for a reaction is an equation showing how the rate depends on the concentration of reactions
What is a generic rate equation
Rate = K[A]^m [B]^n
What is the overall order of a reaction
The sum of the separate orders of reaction
Why can’t the orders of reaction be deduced from the reaction equation
Because it depends on the mechanism of the reaction
What is k in the rate equation
The rate constant
How is the rate constant affected by the temperature and why
k increases as the temperature increases as the proportion of reactant molecules (or ions) with energy greater than the activation energy increases
Does pressure affect the rate constant
no
How does a catalyst change the rate equation
The whole rate equation changes as there is a new pathway with lower activation energy.
What is a general formula for finding the units the rate constant
mol^(1-x) dm^(3x-3) s¯¹
where x is the overall order of the reaction
What is the half life of a reaction
The time taken for the concentration of a reactant to fall to half of its original value
What the rate equation for a zero order reaction
rate = k
Units for the rate constant in a 0 order reaction
moldm¯³s¯¹
What does the graph look like for concentration v time for zero order reaction
A straight line with negative gradient
What does the rate v concentration graph look like for zero order reaction
Straight (horizontal) line with 0 gradient
eg. y=5
What is the trend for successive half lives for zero order reactions
Successive half lives decrease (half)
What is the rate equation for first order reaction with respect to A
Rate = k[A]
What does the concentration v time graph look like for a first order reaction
Curved line of negative gradient
Like the positive part of 1/x
What is the trend for successive half lives for a 1st order reaction
Successive half lives are constant
Units for rate constant for 1st order reaction
s¯¹
Equation for calculating half life for a 1st order reaction
t½= ln2 / k
What does the rate v concentration graph look like for 1st order reaction
Rate is directly proportional to concentration (y=x)
What is the rate equation for a second order reaction with respect to A
Rate = k[A]²
What does the concentration v time graph look like for second order reactions
Curve with negative gradient
What is the trend for successive half lives for a second order reaction
Successive half lives increase
What does the rate v concentration graph look like for second order reaction
Curve with positive gradient (similar to the positive part of y=x²)
