Chemical Kinetics Flashcards
Topic 2, Lectures 5-8 - Colan Hughes
Why reaction rates must be controlled in industry
Fast reactions may be too dangerous and slow reactions can be uneconomical or bad for the environment.
In-situ monitoring
Observation, spectroscopy, chemical sensor, gas pressure
Involves measuring the concentration of reactants within the reaction vessel.
Ex situ monitoring
Refers to measuring rate outside of the reaction vessel. This would require stopping the reaction momentarily through temperature change, neutralisation, etc. or taking a sample from the vessel.
Quenching
A method of stopping reactions. This can be achieved using sudden cooling or neutralisation. When reactions are stopped for monitoring, several repetitions of the experiment must be run, stopping at different times.
Calculating rate
Rate is shown by the rate of change of concentration of reactants over the course of a reaction. Found using the gradient.
Key points when calculating rate
If the gradient is negative, multiply x by -1 to ensure the rate is always positive.
Must divide each reactant by its stoichiometry.
Rate laws
The powers of each concentration gives the order of that reactant.
Sum of orders gives overall order.
Rate constant k is temperature dependent but independent of concentrations.
Units of k are dependent on the rate law.
Rate law cannot be predicted from stoichiometry.
Integrated rate laws
If rate laws show how rate depends on concentration, integrated rate laws show how concentration changes over time based on the order of the reaction.
Integrated rate laws: first order
For a first-order reaction, a graph of ln[A] against t would be linear.
Integrated rate laws: second order
For a second-order reaction, a graph of 1/[A] against t would be linear.
Integrated rate laws: zero order
For a zero-order reaction, a graph of [A] against t would be linear.
Half life
The amount of time taken for the concentration of a reactant to be halved.
Initial rates method
The process of running several experiments with reactants at varying concentrations. Measure the initial rate for each experiment. Compare how the rate changes with each concentration. Using ratios, find the order with respect to each reactant.
Isolation method
One reactant is used in large excess, so the concentration becomes effectively constant. Determine the rate according to the non-isolated reactant. Repeat the experiment for each reactant until you have determined the rate law.
