Kinetics Flashcards
Define the term rate of reaction
Change in concentration (of any reactant or product) per unit time. State what is being monitored (usually production of a product)
At a given instant, how could you calculate the rate of reaction
Rate of reaction = change in concentration / change in time
How could you measure the rate of reaction experimentally
Use a colorimeter at suitable intervals if there is a colour change
If gas is evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture
How would you measure reaction rate for really fast reactions
Use a flash of light to break bronds, use probe flashes to record amount of light absorbed by a species; this can show its concentration.
First used for ClO2 —> ClO* + O*
Can now monitor reactions that occur in times as fast as 10-12 seconds
How would you determine the rate constant and rate expression for a reaction
Only experimentally
What affects the value of the rate constant for a given reaction
Temperature, nothing else
Write a generic rate expression and state what each term means
Rate = k [X]^x [Y]^y
K = rate constant for the reaction
[X] and [Y] are concentrations of X and Y
x and y are the order of reaction with respect to X and Y
Do species need to be in the chemical reaction to be in the rate expression
No - species in the chemical equation may be excluded and species not in the chemical equation e.g. catalysts, may be included
Define the term order of a reaction with respect to a given product
The power to which the species’ concentration is raised in the rate equation
Define the term overall order of reaction
The sum of the orders of reaction of all species in the rate expression e.g (from earlier), total order = x + y
How would you calculate the units of the rate constant
Units of rate are moldm^-3s^-1 and units of concentration are mol dm-3
Rearrange rate equation to get k=
Sub in units and cancel them out
How would you draw a rate concentration graph
Plot [A] against time, draw tangents at different values —> draw a secondary graph of rate against [A]
How could you find the rate expression using the initial rate method?
Do a series of experiments during which you vary concentrations, so the concentration of just one reactant changes each time
Plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of reaction
Compare rates and concentrations between each experiment to find out order of reaction and overall rate equation
