Kinetics II Flashcards
What is the easiest way to measure the change in volume of gas produced?
Gas syringe
What is the easiest way to measure the change in mass of gas produced?
Measured by standing the reaction mixture directly on a balance
What is a colorimeter?
A colorimeter passes light of a selected wavelength through the sample being studied and measures the light absorbed by the reaction mixture. As the concentration of the coloured component increases, it absorbs more light, so less is transmitted
What is the easiest way to measure the change in concentration of gas produced?
Titration
- Cannot be done continuously as the reaction is occurring
What is the easiest way to measure the change in conductivity of gas produced?
The sharp decrease in the concentration of ions will give a corresponding decrease in electrical conductivity of the solution as the reaction proceeds
What is a non-continuous method of detecting change during a reaction
Clock reaction
- The time it takes for the solution to obscure a cross can be measured
What is the iodine clock reaction?
Two colourless solutions are mixed and at first there is no visible reaction. After a short time delay, the liquid suddenly turns to a shade of dark blue. The iodine reaction exists in several variations
What is the hydrogen peroxide variation?
First, slow reaction, iodine is produced:
H2O2 + 2I- + 2H+ -> I2 + 2H2O
Second, fast reaction, iodine is reconverted into iodide ions by the thiosulphate ions:
2S2O3- + I2 -> S4O62- + 2I-
After some time the solution changes colour to blue-black
Why does the solution change to blue-black?
All the thiosulphate ions have been used up, the excess iodine then reacts with the starch - hence the blue-black colour of the iodine-starch complex. If no thiosulphate ions were present the formation of the blue-black starch-iodine complex would happen far too quickly
What is the peroxodisulphate variation?
Uses sodium, potassium or ammonium persulphate to oxidise iodide ions to iodine. Used to reduce iodine back to iodide before the iodine can complex with the starch to form the characteristic blue-black colour
Iodine is generated: 2I- + S2O82- -> I2 + 2SO42-
And is then removed: I2+ 2S2O32- -> 2I- + S4O62-
What does a graph of conc. against rate look like for zero order?
Constant
What does a graph of conc. against rate look like for first order?
Directly proportional - goes through the origin
What does a graph of conc. against rate look like for second order?
Curve going up
What does a graph of time against conc. look like for zero order?
Rate is constant
What does a graph of time against conc. look like for first order?
For first order reactions, the half-life is constant. The half-life is the time taken fro the concentration of the reactant to fall by half
