Measuring rates of reaction. Measuring rates. The rate of a reaction is the rate of change of concentration with time . Δ C. Rate =. Where Δ = difference. Δ t. There are two ways of measuring the rate;. 1) The disappearance of a reactant. 2) The appearance of a product.
The rate of a reaction is the rate of change of concentration with time.
Where Δ = difference
There are two ways of measuring the rate;
1) The disappearance of a reactant.
2) The appearance of a product.
At any given time the reaction rate can be calculated from the gradient of the tangent of the curve.
The slope of the curve is steeper at the beginning, when the rate is greater.
The curve becomes shallower as the rate decreases.
The gas produced is lost lowering the mass.
Eg; The reaction of calcium carbonate with acid.
Some reactions can be followed by recording the mass at regular time intervals.
If a reaction produces a gas its volume can be measured at regular time intervals.
Eg; Hydrogen peroxidedecomposes to form oxygen with a manganese dioxide catalyst.
The concentration of a coloured substance can be measured using a colorimeter.
The transmission or absorbance is read off the scale.
The sample is placed into a cuvette.
The wavelength is set.
The absorption, or transmission, is read off the scale.
The correct filter is choosen.
The machine is zeroed using a blank.
Above 0.3 mol/dm3 absorbance is no longer άconentration, so the solution has to be diluted until it will give an absorbance on the straight line.
Absorbance ά concentration between 0 – 0.3 mol/dm3
If a reaction involves different numbers of ions it can be followed by measuring the conductivity of the solution.
CH3COCH3 + I2 → CH2ICOCH3 + HI
Propanone, iodopropanone and HI are all colourless.
Iodine is yellow/brown
[Iodine] can be analysed by titration against sodium thiosulphate., giving colourless products.
2S2O32- + I2 →S4O62- + 2I-
NB aliquots become increasingly paler as the reaction proceeds.
It is difficult to distinguish the end point so starch is used as an indicator.
So long as iodine is present the solution will be blue/black.
At the end point it will be colourless.