Factor that affect the rate of reaction, among other
:
1.Touch
surface area
To the more surface
area touching, so that there will collision more and more. So, the faster is
the reaction rate.
2.Temperature
The higher of
temperature, the kinetic energy will be even greater, so that the collision
which occurred more frequently and caused faster reaction rate.
3.Concentration
The greater of the
concentration the more substance to react, so the greater the possibility of
collisions, consequently the faster reaction.
4.Catalyst
Catalyst for reaction rate increase
which decrease the activation energy of a reaction. Catalyst is added to a
reaction to speed up the rate of reaction.
Analytical
solutions are made up in volumetric flasks with volumes ranging from one cubic
centimetre to as much as five litres.
Although
they are all accurately calibrated, the largest flask will always give a
solution with the most precisely measured concentration. This is mainly because
any balance has a limit to its accuracy and this will produce a greater
percentage error when weighing out smaller quantities of solute. Thus the
smaller flasks should only be used when the greatest precision is not required.
The very large flasks are used for making up standard solutions of reagents
that will be used many times.
Solutions
may deteriorate, so standard solutions should never be stored for long periods
of time. Furthermore, some solutions may need to be transferred to dark
bottles, to limit photochemical decomposition.
A
common reason for making up a solution of a solid is to determine the solid's
purity or composition - say to determine the percentage copper in this copper
salt. The most commonly used flask for this purpose has a volume of 250 cubic
centimetres. Like all volumetric flasks, it has a stopper, and a single
calibration line which accurately marks the stated volume at room temperature -
20 degrees Celsius.
To
determine the purity or composition of a solid, first weigh out a suitable
quantity on an analytical balance to an accuraccy of four decimal places.
Record the weighing in a notebook. Solids which absorb atmospheric moisture
must first be dried in an oven and then allowed to cool in a desiccator, before
weighing. Tip the solid into a 100 cubic centimetre beaker, then reweigh the
weighing bottle to accurately determine the amount of solid added. Add any
other necessary reagent, such as dilute acid then add about 50 cubic
centimetres of de-ionised water, and stir the mixture until the soolid has
completely dissolved It's bad practice to tip the solid directly into a
volumetric flask - some solids are very slow to dissolve at room temperature.
In such cases, gently warm the beaker carefully stirring the contents from time
to time until the solid dissolves. Do not allow the mixture to boil, in case
some drops of liquid splash out. Once a clear solution has been formed, remove
it from the heat and allow it to cool. When the solution has cooled, first
carefully wash the clock glass and the stirrer rod with de-ionised water. The
remaining procedure is the same for solutions of solids, like the copper salt,
which dissolve easily at room temperature.
Transfer
the solution to a clean volumetric flask using a funnel to make sure that all
the solution gets into the flask. Then rinse out the beaker with more water.
Pour in de-ionised water until the level is about one centimetre below the
calibration mark. Remove the funnel then add water dropwise using a
wash-bottle, until the bottom of the meniscus is at the calibration level.
Place the stopper in the flask, then thoroughly mix the solution by inverting
the flask several times, keeping a finger firmly over the stopper. Notice that
the flask has at all times remained at room temperature. Heating the flask will
alter its calibrated volume so never heat a volumetric flask - it is a
precision piece of equipment.
Solutions
of standard reagents such as potassium dichromate or EDTA can be made up in a
similar fashion. Alternatively, standard ampoules containing concentrated
solutions of standard reagents can be purchased. There are several different
types of ampoule in production, some made of glass and some made of plastic.
It
is sometimes necessary to make up solutions of different known concentrations,
starting from a standard stock solution. The easiest way to accurately dilute
by a factor of 10 is to take 25 cubic centimetres of the stock solution in a
pipette, then transfer it to a 250 cubic centimetre volumetric flask. Dilute
the solution to the mark with deionised water. Then stopper the flask and
thoroughly mix the contents. As the volume containing the solute has been
increased 10 fold its concentration has been reduced 10 fold. If several
different volumes need to be measured out. to give a range of concentrations,
it may be easier to deliver the stock solution from a burette. Note that
because 10 cubic centimetres can be measured out to higher accuracy than 1
cubic centimetre, normally a dilution of 100 fold is better done by diluting 10
cubic centimetres of stock solution to 1 litre, rather than one cubic
centimetre to 100 cubic centimetres.
