Introduction-
This extended experimental investigation is aimed at
investigating the scientific theory of ‘like dissolves like.’ In order to
thoroughly examine this topic and gain both qualitative and quantitative
evidence, the solubility constant of the salt sodium chloride (NaCl) will be
investigated in both polar (water and ethanol) and non-polar (oil) substances.
A Solubility constant is referring to the equilibrium of a solid and its
corresponding ions in a saturated solution. The higher the concentration of
ions from the solid dissociated in the solution, the greater the solubility constant.
The overall solubility of the solute is dependent upon the level of the Ksp
(solubility constant). Additionally, the polarity of the solvent is also
an integral factor in determining the solubility constant of the solute, in
this case Sodium Chloride. Additionally, the solution must be saturated in
order to gain an accurate measure of the Ksp. Temperature is also an
experimental factor investigated, though its ability to either negate or
increase the effects of polarity will be the main focus. Both the polarity of
the solvent and the solubility product of the Sodium Chloride are relatable to
the theory ‘like dissolves like,’ which states that the solubility or
miscibility of a product depends on the degree to which polar substances
dissolve polar substances and non-polar substances dissolve non-polar
substances.
Discussion-
Ever since scientists first developed
the central idea of ‘like dissolves like,’ researchers world-wide have based
and implemented their experiments generally according to this guideline. However,
a metaphorical ‘grey area’ which still exists in comparison to other theories
is the extent to which like dissolves like. As mentioned, there is a key component
being investigated which could determine the level of dissolution of similar substances-
solubility product, in terms of polarity of the solution and the temperature. The substances being used to dissolve the
sodium chloride are water, ethanol and oil.
Water makes up around 70% of the
surface of the earth, either in the form of salt or fresh water (Chemistry in
Use 1, 2006). Water is unusual in terms of its properties, even though each
molecule is made up of the relatively simple structure of two hydrogen atoms
and one oxygen atom. Compared to other similar molecules, water has an abnormal
boiling and freezing point, as well as high surface tension, cohesion and heat
of vaporisation. However, the polarity of both the intra- molecular and
inter-molecular forces within water can be considered the cause for these
anomalies. Each hydrogen atom within a water molecule is covalently bonded to
the oxygen molecule. Oxygen, having space for two more valency electrons in its
outer shell, and each hydrogen having one space available in its outer shell,
allows this bonding to occur. Oxygen has the highest electronegativity (ability
to ‘hold’ electrons) out of the two atoms in the bond, therefore it has a
greater attachment to the shared electrons. This attraction creates a permanent
dipole, where the oxygen atom is always slightly negatively charged and the
hydrogen is slightly more positive. The molecule consequently becomes arranged
as a bent structure, as the positive hydrogen atoms repel each other. The
polarity intra-molecularly increases the inter-molecular forces and gives water
a greater cohesion. In water, this attraction between the highly
electronegative oxygen atom and the corresponding hydrogen atoms, as well as
the inter-molecular forces between oxygen and hydrogen atoms from different
molecules is known as hydrogen bonding (see figure below).
This polarity and high cohesion of
water helps determine the solubility of different substances. According to the
theory ‘like dissolves like,’ water should be able to act as a solvent for
polar molecules (i.e. substances with a permanent dipole and to a lesser extent
an instantaneous dipole). The greater the polarity of the solute, the greater
the ability of the molecules within that solute to ‘break’ the hydrogen bonding
between water molecules.
High temperature also increases the
ability of a solute to dissolve in water. When water is heated, the molecular energy of
the water molecules is increased, meaning that movement is faster and
collisions between molecules contain more force. If the heat energy is great
enough, then the hydrogen bonds between the water molecules will break and the
molecules will move closer to a gaseous state. Through this increase in
molecular motion, solutes with lower polarity in relation to water are able to
dissolve more easily due there being more space between the water molecules
(Does temperature affect dissolving, n/d). However, water (regarded as the
universal solvent) does not contain the same solubility properties as other
substances.
Ethanol (ethyl alcohol-CH3CH2OH) is one such substance very different to water in terms of its ability to
dissolve polar solutes (such as NaCl). The hydroxyl group (OH group) present in
ethanol signifies that the molecule has a certain degree of polarity. To what
degree is yet to be determined in the experiment, however due to the fact that
ethanol is a hydrocarbon and the length of the ‘R’ group is large in comparison
to the OH group, this would decrease polarity and consequently solubility of
polar substances. The oxygen atom present in the hydroxyl group has a high
degree of electronegativity, therefore enabling other polar substances to
dissolve. There is only one oxygen atom per ethanol molecule, however;
therefore, the overall polarity is reduced from if ethanol was a smaller
molecule. Due to the nature of the ethanol molecule, it is also capable of
dissolving other hydrocarbons. This is also indicative of the theory ‘like
dissolves like,’ as the non-polar component of ethanol (CH3CH2) has
the ability to dissolve other non-polar substances.
Temperature
increases the rate of solubility of either a polar or non-polar substance
within ethanol. The higher the temperature, the greater the motion of the
molecules, which weakens the dispersion forces inter-molecularly. This allows
the solute to disperse more easily and to a greater extent throughout the
solution. Although the polarity and consequent solubility of ethanol is much
less than that of water, it is a more versatile solvent and still dissolves
polar substances better than completely non-polar substance like oil.
Olive
oil is a tri-glyceride made up of a variety of different fatty acids and is
predominantly considered to be not polar. Tri-glyceride is a specific type of
lipid, which is also known as fats. Oil contains molecules which are bent and
irregular, therefore weakening the already poor dispersion forces between molecules.
When a polar solute is added to the liquid, the oil molecules tend to clump
together and rise to the top. The molecules clump together because the
dispersion forces of oil are too weak to force apart the strong dispersion
forces or dipoles which hold together polar substances. Additionally, the polar
molecules are not attracted to the oil molecules because they do not hold a
significant charge, let alone any highly electronegative atom. However, in the
case of an ionic substance being used as the solute, a very small dipole is
created when the charged particles are added to the oil, bonding the two
molecules together briefly (Intermolecular and Interatomic particles, n/d). Temperature
has no effect on the solubility of polar substances in oil, as no matter how
much energy is injected into molecule motion and which intermolecular bonds are
broken, the oil will still end up clumped together (because it has nothing to
keep it attracted to polar molecules).
Sodium
Chloride (NaCl) was the ionic (and consequently very polar) substance used
throughout the following investigations to determine its solubility product
when added to the aforementioned solvents. NaCl is essentially two charged
ions. A positively charged sodium ion and a negatively charged chlorine ion
combined together to cancel each other out and create a neutral compound. When
sodium chloride is added to water, the sodium and chloride ions (through the
strength of the hydrogen bonding in water over the ionic bonding in NaCl)
dissociate and become attracted to their oppositely charged particles in water.
The chlorine ion attracts two hydrogen atoms and the sodium attracts the oxygen
in a process known as ion-dipole interaction (Intermolecular and Interatomic
particles, n/d). At 20 degrees, a saturated solution is created when there is
35.7g of salt per 100mL. When sodium chloride dissociates in ethanol the
procedure works much the same way, except there is only one OH group so limited
ion-dipole interactions can occur. Therefore, the consequent solubility product
for a solution containing ethanol and salt should be less than that of water
and salt. Sodium Chloride in oil should have a solubility product equal to
zero, even though the sodium chloride will create a very small dipole upon
contact with the oil molecules. The formula for calculating the solubility
product of NaCl is:
Solubility product itself is independent of
temperature. The rate of the dissolution of a substance in a solvent, however,
is altered depending on the amount of kinetic energy available to molecules. If
time constraints are added, then the solubility product could differ depending
on the amount of time the sodium chloride ions take to dissociate in solution.
If there was a known amount of NaCl within a solution, then the amount of ions
in the solution would not change unless a greater proportion of the precipitate
which was in equilibrium dissolved. However, the solution would then face the
possibility of becoming super-saturated. If the Ksp is equal to
one, an equilibrium is present between the ions and the precipitate. The Q
value, however, can be above or below the equilibrium constant if the substance
is not at equilibrium. A higher Ksp value indicates that more ions have
dissociated in comparison to amount of precipitate formed, while a low Ksp
value means there is a higher relative concentration of the undissolved compound.
In order to determine
the amount of chloride ions in a solution of sodium chloride in either water or
ethanol, a titration must occur. Normally, silver nitrate (AgNO3) is
titrated when investigating salinity, and a potassium dichromate solution is
used as the indicator (Salt concentration by titration, n/d). The substances
react in the titration according to the following equations (the second
equation being with respect to the indicator):
When the brick red
solid precipitate is formed, the reaction is finished. In order to determine
the solubility product of the salt, the chlorine concentration must first be
determined. 