Draft Calculations/ Topic Report draft

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Topic Report drafting-

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 K_sp (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.