Plan to investigate the transition temperature of salt hydrates

by bapritchard94 (student)

Transition Temperatures Of Salt Hydrates

For my investigation, I am attempting to investigate the transition temperatures of salt hydrates. Trying to identify the transition temperature, and what affects them. I shall experiment on a salt hydrate and produce a graph of results. I shall do this for all salt hydrates I am testing.

Salts are chemical compounds that always contain two things: Cation’s and Anion’s Cation’s are positively charged particles (for example a lithium ion or a hydrogen nucleus). The other things are anion’s. Anion’s are negatively charge particles (for example a chlorine ion or a fluorine ion). Salts are ionic compounds that contain a lattice structure of these charged particles. Salts are ionic compounds and thus they are soluble in water, this is due to the water molecules being polar and these molecules effectively dismantle the ionic lattice, by pulling the ion’s (like pulling blocks out of a jenga tower) [1]. When molten or in a solution, salts can conduct electricity, this is because the individual ion’s can move. Charge can therefore be transferred from one area to the other, meaning liquid or aqueous salts are electrically conductive. The body uses salt’s as ‘electrolytes’ to transfer electrical charge from one area to another. This includes along the conductive tissue found in the heart, initiating a heartbeat or opening pores in a cell membrane allowing molecules in or out of the cell [2].

Salt hydrates are salt molecules surrounded by water molecules. The water molecules are made up of an oxygen molecule bonded to 2 hydrogen molecules. The oxygen molecule has a higher electronegativity and thus the electrons around the molecule are attracted to the oxygen more than the hydrogen. Thus the molecule has a slight negative charge over the oxygen and positive charge over the hydrogen’s. The water molecules are then attracted to cation’s and anion’s respectfully . The water molecules are bonded to the salt molecules by intermolecular forces which aren’t as strong as the bonds between individual atoms. This explains why the salt can be heated and the water molecules can evaporate from the hydrate, forming an anhydrous salt (or a salt dehydrate). When the compound is heated, water will evaporate away from it, causing a dehydrate to be formed. This will have a molecular mass of just the salt as it is just the salt. But if you pass water vapour over the salt, it will absorb any water returning to a salt hydrate. This will now have the Molecular mass of the salt plus the molecular mass of the water. To form a slat hydrate, you must place the dehydrate salt in water and then heat gently to evaporate the excess water. The crystals you are left with will now be the hydrated salt. To write a salt hydrate you first write the salt formula followed by a dot, followed by how many water molecules are combined to one unit of salt. An example:- CuSO4·5H2O [3]. This shows there are 5 water molecules for every one copper sulphate molecule. The name of this molecule would be Copper Sulphate Pentahydrate, so to name a salt hydrate, you first write the salt, followed by the prefix which is linked to the number (mono-1, di-2, tri-3…) followed by hydrate.

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Transition temperatures are defined as either the change of one crystalline state into another. The most notably example I know of is super cooling a liquid like water without disturbing it. This doesn’t allow a seed crystal to form, so ice doesn’t form below the freezing point. But if the water is disturbed, ice instantly forms from the seed crystal you introduced by disturbing it. The other notable definition is the temperature at which a solid changes, becoming more brittle or ductile [4]. (The exact temperature can be thought of as equilibrium between the two solid states. At this temperature both states can exist in equilibrium) [5]. But as for my experiment I am going to use the first definition to define the transition temperatures. This is because it shall be easier and more reliable to observe the change in the allotrope (crystal state) of the salt hydrate whilst it is cooling, and note the temperature at which this occurs. Than attempt to alter the physical properties of the salt hydrate by heating and cooling and then gauge how I have affected it. Bring into play quite a large human error risk.

I believe the transition temperatures of the salt hydrates can depend on several factors. These include:

The strength of the intermolecular forces between the salt and water molecules. As there are three different types of intermolecular bonding (from weakest to strongest: - Instantaneous Dipole -Induced Dipole, Permanent Dipole-Permanent Dipole, and Hydrogen Bonding).
The ions found in the salt will determine which type of intermolecular force will occur.
The shape of the salt molecules. This will determine how many water molecules can combine with it.
The salt molecule itself should affect how many water molecules are combined to it.

I will briefly explain what the intermolecular forces are. Atoms are chemically bonded together by either ionic or covalent bonds. Ionic bonds are where one or more atoms donate electrons. And the other atom(s) accepts these electron(s). This process forms ionic molecules which are attracted to each other by their difference in charge (because one atom donated, and one atom accepted, each ion will have a + and – charge respectively). This is the type of bonding found in all salts. (Covalent is the sharing of electrons found on the outer electron layer, by two or more atoms). But since a whole molecule cannot donate one electron to another molecule, there must be something different acting as glue, holding molecules together. This is the intermolecular forces.

The first force is the:

Instantaneous Dipole – Induced Dipole. This force is due to the electrons orbiting different atoms are moving incredible fast, they are thought of as clouds of charge. At any instant moment, the electrons are more likely to be found in one region of space. Making the charge at that region incredibly dense. This causes the molecule to have a temporary dipole. This dipole can induce other dipoles in any neighbouring molecules. But because the electrons are moving incredibly quickly these dipoles are being created and destroyed immediately. This is why this force is the weakest [6].

The second force is the:

Permanent Dipole – Permanent Dipole. This force is due to the idea of polar molecules. A polar molecule is a molecule in which there is a difference in electronegativity between the individual atoms and ions. This will cause the electrons in the bond to be ‘pulled’ more vigorously by one atom / ion and cause a dipole in the molecule. This will cause an electrostatic attraction between molecules. This is a stronger force than the first type [8].

The final force is the:

Hydrogen Bonding. But this is a special type of force. This force only occurs when there is a hydrogen atom covalently bonded to either a nitrogen, fluorine or oxygen atom. Because these three atoms have high electronegativity values, and hydrogen has such a low value, the atoms in the bond will pull the electrons away from the hydrogen atom. The bond between the two atoms is so polarised (due to hydrogen having a high positive charge density), the hydrogen atoms are able to form weak bonds with the lone pairs of Nitrogen, Fluorine or Oxygen’s of other molecules. (But they must be bonded to hydrogen atoms). This is the strongest type of intermolecular force [7].

If there are different forces combining the water and salt, then the transition temperature should be different, as more or less kinetic energy will be needed to form the salt hydrates.

My practical work will have two stages to it. The first aspect will be to determine how hydrated each salt I have is. The second part of my practical will be to test the salt hydrates for their transition temperatures.

To complete the first part of my experiment I intend to first set up apparatus to allow me to heat the salt hydrate, and cause water to evaporate. To do this I will need a Bunsen burner to provide a method of heating, an evaporating dish which will allow the water to evaporate away, and a weight scale to measure the difference in weight. Then by performing some calculations I will be able to find the number of water molecules associated with each salt hydrate.

For the second part of my experiment I intend to set up apparatus that will allow me to heat up the salt hydrate, but then allow it to cool. But whilst it is cooling I will need to find a way to slow down the rate of cooling. This will allow me to detect the transition temperature to a greater degree of accuracy.

As the salt hydrate is heated up the compound will melt. Then as the compound cools, a steady rate of decline will be observable. But as the allotrope (crystal state) [8] changes and a salt hydrate reforms, there will be an interruption in the rate of decline of temperature. This is the temperature at which my salt hydrate has been formed. And is the transition temperature of a salt hydrate.

I believe that the transition temperatures of similar salts like group 2 chloride salts when analyzed using graphical methods, will show a predictable trend, which I can use to predict the more volatile group 2 chloride salts. If I can successfully predict the trends I will consider my investigation a success.