Mr. Lou's Chemistry Website

Daniel McIntyre Collegiate

Lesson 2 - The Process of Dissolving

If we want to understand solutions, we must first examine how solutions are formed. In this lesson, we will learn about the dissolving process; more specifically the dissolving of a solid in a liquid.

The Structure of Water

Water is called the universal solvent because it can dissolve a large number of substances. Many biological processes occur in an aqueous environment. What makes water so different from other solvents? The answer to this question lies in its structure.

Water is made of two hydrogen atoms covalently bound to a single oxygen atom. That is, the hydrogen atoms and the oxygen atom each share a pair of electrons.

The hydrogen atoms are bonded to the oxygen at an angle of 104.5°.

In order to understand why water is unique, we will need to take a closer look at the hydrogen-oxygen covalent bonds and the nature of covalent bonding.

When two like atoms form a covalent bond, like in H2, the electrons are shared equally between the two atoms because both atoms sharing the electrons are identical. Do all covalent bonds involve atoms sharing electrons equally? How atoms share electrons will be discussed on the next page.

Electronegativity

Different atoms have differing abilities to attract electrons in a bond. This ability is called electronegativity (EN) . Electronegativity is the attraction an atom has for the shared pair of electrons in a covalent bond. Fluorine is the most electronegative of all atoms with a value of 4.0 (no units). The least electronegative elements are in the lower left corner of the periodic table (cesium = 0.7). Generally, electronegativity tends to increase as you move across the periodic table from left to right and up the periodic table.

When atoms with different electronegativities form a bond the atom with the higher electronegativity will draw electrons to itself in that bond. If electrons are closer to one atom than another in a bond, the charge of the electrons is not shared equally. As a result, one atom is more negatively charged than the other.

Bonds formed between atoms of differing electronegativities are called polar bonds because one end of the bond has a partial positive charge and the other has a partial negative charge because the electrons are shared unequally. A good example of a polar bond is hydrogen fluoride, HF.

Fluorine is the most electronegative atom. When bonded to hydrogen, the fluorine draws the electrons towards itself. The arrow between the H and F indicates the direction the electrons are drawn towards resulting in a more positive end. As a result, the fluorine end of the bond has a partial negative charge (δ-) and the hydrogen has a partial positive charge (δ+).

The only true covalent bonds are bonds in which electrons are shared equally. This only occurs when two of the same atom (that is atoms with the same electronegativity) form a molecule. Molecules like H2 and O2 form non-polar covalent bonds.

Generally, the greater the electronegativity difference the more polar the bond. If the electronegativity difference is large enough the bonds are considered to be ionic. This is why we use the general rule that metal and non-metal atoms make ionic bonds, since metals usually have low electronegativities and non-metals have high electronegativities.

Click on the links below to view animations that illustrate how electronegativity differences affect the nature of a bond. The blue shaded regions in the animations represent the area occupied by the electron cloud. (Note: Your computer may give a warning window. Allow the computer to download the animation. You may click SAVE or RUN.

 Non-Polar Molecules

Some molecules can contain polar bonds, but because of the shape are considered to be non-polar molecules. A great example of this is carbon tetrafluoride, CF4. Each C-F bond is polar, but because the molecule is symmetrical, no one side is more polar than the other. The polar bonds “balance out” or cancel out each other because they are evenly distributed around the carbon atom. Look at the diagram below.

In three dimensions CF4 looks like this:

Oils and waxes are made of long chains that contain mostly carbons and hydrogens. Since carbon and hydrogen have similar electronegativities most oils and waxes are considered to be non-polar molecules.

The Dissolving Process

When a solute dissolves in a solvent the individual particles of the solute separate from the other particles of the solute and move between the spaces of the solvent particles. The solvent particles must collide with the solute particles and forces of attraction between solute and solvent particles "hold" the solute particles in the spaces. See the diagram below.

There are 3 steps to the dissolving process:

  1. The solvent particles must move apart to make room for solute particles. This process requires energy to overcome forces of attraction between solvent particles. The first step in the dissolving process is endothermic.

 

  1. The solute particles must separate form the other solute particles. This process also requires energy to overcome the forces of attraction between the solute particles. The second step in the dissolving process is endothermic.

    The energy of this step is known as lattice energy. Lattice energy is the amount of energy required to separate the molecules or ions from each other in a solid crystal.
  2. When the solute particles move between the solvent particles the forces of attraction between solute and solvent take hold and the particles "snap" back and move closer. This process releases energy. The final step in the dissolving process is exothermic.

    This is known as the heat or energy of hydration. When water surrounds individual molecules or ions and the molecules or ions are said to be hydrated. When a solvent other than water is used the processes of the solvent particles surrounding the solute particles is called solvation.

Dissolving Sugar vs. Salt in Water

When we dissolve salt and sugar in water, there doesn't appear to be a difference. However, if we look at the molecular level a difference becomes apparent.

To demonstrate this difference, we will use an apparatus shown below.

This apparatus is simply a light-bulb connected to two wire electrodes that interrupt an electrical current. The light bulb does not light until the circuit is completed.

We can start our experiment by putting the apparatus in a container of pure water.

The light-bulb does not light! This shows that pure water is NOT a conductor of electricity.

If we take our apparatus and place it into a container with dissolved sodium chloride (table salt) and another containing sugar, we can see if these solutions conduct an electric current.

Only sodium chloride solution in water conducts an electric current to light the light bulb. A solution of sodium chloride in water is called an electrolyte since it conducts an electric current. However, sugar in water solution is a non-electrolyte since it does not conduct an electric current. These results will serve as our operational definitions of electrolyte and non-electrolyte.

Pure water is a non-electrolyte because it does not contain anything to carry an electric current. In order for a solution to conduct an electric current, charged particles or ions must be present in the solution. The more ions present in a solution the greater the conductivity, up to a certain limit. The conductivity is due to the movement of the ions in solution.

Sodium chloride is an ionic compound (that is, oppositely charged particles held together by electrostatic forces, called ionic bonds) when it dissolves, it breaks up into ions, or dissociates. These ions become surrounded by water molecules, or hydrated, and are free to move and carry an electric current through the solution.

As a solid, sodium chloride does not conduct a current since its ions are held tightly by the ionic bonds in a crystal lattice structure (as seen in the diagram above). However, if we melt the solid sodium chloride crystals, the liquid sodium chloride will conduct a current, since it separates into positively charged sodium ions and negatively charged chloride ions.

All ionic compounds dissociate into positive and negative ions in water and dissociate into ions when melted, forming liquids that conduct electric currents.

Sugar, C12H22O11 , is a molecular compound (all non-metal atoms joined with covalent bonds) and dissolves as whole molecules. These molecules are uncharged so there are no particles to carry a charge. The animation below illustrates how sugar, or other molecular compounds dissolve in water.

Dissociation Equations

A non-electrolyte such as table sugar or sucrose (C11H22O11) dissociates according to the following equation:

C11H22O11(s)  --- >  C11H22O11(aq)

The sugar does not separate into charged particles, but dissolves as whole molecules.

The dissociation or ionization of a substance in water can be shown using chemical equations.

Ionic compounds, such as sodium chloride, dissociate according to the equation shown below:

NaCl(s)  --- >  Na+(aq) + Cl(aq)

The (s) after the sodium chloride represents solid and the (aq) represents aqueous, which means "dissolved in water". When an ionic compound dissolves in water, it no longer exists as a compound, but as freely moving ions that are generally not associated with each other.

Example 1. Write the equation for dissolving solid magnesium chloride, MgCl2, in water.

Solution.

Determine if the compound is ionic or molecular.

Magnesium chloride is composed of a metal, Mg, and non-metal, Cl, atoms. Usually, metal and non-metal atoms will produce an ionic compound.

Write the ions.

Mg2+ and Cl

Use the subscripts to indicate the coefficients.

The subscripts for each ion indicate the number of each ion that will dissociate. Magnesium chloride is composed of 1 magnesium ion and 2 chloride ions.

1 Mg2+ and 2 Cl

Write he equation using the appropriate state for the compound (s) and (aq) for each dissociated ion.

MgCl2(s)  --- >  Mg2+(aq) + 2 Cl(aq)

When magnesium chloride is dissolved in water, the compound exists as freely moving magnesium and chloride ions, not these ions together in a compound.

Example 2. Write the equation for solid aluminum sulphate, Al2(SO4)3 dissolved in water.

Solution.

Determine if the compound is ionic or molecular.

Aluminum sulphate is composed of a metal, Al, and non-metal, S and O, atoms. Usually, metal and non-metal atoms will produce an ionic compound.

Write the ions.

Al3+ and SO42–

Use the subscripts to indicate the coefficients.

The subscripts for each ion indicate the number of each ion that will dissociate. Aluminum sulphate is composed of 2 aluminum ions and 3 sulfate ions.

2 Al3+ and 3 SO42–

Write he equation using the appropriate state for the compound (s) and (aq) for each dissociated ion.

Al2(SO4)3(s)  --- >  2 Al3+(aq) + 3 SO42–(aq)

Example 3. Write the equation for the dissolving of liquid methanol, CH3OH, in water.

Solution.

Determine if the compound is ionic or molecular.

Methanol is composed entirely of non-metal atoms, which is usually an indication of a molecular compound. Molecular compounds do not dissociate, but dissolve as whole molecules.

The exception to this general rule is compounds containing the ammonium ion, NH4+. Compounds containing the ammonium ion are ionic compounds, like ammonium chloride, NH4Cl.

Write he equation using the appropriate state for the compound (l) and (aq) for the dissolved molecule.

CH3OH(l) --- >  CH3OH(aq)