Ch. 08 Chemical reactions: What happens? Module 0803
Categories of reaction in aqueous solution
What species is transferred between reactant molecules or ions?
Categorization (or classification) means to allocate into groups those reactions that have a common characteristic. Then it is convenient, for communication purposes, to give each group a label (e.g., round objects, mammals, invertebrates, igneous, recyclable, brown, six-legged, easy-going, eucalypt, ........)
Categorization of objects, concepts and ideas is very common: adults vs. children; expensive vs. cheap; insects vs. animals; comedies vs. tragedies; ……
Overview
This module refers to four common categories of chemical reactions that take place in aqueous solution. It is a useful, but not exhaustive list. They categories of reaction are:
This module refers to four common categories of chemical reactions that take place in aqueous solution. It is a useful, but not exhaustive list. They categories of reaction are:
- acid-base reactions
- oxidation-reduction reactions
- complexation reactions
- dissolving and precipitation reactions
In later modules, we will discuss each of these types of reaction in some depth, as well as important specific examples of each type. Meanwhile, it will be useful for you to have a basic understanding of these reaction types when we discuss general chemistry principles, such as thermochemistry, chemical equilibrium, chemical kinetics, etc.
So perhaps regard this chapter as the basic level of a spiral program.
So perhaps regard this chapter as the basic level of a spiral program.
KEY IDEAS
All reactions involve some sort of redistribution of the atoms and electrons of the reactants species – so that new species are formed.
In many cases, the redistribution involves a transfer of some chemical species from the atoms, ion, or molecules of one of the reactants to the atoms, ions, or molecules of another reactant.
It is common to categorize reactions in aqueous solution according to which species is transferred from particles of one reactant to the particles of the other: are they protons, electrons, ligands, or ions?
Categorization of chemical reactions in aqueous solution
Acid-base reactions: proton transfer
Acid-base reactions are those reactions in which hydrogen ions (protons) transfer from molecules or ions of one reactant to the ions or molecules of the other reactant.
Acid-base reactions are those reactions in which hydrogen ions (protons) transfer from molecules or ions of one reactant to the ions or molecules of the other reactant.
A chemical equation to represent the reaction between ethanoic acid molecules and aquated hydroxide ions is:
Let's portray the tranfer process explicitly:
Squillions of times per second, there are collisions between hydroxides ions and ethanoic acid molecules in solution. On some of these occasions, there is transfer of a proton from an ethanoic acid molecule to a hydroxide ion that it collides with. New species are formed: an ethanoate ion, and a water molecule (HOH or H2O). This is an acid-base reaction.
Terminology: We call the species from which the proton is transferred an acid, and the species to which the proton is transferred is called a base. In the example above, aquated ethanoic acid molecules are the acid, and aquated hydroxide ions are the base.
Another example of acid-base reactions is “ionisation” of carbonic acid in water. This process involves transfer of protons from aquated carbonic acid molecules to surrounding water molecules:
Aquated carbonic acid molecules are the acid, and water molecules the base.
[The double reaction arrow is used in the chemical equation to indicate that the reaction comes to a condition of chemical equilibrium when there are still significant amounts of the reactants in the reaction mixture: it does not go “to completion”.]
[The double reaction arrow is used in the chemical equation to indicate that the reaction comes to a condition of chemical equilibrium when there are still significant amounts of the reactants in the reaction mixture: it does not go “to completion”.]
And another acid-base reaction example is “neutralisation” of ammonia by hydrochloric acid solution.
The reactant species are aquated ammonia molecules and hydronium ions (formed by complete ionisation of hydrochloric acid molecules in solution).
The reactant species are aquated ammonia molecules and hydronium ions (formed by complete ionisation of hydrochloric acid molecules in solution).
Aquated ammonia molecules are the base, and hydronium ions the acid.
Oxidation-reduction reactions: electron transfer
Oxidation-reduction reactions are those reactions in which electrons transfer from atoms, molecules, or ions of one reactant to the atoms, ions, or molecules of the other.
Oxidation-reduction reactions are those reactions in which electrons transfer from atoms, molecules, or ions of one reactant to the atoms, ions, or molecules of the other.
For example, when a piece of zinc metal is put into a copper sulfate solution, zinc atoms are changed into zinc ions (and the metal “dissolves”), and the copper(II) ions in solution are changed to copper metal (which deposits out of solution). A chemical equation to represent this process is:
This happens by transfer of two electrons from every zinc atom to copper(II) ions, which can be portrayed as follows:
Are you comfortable that if two electrons are taken from a neutral zinc atom, it becomes a Zn2+ ion? And that if a Cu2+ ion gains two electrons, it becomes a neutral Cu atom?
Terminology: Chemists' jargon is that the species from which electrons are transferred is said to be oxidised, and the species to which the electrons are transferred is said to be reduced. In this example, zinc atoms are oxidised, and copper(II) ions are reduced.
Another example is the burning of magnesium metal in oxygen – with a very bright flame. A chemical equation to describe the reaction is:
Every magnesium atom has two electrons taken from it (to form Mg2+ ions). Every oxygen molecule takes four electrons (from two magnesium atoms) and each of the oxygen atoms in the molecule take two electrons, forming O2- ions.
Magnesium atoms are oxidised, and oxygen molecules are reduced. The Mg2+ ions and O2- ions stack together to form a crystalline lattice.
In the above examples, involving reactants (shown on both sides of the equation) that are either atoms or ions, it is relatively easy to “see” the transfer of electrons from the change of electrical charge on the species. However, it is not directly apparent when the reactants are covalent compounds – such as when methane gas burns in air or oxygen:
For every methane molecule that reacts, four electrons are transferred to oxygen molecules. So, methane is oxidised, and oxygen is reduced. The electrons are transferred to, and from, covalent bonds within the molecules, rather than change the electrical charges on the species.
Understanding of such reactions as oxidation-reduction reactions needs understanding of the concept of oxidation state.
Complexation reactions
Complexation reactions are those reactions that involve exchange of ligands. Uh-huh, we need some background ....
A ligand is a molecule or ion that has, on at least one of its atoms, a non-bonding pair of electrons (a “lone pair”) by means of which the ligand can form a covalent bond to metal ions.
Some examples of simple ligands are the following, shown as their Lewis structures:
Representation of the structure of a water molecule (formula H2O), showing the two lone pairs on the oxygen atom.
Representation of the structure of an ammonia molecule (formula NH3), showing the lone pair on the nitrogen atom.
Representation of a carbon monoxide molecule (formula CO), showing the lone pair on each of the atoms.
Representation of a cyanide ion (formula CN-), showing the lone pair on each of the atoms.
And now .... What are complexes (either complex compounds, or complex ions)?
If we stir some copper(II) sulfate in water, the subsequent dissolution process is usually represented by the equation:
If we stir some copper(II) sulfate in water, the subsequent dissolution process is usually represented by the equation:
The notation (aq) means aquation (or hydration), which is often taken to mean that the ions are surrounded by water molecules. A better interpretation of the meaning of Cu2+(aq) is a complex ion, in which four water molecules are joined by covalent bonds to each of the copper ions, using the lone pairs on the water molecules to provide the electrons of the covalent bonds.
One of these bonds can be represented in either of the following ways:
One of these bonds can be represented in either of the following ways:
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And the complex ion formed when four water molecules bond to a copper ion:
A representation of the structure of a copper ion-water complex ion.
The formula of this complex ion is written as:
The water molecules in this species are written as OH2 (rather than H2O) to remind us that the oxygen atoms are joined to the copper ion.
The square brackets [ ] and superscript 2+ just mean that the species written inside the brackets has, overall, a charge of 2+ (two more protons than electrons), without any information about the location of the charge.
The square brackets [ ] and superscript 2+ just mean that the species written inside the brackets has, overall, a charge of 2+ (two more protons than electrons), without any information about the location of the charge.
Now, ammonia molecules can also form bonds to copper(II) ions, forming a complex ion with four ammonia molecules bonded to each copper(II) ion. The formula of this complex ion is
When an aqueous ammonia solution is added to a copper sulfate solution, a complexation reaction occurs: ammonia molecules (ligands) “kick out” and replace the water molecule ligands.
Evidence for this change can be seen in a video demonstration on YouTube.
A chemical equation for this complexation reaction can be written as:
Evidence for this change can be seen in a video demonstration on YouTube.
A chemical equation for this complexation reaction can be written as:
Another complexation reaction occurs if an aqueous solution containing cyanide ions is added to the dark blue solution containing the copper(II)-ammonia complex: the ammonia ion ligands are “kicked out” and replaced by cyanide ion ligands, forming a colourless complex ion. A chemical equation to represent this complexation reaction is:
You will have noticed that the charge on the cyanide complex ion is different from that on the ammonia complex ion because the charges on the ligands are different.
Well, that is the essence of complexation reactions, but hardly delves deeply. There is more, lots more .....
Well, that is the essence of complexation reactions, but hardly delves deeply. There is more, lots more .....
Dissolving and precipitation reactions of ionic salts
These are reactions in which ions are transferred from fixed positions in a crystal lattice, to a solution, each ion becoming surrounded (aquated) by water molecules (ie, dissolving). Or vice versa (precipitation, or crystallization).
The process of dissolution of sodium chloride, for example, is discussed in Module 0905: Dissolution of ionic salts in water: a competition. A chemical process to represent the dissolution process is:
An example of the opposite process (precipitation or crystallization) occurs when a solution of silver nitrate is mixed with a solution of sodium chloride. The silver cations and chloride anions in the solution come out of solution, 'growing' a solid structure of orderly stacked ions. A chemical equation is:
You can find discussions in other places about whether these changes are chemical processes or physical processes – sometimes with evangelical fervour. Give us a break! What happens is more important than any human categorization.
Despite that, I accept the IUPAC definition of chemical change as a process in which new chemical species are formed. Aquated chloride ions are a vastly different species (they have different properties) from chloride ions packed in a solid lattice structure interspersed with sodium ions. So these are chemical processes in my view – but I don’t lie in bed pondering the issue at night. That time is taken up pondering upon the meaning of life.
Despite that, I accept the IUPAC definition of chemical change as a process in which new chemical species are formed. Aquated chloride ions are a vastly different species (they have different properties) from chloride ions packed in a solid lattice structure interspersed with sodium ions. So these are chemical processes in my view – but I don’t lie in bed pondering the issue at night. That time is taken up pondering upon the meaning of life.
