:






Oxydation-reduction reactions (redox-reactions).






Reactions, in which one or more electrons are transferred, are called oxidationreduction reactions or redox reactions.

We defi ne the oxidation states (or oxidation numbers) of the atoms in a covalent compound as the imaginary charges the atoms would have if the shared electrons were divided equally between identical atoms bonded to each other or, for different atoms, were all assigned to the atom in each bond that has the greater attraction for electrons.

Oxidationreduction reactions are characterized by a transfer of electrons. In some cases, the transfer occurs in a literal sense to form ions.

Rules of assigning oxidation states.

It is worthwhile to note at this point that the convention is to write actual charges on ions as n+ or n-, the number being written before the plus or minus sign. On the other hand, oxidation states (not actual charges) are written +n or -n, the number being writtenafter the plus or minus sign.

With this background, we can now defi ne some important terms. Oxidation is an increase in oxidation state (a loss of electrons). Reduction is a decrease in oxidation state (a gain of electrons). Thus in the reaction

sodium is oxidized and chlorine is reduced. In addition, Cl2 is called the oxidizing agent

(electron acceptor), and Na is called the reducing agent (electron donor).

Concerning the reaction

we can say the following:

carbon is oxidized because there has been an increase in its oxidation state (carbon has formally lost electrons).

oxygen is reduced because there has been a decrease in its oxidation state (oxygen has formally gained electrons).

CH4 is the reducing agent,

O2 is the oxidizing agent.

Note that when the oxidizing or reducing agent is named, the whole compound is specifi ed, not just the element that undergoes the change in oxidation state.

Oxidation is an increase in oxidation state. Reduction is a decrease in oxidation state. A helpful mnemonic device is OIL RIG (Oxidation Involves Loss; Reduction Involves Gain). Another common mnemonic is LEO says GER. (Loss of Electrons, Oxidation; Gain of Electrons, Reduction). An oxidizing agent is reduced and a reducing agent is oxidized in a redox reaction.

A summary of an oxidationreduction process in table form:

OXIDIZED REDUCED
Loses electrons Gains electrons
Oxidation state increases Oxidation state decreases
Redusing agent Oxidising agent

Balancing OxidationReduction Equations

Oxidationreduction reactions in aqueous solutions are often complicated, which means

that it can be diffi cult to balance their equations by simple inspection. In this section we

will discuss a special technique for balancing the equations of redox reactions that occur

in aqueous solutions. It is called the half-reaction method.

The Half-Reaction Method for Balancing OxidationReduction

Reactions in Aqueous Solutions

For oxidationreduction reactions that occur in aqueous solution, it is useful to separate the reaction into two half-reactions: one involving oxidation and the other involving reduction. For example, consider the unbalanced equation for the oxidationreduction reaction between cerium(IV) ion and tin(II) ion:

This reaction can be separated into a half-reaction involving the substance being reduced,

and one involving the substance being oxidized,

The general procedure is to balance the equations for the half-reactions separately and then to add them to obtain the overall balanced equation. The half-reaction method for balancing oxidationreduction equations differs slightly depending on whether the reaction takes place in acidic or basic solution.

Elements and compounds react with each other in numerous ways. Memorizing every type of reaction would be challenging and also unncecessary, since nearly every inorganic chemical reaction falls into one or more of four broad categories.

1. Combination Reactions

Two or more reactants form one product in a combination reaction. An example of a combination reaction is the formation of sulfur dioxide when sulfur is burned in air:

S (s) + O2 (g) → SO2 (g)

Decomposition Reactions

In a decomposition reaction, a compound breaks down into two or more substances. Decomposition usually results from electrolysis or heating. An example of a decomposition reaction is the breakdown of mercury (II) oxide into its component elements.

2HgO (s) + heat → 2Hg (l) + O2 (g)

1. Single Displacement Reactions

A single displacement reaction is characterized by an atom or ion of a single compound replacing an atom of another element. An example of a single displacement reaction is the displacement of copper ions in a copper sulfate solution by zinc metal, forming zinc sulfate:

Zn (s) + CuSO4 (aq) → Cu (s) + ZnSO4 (aq)

Single displacement reactions are often subdivided into more specific categories (e.g., redox reactions).

2. Double Displacement Reactions

Double displacement reactions also may be called metathesis reactions. In this type of reaction, elements from two compounds displace each other to form new compounds. Double displacement reactions may occur when one product is removed from the solution as a gas or precipitate or when two species combine to form a weak electrolyte that remains undissociated in solution. An example of a double displacement reaction occurs when solutions of calcium chloride and silver nitrate are reacted to form insoluble silver chloride in a solution of calcium nitrate.

CaCl2 (aq) + 2 AgNO3 (aq) → Ca(NO3)2 (aq) + 2 AgCl (s)

A neutralization reaction is a specific type of double displacement reaction that occurs when an acid reacts with a base, producing a solution of salt and water. An example of a neutralization reaction is the reaction of hydrochloric acid and sodium hydroxide to form sodium chloride and water:

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

23.Concentration units.

Solution - a mixture consisting of a solute and a solvent

Solute - component of a solution present in the lesser amount

Solvent - component of a solution present in the greater amount

Concentration - amount of a solute present in a solution per standard amount of solvent

There are numerous ways of expressing concentrations. It will be important to know the units used to express each concentration, as these units essentially define the concentration.

Percent by Mass (Weight percent, ω %)

The percent by mass of a solute is simply the number of grams of solute in exactly 100 g of solution. It can be calculated as exactly 100% times the mass of the solute divided by the mass of the entire solution: