📝 Redox reaction

Task

🎧️ Listen to the recording and mind pronunciation of words.

Oxidation is the addition of oxygen to a substance, and reduction is the removal of oxygen from a substance. Reduction and oxidation always take place together. If an atom is oxidized, its oxidation number increases. If an atom is reduced, its oxidation number decreases.
Oxidation numbers can be used to balance equations and to define the processes of oxidation and reduction. The oxidation state of an element in a compound is equivalent to the number of electrons it has lost or gained by forming bonds. In molecules or compounds, the sum of the oxidation numbers on the atoms is zero. In complex ions, the sum of the oxidation numbers on the atoms is equal to the overall charge on the ion. In elements in their standard states, the oxidation number of each atom is zero.
A redox reaction is a reaction in which one substance is reduced, and another is oxidized. Redox reactions involve the transfer of electrons. Equations written to show what happens to the electrons during oxidation and reduction are called half-equations. Some reactions involve one substance being oxidized and reduced. These reactions are called disproportional reactions.
Half of an electrochemical cell is where either oxidation or reduction occurs. It consists of an electrode in contact with a solution of ions. The simplest half cells are a metal electrode immersed in a solution of metal ions. The electrochemical series is a list of standard electrode potentials (E^θ). The equilibria are written with the electrons on the left of the arrow, i.e. as a reduction. A standard hydrogen electrode is a half-cell in which hydrogen gas at a pressure of 101 kPa bubbles through a solution of 1.00 mol dm^–3 H⁺_(aq) ions. The standard electrode potential of a half-cell is the voltage of the half-cell under standard conditions compared with a standard hydrogen electrode.
Two half-cells joined together to form an electrochemical cell. An electrochemical cell can be represented in a shorthand way by a cell diagram. The double vertical lines represent a salt bridge. The single lines represent a phase change between the solid metal and the aqueous metal ions
The half-cell with the greatest negative potential is on the left of the salt bridge. The left cell is being oxidized while the right is being reduced. The positive electrode is taken to be the least negative half-cell, and the negative electrode is the most negative half-cell. When a pair of electrodes are connected, electrons flow from the more negative to the more positive.
The standard cell potential (E^θ_cell) is the voltage developed under standard conditions when two half-cells are joined. The standard cell potential is calculated from the difference between the standard electrode potentials of two half-cells. E^θ_cell = E^θ (positive electrode) – E^θ (negative electrode) or E^θ_cell = E^θ_{right cell} – E^θ_{left cell}
Electrodes with more negative electrode potentials have a lower tendency to accept electrons. The signs of the electrodes can be used to predict the direction of the reaction. A redox reaction will occur if the E^θ of the half-equation involving the species being reduced is more positive than the E^θ of the half-equation of the species being oxidized. Redox equations can be constructed by combining the relevant half-equations. The value of the E^θ of a half-cell containing a metal in contact with its aqueous ions becomes more negative as the concentration of the aqueous ion decreases.

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