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Redox Equations

Be able to apply the electron transfer model of redox, including oxidation states and half equations to D block.
by

Josh Smith

on 23 October 2012

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Transcript of Redox Equations

Redox is an abbreviation of reduction and oxidation. Redox Equations Redox processes in terms of electron transfer When metals react with oxygen they form oxides.
E.g. 2Mg+O2--->2Mg2++O2-
4Na+O2--->2(Na+)2O2- Electron Transfer in Redox Reactions When powdered zinc is added to copper(II) sulphate solution an exothermic reaction occurs. Copper ions are reduced to a deposit of red-brown and zinc goes into solution as zinc ions. The flow of electrons can be shown in a circuit, as shown in the diagram below. They normally occur simultaneously.
Redox reactions include processes such
as burning, rusting and respiration. Redox is the
process of
transfer. When the electrons
are transferred,
energy is released in
the form of electrical
energy. Red=Charge The metal is oxidized and the oxygen is reduced. During this process the metal atoms lose electrons to from positive ions and oxygen gains electrons to form negative oxide ions. The oxygen takes up the electrons given up by the metal. 2Mg--->2Mg2++4e-
O2+4e- --->2O2- Half equations!! Electron transfer like this is called redox reactions. Reduction is the gain of electrons Oxidation is the loss of electrons. Reducing agents donate electrons. Oxidizing agents accept electrons. At the zinc rod rod, zinc atoms give up electrons and form zinc ions which go into solution as Zn2+. The electrons flow from the zinc rod through the external circuit including the bulb to the copper rod where they combine with Cu2+ ions to form copper atoms.
This flow of electrons causes the light bulb to light up! There are three simple steps to balancing a redox reaction. These are:

1.Write down the oxidizing and reducing agents and their products.

2.Write separate half-equations for the oxidation and reduction processes and balance these with respect to atoms and charge.
3.Combine the half-equations to obtain the overall. Balancing Redox Equations 1.The oxidizing agent is Br2, which forms Br- on reaction. The reducing agent Fe2+, which forms Fe3+ on reaction.

2.The balanced Br2/Br- half equation is:
Br2+2e--->2Br-.
Of course, one Br2 can form 2 Br- ions and consequently the Br2 requires 2e- to undergo reduction to 2Br-. The balanced Fe2+/Fe3+ half-equation is:
Fe2+-->Fe3++e-.

3. You then times the Fe half equation by 2, cancel the electrons and then add the two half equations together. Worked example,
bromine + Iron (II) ions If a metal rod is dipped into a solution containing its' own ions, an equilibrium is set up.
The equilibrium will be far to the right. The metal will gain a negative electrical potential (or a negative charge). This arrangement is called an electrode. Half Cells We cannot measure electrical potential directly, only potential difference (voltage). We can measure the potential difference by connecting different electrodes and using a voltmeter to measure the potential difference.. The circuit is completed by a salt bridge.
If you want to compare the tendency of different metals to release electrons, there must be a standard electrode. The half cell chosen is called the standard hydrogen electrode The Hydrogen electrode To do this we bubble hydrogen gas into a solution containing H+ ions. Hydrogen doesn't conduct, therefore electrical contact is made using a piece of unreactive platinum metal. The electrode is used under standard conditions. The potential of the standard hydrogen electrode is defined as zero. This means that if it is connected to another electrode, the measured voltage, EMF, is the electrode potential of that cell. The Electrochemical Series The equilibria are written with the electrons on the left of the arrow. These are called electrode potentials (or reduction potentials).
Arranged in this order with the most negative values at the top , this list is called the electrochemical series. There is a shorthand for writing down the cell formed by connecting electrodes.
For Example:
-A vertical solid line, | , indicates a phase boundary. E.g. between a solid and solution.
-A double vertical line shows a salt bridge.

Cell Representation This is followed by the Emf value.
The EMF=EMF of right - EMF of left.
If the Emf value is positive the electrons
are flowing towards the copper half cell
which is positive.
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