Redox Reactions

Metals in Contact With Aqueous Solutions

When a metal rod is placed in a beaker containing a solution of its own metal ions, two solutions can arise.

1. Metal atoms leave the rod and become metal ions in solution

M = M + e

2. Metal ions leave the solution and become metal atoms on the surface of the rod.

M + e = M

In the first case the electrons are released when the metal ions are formed stay on the rod, so the rod carries a negative charge. We say it has a negative potential. Zinc and Magnesium are examples of metals that behave in this way.
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In the second instance, electrons are attracted out of the rod into the solution so the rod carries a positive charge. We say it has a positive potential. Copper and Silver are examples of metals that behave in this way.

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Half Cells
The examples shown above each exhibits a half cell. This is because any two of them can be joined to form a whole cell.
This figure shows an attempt to link tow half cells to make a whole cell.
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– The electrons repel each other from election rich zinc to electron deficient copper
– Ions (electrons) transfer negative charge through the electrolyte in the salt bridge without the two solutions mixing together.
– The salt bridge completes the circuit. This is called a Daniell Cell. The salt bridge is a tube containing (or a strip of filter paper soaked in) an aqueous solution of a good electrolyte example. Potassium Nitrate KNO3 or Potassium Chloride KCl. The salt bridge is usually made up with Potassium Nitrate solution because the salt does not react with other ions commonly used in electro chemical cells.

This enables charge to be transferred without the solution zinc sulphate and copper sulphate being mixed.
NB. The electrons pass through the wire while the ions move along the salt bridge.
Electrolytes are solutions, which decompose at the electrode when an electric current passes through. However, there is no decomposition in the salt bridge.

Functions Of The Salt Bridge
– One obvious function of the salt bridge is to complete the circuit without allowing the two solutions to mix.
– It has another important function. Without it the zinc half-cell would slowly become positively charged as electrons leave it, and the copper half cell would become negatively charged. With the salt bridge in place ions from the salt bridge are able to move in and out of the solutions to neutralize any build up of charge.

Non-Metals in Contact With Aqueous Solutions

It is convenient to make half cells of non-metals such as chlorine and hydrogen. Hydrogen half cell would consist of a layer of hydrogen atoms in contact with hydrogen ions in solution just as a copper electrode consists of copper atoms with in contact with copper ions in solutions.
A layer of immobile hydrogen atoms is created by solidifying it at negative 260 degrees celcius which will be in contact with hydrogen ions in solution.
Eg. HCL, H+, Cl –
Platinum is used to hold the layer of hydrogen atoms. Platinum absorbed molecules of gases onto its surface that is, it holds them in place when they come in contact with the metal.
In a commercial hydrogen electrode the platinum is not shinny but black. The surface of the electrode is porous and pitted. This creates a very large surface area for the hydrogen to be absorbed into.

The Standard Hydrogen Electrode (S.H.E)

The electromotive force (E.M.F) of the daniell cell is the maximum potential difference between the potential of the zinc half-cell and the potential of the copper half -cell. The question is how do we find the potential of each half-cell individually?
To do this we use the standard hydrogen electrode, which is given a potential of zero volts. When it is connected to another half cell the E.M.F between the S.H.E and the second half-cell is equal to the potential of the second half-cell.
The S.H.E is actually the standard hydrogen half-cell.
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It consist of hydrogen gas bubbling over a platinum electrode immersed in a solution of hydrochloric acid supplying hydrogen ions.
The Reaction that takes place is    2 H+ + 2 e- = H2
The platinum electrode simply produces an inert metal connection between hydrogen gas and hydrogen ions in solution.

When the S.H.E is being used to establish the potential of a Redox system, standard conditions must apply. These are:
– The hydrogen must be at a pressure of 1ATM
– The condition of the hydrogen ions must be 1mol dm3
– The temperature must be 25 degrees Celsius (298k)

Under these conditions the potential is defined as exactly zero volts.

Rate Of Reaction

The rate of reaction or sometimes called the speed of reaction refers to how slow or fast a reaction takes place. The following concepts are associated with reaction rate and that you will see often.

Reaction Rate

Reaction rate is defined as the change in concentration of a substance divided by the time taken for that change to take place it is measured in mol dm-3 s-1 .

Rate constant

Proportionality constant in the relationship between reaction rate and reactant concentration

Rate = k [A] [B]

k = rate constant

Order of Reaction

Reactions have an order with respect to each reactant. E.g.The rate equation has been determined experimentally.

Rate = k [NO] O3

The reaction is first order with respect to NO, which means that the rate depends on the concentration of NO raised to the first power [NO]1.

It is also first order with respect to O3and that is [O3]1 . The sum of the individual orders gives the overall reaction order.

Rate – Determining Step

This is the slowest step in a reaction mechanism and is the one that determines the overall rate.

Half Life

Half life of a reaction is the time required for a reactant to reach half of its original concentration.

Activation Energy

Activation Energy Ea, is the minimum collision energy required to activate the molecules into a state from which reactant bonds can change into product bonds.
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Collision Theory

For a chemical reaction to occur the reacting molecules must collide with each other.
This forms the basis of the collision theory of chemical kinetics.
Basically, this theory states that the rate of a reaction is proportional to the number of collisions occurring each second between reacting molecules.
Rate is prpportional to Number Of Collisions divided by Seconds

It is important that you make note of the following:
– Molecules will only react if they collide with each other.
– Reactions will occur if there is enough energy in the collision
– Increased concentration increases the likely hood of collision, which increases reaction rate.
– Increased temperature increases the average energy of collisions, which increases the reaction rate.
– NB. The molecules must collide in the correct orientation to produce a chemical change.
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This collision that can lead to a net reaction is known as an effective collision.

 

NB. Molecules must possess enough energy when they collide to produce a chemical change.
When two slow moving molecules collide, their electron clouds cannot inter-penetrate much and they just bounce on each other, chemically unchanged.
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When fast moving molecules collide atoms approach each other much more closely and their electron clouds inter-penetrate. This can lead to bond breaking and bond making. The net change here is  A B + C = A + B C

Catalysis
A catalyst is something added to a reaction that increases its rate, but does not change in concentration. This means the same amount of catalyst remains after the reaction as before.
Catalyst increases reaction rate by lowering the activation barrier for the reaction without altering the products of the reaction.
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Enzymes

A vast variety of significant catalysts are provided by enzymes. They occur in living systems and allow reactions to occur at relatively low temperatures like 37 degrees Celsius) our body temperature) and at relatively low pressure.
They are organic compounds that catalyze the reactions involved in the vital processes within animals and plants.
Enzymes are proteins, long chains of amino acids linked together by peptide bonds. Enzymes are involved in almost every reaction in your body. For example they help you to digest food and protect you from dangerous waste products that form in your body.
Enzymes are often used in industrial processes to catalyze biochemical reactions.