Electrochemistry - Part 1

Electrochemistry is concerned with the interconversion of chemical and electrical energy. The basic unit of an electrochemical process is a redox reaction, in which one reactant is reduced (gains electrons), and another reactant is oxidised (loses electrons). Redox reactions occur in many situations, and are used in many applications e.g. how batteries work, how nerve cells operate and how aluminium is produced!

Useful concepts for describing electricity:

Electrochemistry deals with the movement of electrical charge, so we need to know some ways of describing this movement.

Charge (usually represented as Q):

The SI unit of electrical charge is the coulomb C. An electron has a charge of –1.60218 x 10^-19 C (often shown as e). A singly charged positive ion carries a charge of 1.60218 x 10^-19 C)

A mole of electrons has a charge one Farad, also known as the Faraday constant. It is given by:

F = NA (avagdro’s constant) x e = 6.02214 x 10^23 /mol x 1.60218 x 10^-19 C = 96485 C/mol

Current (usually represented as I):

The rate of movement of charge is described as the electric current. The SI unit of electric current is the ampère A (also called amp). One ampère is the current which flows when one coulomb of charge flows for one second, such that:

1 A = 1 C/s

A useful formula is Q = I x t 

Potential difference or voltage (sometimes represented as V):

A current flows between two points if there is a potential difference between 

them – this is analagous to a ‘driving force’, which pushes charges from 

one point to another. The SI unit of potential difference is the volt V, such 

that :

1 V = 1 J/C

Resistance (sometimes represented as R):

A material through which electricity might flow usually exhibits a resistance. Ohm’s law* states that the current flowing through a length of material is proportional to the potential difference between the two points, and inversely proportional to the resistance. The SI unit of resistance is the Ohm Ω, such that:

1Ω = 1V/A

A useful formula is V = I x R

Getting Started:

What happens when we put zinc in water? One zinc atom might dissolve in the solution as a Zn2+ ion. In doing so, it would leave its electrons behind on the zinc metal:

If a few zinc atoms do this, the consequences of dissolution are:

  • A build-up of positive charge in the solution
  • A build-up of negative charge on the zinc metal

This charge separation is not favoured – this is known as the “electroneutrality principle”.

Why is this?

This inhibition is a consequence of the fact that unlike charges attract one another spontaneously, but we have to do work (put energy in) if we want to either:

  • separate unlike charges (e.g. separating Zn2+ from its electrons)
  • bring like charges together (e.g. pushing Zn2+ into a solution which already contains some Zn2+)

So, when we put zinc in water, nothing happens! There just isn’t enough energy for anything to happen.

Structure and Bonding



Types of Bonding in Atomic and Molecular Species:

A word that comes up very often in Chemistry, especially when talking about shape and structure, is electronegativity. In basic terms, the more electronegative an atom is, the more it likes to have electrons. Electronegativity of atoms increases as you go from left to right along the periodic table, and from the bottom to the top. 


In very general terms we may consider the bonding between different type of atoms to belong to a number of different classes.  

  1. Covalent Bonding.  Covalent bonding involves the formal sharing of electrons between two atoms.  In general, covalent bonds are found in elements with high electronegativity (i.e. towards the top right of the periodic table). In heteroatomic molecules (molecules which contain different kinds of atoms, for example, CO, carbon monoxide), covalent bonding occurs when the different atoms have a small difference in electronegativity.
  2. Ionic Bonding.  This type of bonding occurs in compounds in which the constituent elements have a relatively large difference in electronegativity. 
  3. Metallic Bonding.  This is found in elements which have a low electronegativity, i.e. towards the bottom left of the periodic table. Obviously, all metals have metallic bonding!
  4. Hydrogen Bonding.  This is primarily an electrostatic interaction (an interaction between something positively charged and something negatively charged, think magnets!), traditionally between a hydrogen atom attached to an electronegative atom and a second electronegative atom.  It is an example of a non-covalent interaction.

In each case there a number of different models which may be used to describe these types of bonding.  Often, the models which best describe reality are by far the most complex. 

     

    Building Organic Architectures - Drawing Molecules

    One of the things people often struggle with when they begin learning more advanced chemistry is drawing molecules in a simple way. Chemists use a method called skeletal structuring. Here is a simple guide about how to use it.

    Example 1: 

    Rule 1 - Draw carbon chains as a zig-zag line

    Rule 2 - Do not label carbon and hydrogen atoms


     

    Example 2:

    Rule 3 - Label ‘unusual’ atoms

    Rule 4 - Label hydrogens attached to them

          

    Example 3:



    Rule 5 - Draw longest chain first and attach side chains later

    Test yourself: try and find the number of Carbon and Hydrogen atoms in this complex molecule!


      

    There are 31 Carbon atoms and 38 Hydrogen atoms.

    Onwards, with example 4!

     

    Rule 6 – Remember that each carbon should have four bonds

    You may need to use multiple bonds

    Example 5:

    Rule 7 - Remember stereochemistry