Ionic Chemistry

Acid-Base Reactions

The acidity of a solution is the concentration of H (protons)
Acidity relates specifically to the concentration of H+ , H-
An acid is a substance which can lose one or more equivalents H+. and a base is a substance which can pick up H+
When an acid reacts with a base, H+ is transferred
Water can behave as an acid or a base (amphoteric)
Water can also ionize itself
As most acid/ base reactions take place in water, it is common to not include it in any equations.

Equilibrium – Acid Base Reactions

Equilibrium is relevant when talking about acidity
Many reactions can go backward as well as forward will remain stable- the system is said to be at equilibrium
After some time, the speed of the forwards reaction will be the same as the backwards reaction
The concentrations of the reactants and the products
Acids which only dissociate to a small extent are called weak acids

Strength and Concentration – Acid-Base Reactions

Strength and concentrations are different
Strength refers to the extent of dissociation
Concentration refers to the amount of the acid in solution
It is possible to have a concentrated solution of a weak acid or dilute solution of a strong acid.
Concentration is measured in mol dm^-3
Acid strength could be measured by using Ka values but these are generally not convenient numbers

Acids in water

When an acid is added to water H will change
The acidity of a solution is conveniently expressed by pH
Pure water has a pH of 7
Acidic solutions will have greater H and pH < 7
Basic solutions will have smaller H and pH > 7

Bases – Acid- Base reactions

Bases can accept protons
As with the acids the strength of the base can be shown by the equilibrium constant
More commonly base strength is represented by pKa by considering the reaction of the conjugate acid

Strong and weak bases

Strong bases include NaOH and KOH- these will be completely dissociated in water
Amines are examples of weak bases

Polyprotic Acids

Some acids can lose more than one proton and hence have more than one Ka value
Successive removals become more difficult as the molecule has to bear a greater and greater charge
It often forms the basis of buffer solutions

Buffer Solutions

Are specific recipes containing a weak acid and its conjugate base or a weak base and its conjugate acid
Buffers have the property of being able to oppose changes in pH despite the addition of acid/base
Important in maintaining pH in experimental set ups (either chemical or biological)
Studies involving proteins require buffered solutions to preserve their structures
Enzymes are often highly sensitive to changes in pH
Buffers can also help to preserve drugs in solutions for periods of time by preventing acidic/basic degradation.
Water has no buffering capacity
A buffered solution would be able to maintain its pH within a few hundredths of a pH unit after the addition of similar amounts of acid or base.

Buffer Solution in action

A common buffer system is the ethanoic acid/ sodium ethanoate solution

This equation is useful for buffer calculations but during its derivation some assumptions have been made
The biggest assumption is that the equilibrium concentrations are the same as the concentrations of the components which are mixed to make the buffer
To ensure that the effects of the assumptions are minimized, the concentrations of the components should exceed Ka by at least 100 times.

Buffer Capacity and Ranges

Buffers do not have limitless ability to resist change in pH
Addition of sufficient acid or base will exceed the capacity of the buffer and a significant change in pH will be observed.
This will happen before one of the buffer components is completely changed to the other
The more concentrated the buffer the greater its capacity
The most effective buffer will be one which has identical concentrations of acid/ conjugate base
The buffer will be effective one unit either side of this point

Buffer Solution Ranges

The ethanoate buffer gave a pH of 4.74 with equal concentrations of acid and conjugate base so ethanoate buffers will be effective in the pH range 3.74 – 5.74
Buffer recipes can be found in books to ensure sufficient capacity, giving solutions of various pH

Other Ionic Reactions

Carbonates react with acids to form Carbon Dioxide gas
Sulfites react with acids to form sulfur dioxide gas
The above two reactions are overall reactions of the actual molecular events.
Sulfides react with acids to form hydrogen sulfide gas

Aims

To introduce the concept of receptors as drug targets
To introduce the concepts of agonists and antagonists
To consider the types of interaction which drugs may make with their targets
To consider the role of water in drug-receptor binding
To consider the roles of conformation and shape in receptor binding

Solutions and Molarity

The majority of ionic chemical reactions take place in aqueous solution
Molarity is the measurement of the concentration of a chemical in solution
The unit of molarity is the Molar (M)
In diluting a solution, the number of moles remains constant.

Redox Reactions

RedOx reactions are by far the most important type of reactions
RedOx reactions involve the transfer of electrons from one species to another
Oxidation is defined as the loss of electrons
Reduction is defined as the gain of electrons
Oxidation and reduction must occur simultaneously since the electrons lost in the Oxidation must go somewhere.
The Net Ionic Equation shows the reaction of iron metal with CU2 to produce iron and copper metal

Oxidizing and Reducing Agents

The species that is reduced itself and causes another species to be oxidized therefore known as the oxidizing agent
The species that are oxidized and causes another to be reduced is therefore known as the reducing agent

Oxidation Number

The concept of oxidation numbers is a simple way of keeping track of electrons in a reaction.
The oxidation number of an atom in a substance is the actual charge of the atom if it exists as a monatomic ion.

Rules for Assigning Oxidation Numbers

It is possible to predict the upper and lower limits of main group elements
The upper limit is equal to the group number
The lower limit is the group number -8
Oxygen will never have an ON = +6 and Flourine will never have an ON = +7
There are two ways to deal with RedOx Reactions (1) Treat them as any other reactions (2) We can write this reaction in terms of two half- reactions.
A half-reaction is one of the two parts of an oxidation-reduction reaction. One involves the loss of electrons and the other involves the gain of electrons (reduction)

Combination Reactions

A combination reaction is a reaction in which two substances, usually two elements, combine to form a third substance
A decomposition reaction is a reaction in which a single compound reacts to give two or more substances
A displacement reaction is a reaction in which an element reacts with compound, displacing an element from it.
A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame

Balancing RedOx Reactions

We can split the reaction into two half-cells before balancing
We will balance RedOx reactions using the Oxidation Number method

Steps:

Assign ON to ALL elements in reaction
Identify the species that are oxidized/ reduced
Compute the number of e s lost in OX and gained RED draw lines between the two pairs including the number of e s lost/ gained
Multiply one or both reactions so that both numbers match, use these factors as balancing coefficients
Balance other species that were not involved in the electron exchange

Quantitative Analysis

Analytical chemistry deals with the determination of composition of materials that is the analysis of materials
Involves the determination of the amount of a substance or species present in a sample of material

Gravimetric analysis

Is a type of quantitative analysis in which the amount of a species in a material is determined by converting the species into a product that can be isolated and weighed.
Precipitation reactions are often used in gravimetric analysis
The precipitate from these reactions is then filtered, dried and weighed.

Summary:

Most reactions are either precipitation reactions, acid-base reactions or oxidation-reduction reactions
Acid base reactions are non-proton reactions
Oxidation-reduction reactions involve a transfer of electrons from one species to another
Oxidation reduction reactions are the most important type of reactions
Oxidation reduction reactions usually fall into the following categories: combination reactions, decomposition reactions, displacement reactions and combustion reactions
Molarity is defined as the number of moles of solute per liter of solution. Knowing the molarity allows you to calculate the amount of solute in a given volume of solution
Quantitative analysis involves the determination of the amount of a species in a material