- 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
- 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
- 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
- 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 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
- 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)
- 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
- 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
- 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
- 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.
- 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