L2 - Properties of Acids & Bases

objectives

Understandings:

• Most acids have observable characteristic chemical reactions with reactive metals, metal oxides, metal hydroxides, hydrogen carbonates and carbonates.

• Salt and water are produced in exothermic neutralization reactions.

Applications and skills:

• Balancing chemical equations for the reaction of acids.

• Identification of the acid and base needed to make different salts.

• Candidates should have experience of acid-base titrations with different indicators.

Guidance:

• Bases which are not hydroxides, such as ammonia, soluble carbonates and hydrogen carbonates should be covered.

• The colour changes of different indicators are given in the data booklet in section 22.

Properties of Acids and Bases

Reactions of Acids and Bases

The chemical reaction between an acid and an alkali is called neutralization

These neutralisation reactions are exothermic

• Most acids react with metals, metal oxides, hydroxides, hydrogencarbonates and carbonates
• All these reactions produce a salt which is a compound of anion and cation
• Standard enthalpy change of neutralization: the energy change associate with the formation of 1 mol of water from the reaction between a strong acid and a strong base under standard conditions
• Negative value because neutralization is exothermic

Naming Salts

When Acids and Alkalis react they make Salts

Salts are Ionic in Nature and have a mixture of Cations and Anions that often dissolve in water

These salts are sometimes called Metal Salts as many contain a metal cation and an anion.

The Metal or Cation often comes from the Alkali

The Anion comes from the Acid.

Cation Anion = Salt

Example - Sodium Chloride . - Here Sodium is the Cation and Chloride is the Anion - together they are a salt.

Examples of the Anions from Acids can be seen below in the tables:

We can use this acid table in conjunction with the following Alkali Tables to Name and deduce chemical formulae of a range of salts.

In the Acid-Alkali reactions we need to know how Metals, Metal Hydroxides & Metal Oxides (BASES), Metal Carbonates & Metal Hydrogen Carbonates (simplified to Carbonates) react with Acids.

MASH₂ - Metals and Acids

Everytime a Metal that is MORE REACTIVE THAN HYDROGEN on the Reactivity Series reacts with an Acid we get the same products - a Salt and H₂ Hydrogen Gas. This can be observed in the below word equation:

Metal + Acid --> Salt + Hydrogen

As we can see that Metals make the same products we can use the acronym - MASH₂.

BASH₂O - Bases and Acids

Bases are soluble alkalis - for the DP (and iGCSE) we need to know that Bases are either Metal Hydroxides or Metal Oxides. aLthough they have different formulae - Metal Bases always react to produce Salts and Water. This can be observed in the below Equation. Note from the table that we really are only interested in the metal part of the Base for naming salts.

Base + Acid --> Salt + H₂O

As we can see that Bases make the same products we can use the acronym - BASH₂O.

Ammonia (NH₃) is also a base but is contains no metal ion. The Cation that reacts is the Ammonium ion (

NH₄⁺). This would therefore have the Prefix Ammonium.

CASH₂OCO₂ - Carbonates/ Hydrogen Carbonates and Acids

Carbonates and Hydrogen Carbonates are basic solids that can react with acids to produce a salt, water and carbon dioxide. This can be observed in the below Equation. Note from the table that we really are only interested in the metal part of the Carbonate/Hydrogen Carbonate for naming salts.

Carbonate + Acid --> Salt + H₂O + CO₂

As we can see that Carbonates (and Hydrogen Carbonates) make the same products we can use the acronym - CASH₂OCO₂ .