Acid-Base Reactions

• An acid is a substance that turns litmus paper red, reacts with carbonates to give CO₂ and is neutralized by bases.
• Acids have the ability to transfer H⁺ ions to something else.
• The substance which accepts the ions is called a base.

For example:

• In this reaction the HCl has transferred H⁺ ions and the NH₃ has accepted the ions. The HCl is the acid, and the NH₃ is the base.
• In the above reaction, the oxidation states of the elements remain unchanged.
• Since H⁺ ions only consists of only one proton, we can refer to acids as proton donors, and bases as proton acceptors.
• The theory of H⁺ transfer is known as the BrØnstead-Lowry theory of acids and bases.

Acids and Bases in Solutions

• Water consists mainly of H₂O molecules, and hydrogen chloride is a gas that contains HCl molecules.
• When these two substances mix, the solution (Hydrogen Chloride in water) easily conducts electricity, so it must contain H⁺ ions.
• The reaction between the hydrogen chloride molecules and the water molecules produces these ions:

• In this reaction, the hydrochloric acid is the acid (it is the proton donor) and the water is the base (proton acceptor).
• The H₃O⁺ ion is called the oxonium ion and it is present in every acidic solution. The acid donates H⁺ to H₂O forming H₃O⁺.
• The H₃O⁺ can now act as an acid; it can donate an H⁺ ion to a base.
• The properties of an acid are the same as the properties of the H₃O⁺ ion.
• The H₃O⁺ ion is often shortened to just H⁺

• An acid dissolves in water to form H⁺ ions.
• An Alkali dissolves in water to form OH^- ions.
• Some alkalis such as sodium hydroxide contain OH^- ions, whereas others, such as ammonia, form OH^- ions when they react with water.

Acid-base pairs

• After an acid has donated its H⁺ ion, it can then accept it again:

• When ethanoic acid acts as an acid, it donates its H⁺ ion (above).
• If a stronger acid is then added to the solution containing the ethanoic ions (CH₃COO^-), then they can act as a base, and accept H⁺ ions:

The general formula for a conjugate acid-base pair is summarised below.HA_(aq) H + A^-

Strengths of acids and bases

• Acids can have different strengths, some are powerful H⁺ donors, whereas others aren’t quite as strong.
• Powerful H⁺ donors are called strong acids.
• Weak H⁺ donors are called weak acids.
• A strong acid has a weak conjugate base, and a strong base has a weak conjugate acid.

Indicators

• Most acid-base indicators are weak acids. The conjugate acid and base forms have different colours.

• When acid (H₃O⁺) is added the position of equilibrium shifts to the left, producing colour A.
• When Alkali (OH^-) is added the psoition of equilibrium shifts to the right, producing colour B.
• Litmus is an example of this, showing red in its base state when an acid is present, and blue in its acid state, when a base is added.

Useful books for revision:

Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)

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