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AQA A-level Chemistry Acids and alkalis

Acids and alkalis

This page covers the following topics:

1. Reactions of acids with metals
2. BrΓΈnsted-Lowry acids and bases
3. Buffers

Acids react with metals to form salts and hydrogen gas. The higher up the metal is on the reactivity scale, the faster the reaction. Acid-metal reactions are redox reactions. This is a type of chemical reaction that involves a transfer of electrons between two species. The electron donor is called the reducing agent, and the electron acceptor is the oxidising agent.

Reactions of acids with metals

A Bronsted-Lowry acid is a chemical compound capable of donating a proton in the form of a positive hydrogen ion and a Bronsted-Lowry base is a chemical compound capable of accepting a proton, meaning it has a lone pair of electrons to bond with the hydrogen ion. When the acid donates the proton, the conjugate base is formed. When the base accepts the proton, the conjugate acid is formed. Strong acids and bases ionise completely in aqueous solution, while weak acids and bases only ionise partially. When acids and bases ionise partially, they dissociate and recombine simultaneously, so the products and reactants are in a state of chemical equilibrium.

BrΓΈnsted-Lowry acids and bases

Buffer solutions consist of a weak acid, HA and its conjugate base, A⁻. These are in a state of equilibrium: HA ↔ H⁺ + A⁻. The pH of buffer solutions change very little when small amounts of strong acids or bases are added to it, as when there is an external effect on the equilibrium it shifts in a way that compensates for it, so keeping the pH unchanged. Buffers solutions are mainly used to keep the pH at a constant value in chemical and biological applications. Basic buffers also exist, made of a weak base and its conjugate acid.

Buffers

1

Describe what happens on the image.

Describe what happens on the image.

2

Acetic acid is a weak acid. Write the equation for its dissociation in water. Name the conjugate acid and base.

Acetic acid is a weak acid. Write the equation for its dissociation in water. Name the conjugate acid and base.

3

Magnesium reacts with hydrochloric acid to form magnesium chloride. Write the reduction half-equation for the reaction described.

Magnesium reacts with hydrochloric acid to form magnesium chloride. Write the reduction half-equation for the reaction described.

4

Write a balanced equation for the reaction of magnesium with hydrochloric acid.

Write a balanced equation for the reaction of magnesium with hydrochloric acid.

5

Explain how water can act both as a Bronsted-Lowry base and as an acid, therefore it is amphoteric.

Explain how water can act both as a Bronsted-Lowry base and as an acid, therefore it is amphoteric.

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