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OCR A-level Chemistry Acids and alkalis

Acids and alkalis

This page covers the following topics:

Acids are ionic compounds that form positive hydrogen ions when dissolved in water. Acids have a pH lower than 7, can conduct electricity when dissolved in water and they react with metals to produce hydrogen gas. When soluble non-metal oxides are dissolved in water, acidic solutions are formed. Common acids include: hydrochloric acid (HCl), nitric acid (HNO₃), sulfuric acid (Hβ‚‚SOβ‚„), carbonic acid (Hβ‚‚CO₃), acetic acid (CH₃COOH) and phosphoric acid (H₃POβ‚„).

Acids

Alkalis are ionic compounds that form hydroxide ions when dissolved in water. Alkalis have pH higher than 7, can conduct electricity when dissolved in water and can be produced by dissolving metal oxides in water. Common bases include: potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)β‚‚) and ammonia (NH₃).

Alkalis

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

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Write a balanced equation for the reaction of calcium with nitric acid. Write down the ion-electron equations of the reduction and oxidation that occurs.

OCR A-level Chemistry Acids and alkalis Write a balanced equation for the reaction of calcium with nitric acid. Write down the ion-electron equations of the reduction and oxidation that occurs.
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2

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

OCR A-level Chemistry Acids and alkalis Write a balanced equation for the reaction of magnesium with hydrochloric acid.
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3

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

OCR A-level Chemistry Acids and alkalis Explain how water can act both as a Bronsted-Lowry base and as an acid, therefore it is amphoteric.
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4

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Which of the following are acid-base reactions based on the Bronsted-Lowry theory? a, Fe + Hβ‚‚SOβ‚„ β†’ FeSOβ‚„ + Hβ‚‚ b, Hβ‚‚O + Hβ‚‚O ↔ H₃O⁺ + OH⁻ c, HNO₃ + KOH β†’ Hβ‚‚O + KNO₃ c, 2Na + Clβ‚‚ β†’ 2NaCl

OCR A-level Chemistry Acids and alkalis Which of the following are acid-base reactions based on the Bronsted-Lowry theory? a, Fe + Hβ‚‚SOβ‚„ β†’ FeSOβ‚„ + Hβ‚‚ b, Hβ‚‚O + Hβ‚‚O ↔ H₃O⁺ + OH⁻ c, HNO₃ + KOH β†’ Hβ‚‚O + KNO₃ c, 2Na + Clβ‚‚ β†’ 2NaCl
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5

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When nitric acid reacts with water, it forms hydronium and nitrate ions, leaving no undissociated molecules of nitric acid in the solution. Is nitric acid a strong or a weak acid?

OCR A-level Chemistry Acids and alkalis When nitric acid reacts with water, it forms hydronium and nitrate ions, leaving no undissociated molecules of nitric acid in the solution. Is nitric acid a strong or a weak acid?
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6

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Our blood uses a buffering system to keep its pH constant. This buffer involves carbonic acid and bicarbonate ion. Write down the equation for this equilibrium. Why is it important that the pH of our blood remains constant?

OCR A-level Chemistry Acids and alkalis Our blood uses a buffering system to keep its pH constant. This buffer involves carbonic acid and bicarbonate ion. Write down the equation for this equilibrium. Why is it important that the pH of our blood remains constant?
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7

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A common example of a buffer is a solution of ethanoic acid and sodium ethanoate. Give the equation for its equilibrium.

OCR A-level Chemistry Acids and alkalis A common example of a buffer is a solution of ethanoic acid and sodium ethanoate. Give the equation for its equilibrium.
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8

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Describe what happens if a base is added to a weak base buffer, such as the solution of ammonia and ammonium chloride.

OCR A-level Chemistry Acids and alkalis Describe what happens if a base is added to a weak base buffer, such as the solution of ammonia and ammonium chloride.
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