AQA GCSE Chemistry Acids and alkalis
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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 (H₂SO₄), carbonic acid (H₂CO₃), acetic acid (CH₃COOH) and phosphoric acid (H₃PO₄).
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)₂) and ammonia (NH₃).
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.
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.
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.
We have 0.2 moles of NaOH dissolved in 500 ml of water, and 0.05 moles of Ca(OH)₂ dissolved in 500 ml of water. Which solution conducts electricity better? Assume that the electrical conductivity is proportional to the concentration of ions in a solution and both compounds dissociate fully.
Write a balanced equation for the reaction of the metal zinc and hydrochloric acid. Draw a diagram of the redox reaction, showing the roles of the reactants and products and the transfer of electrons.
Single displacement reactions occur the following way (image). Given an example of a reaction between an acid and a metal that is also a single displacement reaction and write down the ion-electron equations. Why is it that when 2 metals replace each other in a displacement reaction, the reaction proceeds only one way but not the other?
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