Chemistry - 9202 OxfordAQA

Preparation Of Salts

Aperçu

Somewhere in the building you are sitting in there is almost certainly a bag of plaster, a tub of Epsom salts, a fertiliser sack or a packet of baking powder. Every one of them is a salt, and every one of them was made on purpose by a chemist who first had to answer a deceptively small question: does the thing I am trying to make dissolve in water or not? That single answer decides everything else. It chooses the starting materials, it chooses the apparatus, and it decides whether the product arrives as crystals grown slowly from a warm liquid or as a cloudy solid that appears in a fraction of a second the instant two clear solutions meet.

This lesson gives you the whole decision tree and then walks each branch of it in detail. You will learn the solubility rules that let you sort any salt into soluble or insoluble before you touch a single bottle, the three acid-based routes to a soluble salt and why a route that works beautifully for copper would be dangerous for sodium, the reason chemists deliberately add too much of one reactant and then throw the leftovers away, and the patient technique that turns a colourless solution into a dish of grown crystals. By the end you will be able to plan a preparation for a salt you have never made, which is precisely what the examiner asks you to do.

Objectifs

  1. Soluble salts can be made from acids by reacting them with: metals - not all metals are suitable; some are too reactive and others are not reactive enough; insoluble bases - the base is added to the acid until no more will react and the excess solid is filtered off; alkalis - an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. Students should be able to suggest methods to make a named soluble salt.
  2. Salt solutions can be crystallised to produce solid salts.
  3. Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed. Precipitation can be used to remove unwanted ions from solutions: for example, in treating water for drinking or in treating effluent. Students should be able to name the substances needed to make a named insoluble salt.

Carte mentale

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Note de cours

A salt is what remains when the hydrogen of an acid has been handed over to a metal, or to the ammonium group, and something else has carried the hydrogen away. Sodium chloride is a salt. So are the calcium sulfate in a plaster cast, the ammonium nitrate spread on a field, the copper(II) sulfate that stains a school beaker blue, and the barium sulfate a patient swallows before an abdominal scan. The name of any one of them is assembled from two parts, and both parts are chosen for you by your starting materials: the metal at the front comes from whatever you neutralise the acid with, and the family name at the back comes from the acid itself.

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Évaluation de la leçon

Félicitations, vous avez terminé la leçon sur Preparation Of Salts. Maintenant que vous avez exploré le concepts et idées clés, il est temps de mettre vos connaissances à lépreuve. Cette section propose une variété de pratiques des questions conçues pour renforcer votre compréhension et vous aider à évaluer votre compréhension de la matière.

Vous rencontrerez un mélange de types de questions, y compris des questions à choix multiple, des questions à réponse courte et des questions de rédaction. Chaque question est soigneusement conçue pour évaluer différents aspects de vos connaissances et de vos compétences en pensée critique.

Utilisez cette section d'évaluation comme une occasion de renforcer votre compréhension du sujet et d'identifier les domaines où vous pourriez avoir besoin d'étudier davantage. Ne soyez pas découragé par les défis que vous rencontrez ; considérez-les plutôt comme des opportunités de croissance et d'amélioration.

  1. Which salt is produced when copper(II) oxide reacts with dilute nitric acid? A. copper(II) nitrate B. copper(II) sulfate C. copper(II) chloride D. copper(II) carbonate Answer: A
  2. Which of these salts is insoluble in water? A. sodium carbonate B. potassium sulfate C. barium sulfate D. zinc nitrate Answer: C
  3. A student adds excess magnesium oxide to warm dilute hydrochloric acid, then filters the mixture. What is present in the filtrate? A. magnesium oxide only B. magnesium chloride solution C. hydrochloric acid only D. solid magnesium chloride Answer: B
  4. Which pair of solutions would produce a precipitate of silver chloride? A. silver nitrate and sodium nitrate B. sodium chloride and potassium nitrate C. silver nitrate and sodium chloride D. silver chloride and sodium nitrate Answer: C
  5. Why is a titration used to prepare potassium nitrate rather than adding excess potassium hydroxide to nitric acid and filtering? A. Potassium nitrate is insoluble in water. B. Potassium hydroxide dissolves, so an excess of it cannot be filtered off. C. Nitric acid does not react with potassium hydroxide. D. The reaction produces a gas that must be measured. Answer: B

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