Chapter 7 - Acids, bases and salts Flashcards
What are acids?
Acids have a pH value below 7, have a sour taste and are corrosive. When acids are added to water, they form positively charged H+ ions. The presence of H+ ions is what makes a solution acidic. Acids are proton donors as they ionise in solution producing protons which are H+ ions.
How do acids react with metals?
Only metals above hydrogen in the reactivity series will react with dilute acids. When acids react with metals, they form a salt and hydrogen gas. Acid + metal → metal salt + hydrogen
How do acids react with bases?
When acids react with bases, a neutralisation reaction occurs. In all acid-base neutralisation reactions, salt and water are produced. Acid + base → metal salt + water
How do acids react with metal carbonates?
When acids react with metal carbonates, they form the corresponding metal salt, carbon dioxide and water. Acid + metal carbonate → salt + carbon dioxide + water
How do acids cause changes in indicators?
Acids turn litmus paper to red, thymolphthalein to colourless and methyl orange to red.
What are bases?
Bases are oxides or hydroxides of metals. They have pH values above 7. Bases are proton acceptors as they accept the protons which are donated by the acid.
What are alkalis?
Alkalis are soluble bases. When they are added to water, they form negative hydroxide ions OH-. The presence of the OH- ions is what makes the aqueous solution an alkali
How do bases react with acids?
When bases react with an acid, a neutralisation reaction occurs. They produce a salt and water. Acid + base → metal salt + water
How do ammonium salts react with alkalis?
Ammonium salts undergo decomposition when warmed with an alkali. Even though ammonia is a weak base, it is very volatile and can easily be displaced from the salt by another alkali. In this reaction, a salt, water and ammonia are produced. Alkali + ammonium salt → metal salt + water + ammonia (NH3)
How do bases cause changes in indicators?
Bases turn litmus paper to blue, thymolphthalein to blue and methyl orange to yellow.
What is a strong acid?
A strong acid is an acid that is completely dissociated (or ionise) in aqueous solution. Hydrochloric acid is an example of a strong acid. HCl (aq) → H+ (aq) + Cl- (aq)
What is a weak acid?
A weak acid is an acid that is partially dissociated (or ionise) in aqueous solution. Ethanoic acid is an example of a weak acid. CH3COOH(aq) ⇌ H+ (aq) + CH3COO- (aq)
What is the neutralisation reaction between an acid and an alkali to produce water?
The neutralisation reaction between an acid and an alkali to produce water: H+ (aq) + OH- (aq) → H2O (l)
What are oxides?
Oxides are compounds made from one or more atoms of oxygen combined with one other element.
What are acidic oxides?
Acidic oxides are formed when a non-metal element combines with oxygen. They react with bases to form a salt and water. When dissolved in water, they produce an acidic solution with a low pH. Common examples include CO2, SO2, NO2 and SiO2
What are basic oxides?
Basic oxides are formed when a metal element combines with oxygen. They react with acids to form a salt and water. When dissolved in water they produce a basic solution with a high pH. Common examples include CuO and CaO.
What are neutral oxides?
Neutral oxides do not react with either acids or bases and thus are said to be neutral. For example, N2O, NO and CO.
What are amphoteric oxides?
Amphoteric oxides are a curious group of oxides that can behave as both acidic and basic. In both cases, salt and water are formed. The most common examples of amphoteric oxides are zinc oxides, ZnO, and aluminium oxide, Al2O3.
What is a salt?
A salt is a compound that is formed when the hydrogen atom in an acid is replace by a metal. When naming a salt, you use the first part from the metal and the second part from the acid.
What are two methods you can use to prepare a salt?
You can prepare a salt solution by method 1: adding acid to a solid metal, insoluble base or insoluble carbonate or method 2: reacting a dilute acid with an alkali (soluble base)
How can you prepare a salt by adding an acid to a solid metal, insoluble base or insoluble carbonate?
Method 1: Add dilute acid into a beaker and heat using a Bunsen burner flame
Add the insoluble metal, base or carbonate, a little at a time, to the warm dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base forms in the acid)
Filter the mixture into an evaporating basin to remove the excess base
Heat the solution to evaporate water and to make the solution saturated. Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end Leave the filtrate in a warm place to dry and crystallize
Decant excess solution and allow crystals to dry or blot to dry with filter paper
How can you prepare a salt by reacting a dilute acid with an alkali (soluble base)
Method 2: Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (thymolphthalein or methyl orange)
Add the acid into the burette
Record the starting volume of acid in the burette
Add the acid very slowly from the burette to the conical flask until the indicator changes to the appropriate colour
Record the final volume of acid in the burette
Calculate the volume of acid added. Final volume of acid - initial volume of acid
Add this same volume of acid to the same volume of alkali without the indicator
Heat the resulting solution in an evaporating basin to partially evaporate, leaving a saturated solution (crystals just forming on the sides of the basin or on a glass rod dipped in and then removed)
Leave to crystallise, decant excess solution and allow crystals to dry
How can you prepare an insoluble salt using a precipitation method?
Method: Dissolve soluble salts in water and mix together using a stirring rod in a beaker
Filter to remove precipitate from mixture
Wash the residue with distilled water to remove traces of other solutions
Leave in an oven to dry
What are hydrated substances?
Hydrated substances are substances that are chemically combined with water.
What are anhydrous substances?
Anhydrous substances are substances containing no water.
What is water of crystallisation?
Water of crystallisation are the water molecules present in hydrated crystals. When writing the chemical formula of hydrated compounds, the water of crystallisation is separated from the main formula by a dot. Hydrated copper(II) sulphate is CuSO4∙5H2O and hydrated cobalt(II) chloride is CoCl2∙6H2O.
Solubility rules table
