Chapter 5: Chemical changes

Question 1

The reactivity series

Answer 1

• The metals can be placed in order of reactivity by their reactions with water and dilute acid
• Hydrogen gas is given off if metals react with water or dilute acids. The gas ‘pops’ with a lighted splint

See picture in documents

Question 2

Displacement reactions

Answer 2

• A more reactive metal will displace a less reactive metal from its aqueous solution
• The non-metals hydrogen and carbon can be given positions in the reactivity series based on displacement reactions
• Oxidation is the loss of electrons
• Reduction is the gain of electrons

Question 3

Extracting metals

Answer 3

• A metal ore contains enough of the metal to make it economic to extract the metal. Ores are mined and might needed to be concentrated before the metal is extracted and purified
• Gold and some other unreactive metals can be found in their native state
• The reactivity series helps you decide the best way to extract a metal from its ore. The oxides of metals below carbon in the series can be reduced by carbon to give the metal element.
• Metals more reactive than carbon cannot be extracted from their ores using carbon. They are extracted by electrolysis of the molten metal compound

Question 4

Salts from metals

Answer 4

• Metal + acid –> a salt + hydrogen
• A salt is a compound formed when the hydrogen in an acid is wholly or partially replaced by metal or ammonium ions
• Salts can be made by reacting a suitable metal with an acid. The metals must be above hydrogen in the reactivity series, but not dangerously reactive
• The reaction between metal and an acid produces hydrogen gas as well as a salt. A sample of the salt made can then be crystallised out of the solution by evaporating water
• The reaction between a metal and an acid is an example of a redox reaction. The metal atoms lose electrons are oxidise, and hydrogen ions from the acid gain electrons and are reduced

Question 5

Salts from insoluble bases

Answer 5

• A base is any substance that can react with acids to neutralise them to make slat and water (i.e. metal oxides or metal hydroxides)
• When an acid reacts with a base, a neutralisation reaction occurs
• The reaction between and acid and a base produces salt and water
• The sum of the changes on the ions in a slat add up to zero. This enables you to work out the formula of salts, knowing the charges on the ions present
• A pure, dry sample of salt made in an acid-base reaction can be crystallised out of the solution by evaporating the water, and drying with filter paper is necessary

Question 6

Making more salts

Answer 6

• Acid + alkali –> a salt + water
• An indicator is needed when a soluble salt is prepared by reacting an alkali with an acid
• The titration can be repeated with out the indicator to make a salt, then a pure, dry sample of its crystals prepared
• A carbonate reacts with an acid to produce a salt, water, and carbon dioxide gas:

Acid + a carbonate –> a salt + water + carbon dioxide

Question 7

Neutralisation and the pH scale

Answer 7

• Acids are substances that produce H+(aq) ions when you add them to water
• Bases are substances that will neutralise acids
• An alkali is a soluble hydroxide. Alkalis produce OH- (aq) ions when you add them to water
• You can use the pH scale to show how acidic or alkaline a solution is
• Solutions with pH values less than 7 are acidic, pH value more than 7 are alkali and pH value which are 7 are neutral.

Question 8

Strong and weak acids

Answer 8

• Aqueous solutions of weak acids, such as carboxylic acids, have a higher pH value than solutions of strong acids with the same concentration
• As the pH decrease by one unit, the hydrogen ion concentration of the solution increases by a factor of 10 (i.e. one order of magnitude)