C5: CHEMICAL CHANGES
The Reactivity Series
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 gives off a "squeaky pop" with a lighted splint
Displacement Reactions
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 on the basis of displacement reactions
Oxidation is the loss of electrons
Reduction is the gain of electrons
An ionic equation shows only the atoms and ions that change in a reaction
A half equation is like an ionic equation, but it shows what happens to each reactant in the reaction (eg. Fe --> Fe2+ +2e-)
Extracting Metals
A metal ore contains enough of the metal to make it economic to extract the metal.
Ores are mined and might need 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 reactivity series can be reduced by carbon to give the metal element
Metals more reactive than carbon cannot be extracted from their ores using carbon, but rather through the electrolysis of the molten metal compound/an aqueous version of the compound
Salts From Metals
A salt is a compound formed when the hydrogen in an acid is wholly or partially replaced by ammonium ions
Salts can be made by reacting a suitable metal with an acid. The metal must be above hydrogen in the reactivity series, but not dangerously reactive
The reaction between a metal and an acid produces hydrogen gas as well as a salt. A sample of the salt made can then be crystallized out of solution by evaporating off the water
The reaction between a metal and an acid is an example of a redox reaction
The metal atoms lose electrons and are oxidised, and hydrogen ions from the acid gain electrons and are reduced
metal + acid --> a salt + hydrogen
Salts From Insoluble Bases
When an acid reacts with a base, a neutralisation reaction occurs
The reaction between an acid and a base produces a salt and water
The sum of the charges on the ion in a salt 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 the salt made in an acid-base reaction can be crystallised out of solution by evaporating off most of the water, and drying with filter papers if necessary
acid + base --> salt + water
Making More Salts
acid + alkali --> a salt + water
An indicator is needed when a soluble salt is prepared by reacting a salt with an acid
The titration can be repeated without the indicator to make a salt, then a pure, dry sample of its crystals prepared
carbonate + acid --> salt + water +carbon dioxde
Neutralization and the pH scale
Acids are substances which produce H+ (aq) ions when you add them to water
Bases are substances which neutralize acids
An alkali is a soluble hydroxide which produces OH- (aq) ions when added to water
The pH scale can be used to show how acid or alkaline a solutions is
ph < 7 = acidic, ph > 7 = alkaline, ph = 7 = neutral
Strong and Weak Acids
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 decreases by one unit, the hydrogen ion concentration reduces by a factor of 10 (ie. pH x 10 (to the power of)
A strong aid has all atoms ionised (with a charge) when n a solution
A weak acid has its atoms partially ionised when in a solution
eg. hydrochloric acid
ef. citric acid
Concentration refers to the relative amount of a particular substance contained within a solution or mixture
The more dilute a solution or mixture is, the less particles there are in it
The more concentrated a solution or mixture is, theless particles there are in it
concentrated does not mean strong