Chemical Changes

Acids and Bases

• An acid is a substance that forms an aqueous solution of less than pH 7.
• A base is a substance with a pH greater than 7
• An alkali is a base that dissolves in water to form a solution
• The reaction between acids and bases is called neutralisation: acid+base ---> salt + water
• When an acid neutralises a base, the pH is 7, which is neutral.
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Strong and Weak Acids

• Strong acids ionise completely in water, whereas weak acids do not fully ionise.
• The ionisation of a weak acid is a reversible reaction
• Strong acids are more reactive than weak acids, as they have a higher concentration of H+ ions.
• The pH of a substance is a measure of the concentration of H+ ions.
• Every time the pH scale decreases by 1, the concentration of H+ ions increases.
• Remember, concentration is different to strength.
• Concentration measures the amount of acid in a certain volume.
• Strength is the amount of acid that ionises in water.
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Reactions of acids

• All metal oxides and metal hydroxides react with bases to form a salt and water
• Acid + metal oxide or metal hydroxide ------> salt + water
• Metal carbonates also react with bases, but as well as salt and water, they also form carbon dioxide.
• Acid + metal carbonate -----> salt + water + carbon dioxide

The Reactivity Series

• Reactivity in metals is determined by how easily a metal can lose its electrons to form ions. The easier it is, the more reactive it is.
• Some metals react with acids to form salt and hydrogen, and this is determined by the rate at which bubbles are formed.
• Very reactive metals can react explosively, whereas ones which aren't as reactive react less violently.
• Lithium, potassium, sodium and calcium will all react with water, whereas zinc, iron and copper wont.
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Separating metals from metal oxides

• Metals often react with oxygen to form oxides, where they gain oxygen. These oxides are often ores that metals need to be extracted from. A reaction that separates metals and oxygen is a reduction reaction.
• An example of an oxidation reaction would be: 2Mg + O2 ----> 2MgO. As we can see, magnesium has been oxidised (it has gained oxygen)
• An example of a reduction reaction would be: 2CuO + C -----> 2Cu + CO2. Here we can see that copper oxide is being reduced (oxygen is lost)
• Metals that are higher up than carbon in the reactivity series have to be reduced using electrolysis. Metals below carbon can be extracted using carbon.
• Some metals are only reactive that they can only be found in their purest form.

Redox Reactions

• In terms of oxygen, reduction is loss, and oxidation is gain. However, in terms of electrons its different. A loss of electrons is oxidation, and gain of electrons is loss. This can be remembered using the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain.
• When electrons are transferred its called a Redox reaction, as it is both reduction and oxidation.
• A more reactive metal will displace a less reactive metal from its compound. If you put a reactive metal into the solution of a dissolved metal compound, the reactive metal will replace the less reactive in the compound.
• For example, if you put iron in a solution of copper sulfate, iron will displace copper as it is less reactive. The end result will be iron sulfate, with metal in its pure form. The equation would look something like this: iron + copper sulfate ----> iron sulfate + copper
• Ionic equations only show the products being reacted and the products they formed.

Electrolysis

• During electrolysis, an electric current is passed through an electrolyte (a molten or dissolved ionic compound). The ions move towards the electrodes, where they react, and the compound decomposes. Positive ions move towards the cathode, and are reduced (gain electrons). Negative ions move towards the anode, and are oxidised (lose electrons). This can be remembered as PANIC: Positive Anode Negative Is Cathode.
• This creates a flow of charge through the electrolyte s ions travel to the electrodes.
• An ionic solid can't be electrolysed because the ions are fixed into positions and cant move. Molten ionic compounds move freely so they can be extracted, as they conduct electricity.

Using Electrolysis to Extract Metals