Unit 4 - Chemical Changes

Acids and Bases

The pH scale is a measure of how acidic or alkaline a solution is and ranges from 0 to 14. The lower the pH of a solution, the more acidic it is. Whereas the higher the pH of a solution, the more alkaline it is. A neutral substance e.g. pure water has a pH of 7.

You can measure the pH of a solution:

  • an indicator is a dye that changes colour depending on wether it's above or below a certain pH e.g. Universal Indicator
  • a pH probe attached to a pH meter can also be used to measure the pH electronically. The probe is placed into the solution you are measuring and the pH is given on a digital display asa a numerical value, meaning it's more accurate than an indicator.

An acid is a substance that forms a aqueous solution with a pH less than 7. Acids form H ions in water. A base is a subtance with a pH greater than 7. An alkali is a base that dissolves in water to form a solution with a pH greater than 7. Alkalis form OH  ions in water.

The reaction between acids and bases is called neutralisation and the products are always neutral.

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Titrations

Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali or acid. You can then use this data to work out the concentration of the acid or alkali.

  • Using a pipette and pippette filler, add a set volume of the alkali to a conical flask. Add two or three drops of indicator too.
  • Use a funnel to fill a burette with some acid of known concentration, making sure to do this below eye level to reduce the risk of acid getting in your yes.
  • Record the intial volume of acid in the burettte.
  • Using the burette, add the acid to the alkali a little bit at a time giving the conical flaska swirl each time, take care when a colour change begins to occur.
  • The indicator will change colour when all the alkali has become neutralised. Phenolphthalein is pink in alkaline conditions but colourless in acidic conditions.
  • Record the final volume of acid in the burette and use it, along with the intial reading, to calculate the volume of acid used to neutralise the alkali.

To increase the accuracy of your titration and to spot any anomalous results, you need several consistent readings. The first titration should be a rough trial to get an idea of when the alkali starts to neutralise. You then need to repeat it a few times making sure its consistent and calculate an average.

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Strong Acids and Weak Acids

Strong acids ionise completely in water and all particles dissociate to release H  ions e.g. sulfuric, hydrochloric and nitric acid.

Weak acids don't fully ionise in solution and only a small proportion of acid particles dissociate to release H  ions e.g. ethanoic, citric and carbonic acid. The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid.  

Acid strength tells you what proportion of the acid molecules ionise in water.

The concentration of an acid measures how much acid there is in a certain volume of water. The larger the quantity of acid there is in a certain volume of liquid, the more concentrated the acid is. So you can have a dilute strong acid or a concentrated weak acid. pH will decrease with increasing acid concentration regardless of whether it's a strong or weak acid.

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Reactions of Acids

Metal Oxides and Metal Hydroxides are bases and some of them dissolve in water, these soluble compounds are alkalis. All metal oxide and metal hydroxides react with acids to form a salt and water.

  • acid + metal oxide -----> salt + water
  • acid + metal hydroxide -----> salt + water

For example:

hydrochloric acid + copper oxide -----> copper chloride + water

 Metal Carbonates are also bases. They react with acids to produce a salt, water and carbon dioxide.

  • acid + metal carbonate -----> salt + water + carbon dioxide 

For example:

hydrochloric acid + sodium carbonate -----> sodium chloride + water + carbon dioxide

 

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The Reactivity Series

The reactivity series lists metals in order of their reactivity towards other substances. For metals, their reactivity is determined by how easily they lose electrons - forming positive ions. The higher up the reactivtiy series the easier they form positive ions and easier they react with water or acid meaning they are more reactive.

  • acid + metal -----> salt + hydrogen
  • water + metal -----> metal hydroxide + hydrogen

The Reactivity Series:

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Separating Metals from Metal Oxides

Metals often have to be seperated from their oxides. Lots of common metals like iron and aluminium react with oxygen to form oxides. This process is an examples of oxidation. These oxides are often the ores that the metals need to be extracted from.

A reaction that seperates a metal from its oxides is called a reduction reaction. 

Formation of a metal ore is oxidation as oxygen is gained e.g. magnesium is oxidised to make magnesium oxide.

Extraction of metal is reduction as oxygen is lost e.g. copper oxide is reduced to copper.

Metals higher than carbon in the reactivity series have to be extracted using electrolysis. Whereas metals below carbon can be extracted by reduction using carbon. This is because carbon can only take the oxygen away from metasl which are less reactive than carbone itself.

Some metals are so unreactive they are in earth as the metal itself e.g. gold is mined as its elemental form.

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Redox Reactions

A loss of electrons is called oxidation. A gain of electrons is reduction. REDuction and OXidation happen at the same time hence the term 'REDOX'.

Displacement reactions are Redox reactions. A more reactive metal with displace a less reactive metal from its compound. In Displacement reactions, it's always the metal ion that gains electorns and is reduced. The metal atom always loses electrons and is oxidised. 

For example:

iron + copper sulfate -----> iron sulfate + copper

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Electrolysis

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Electrolysis of Aqueous Solutions

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