C4 - Chemical Economics

OCR GCSE Chemistry: Chemical Economics


  • Acids and bases
  • Reactions of Acids
  • Relative Atomic and Formula Mass
  • Calculating Masses In Reactions
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Acids and Bases

An acid is a substance with a pH of less than 7. They form H+ ions in water

A base is a substance with a pH of greater than 7.

An alkali is a base that dissolves in water. They form OH- ions in water.

The reaction between acids and bases is called neutralisation.

acid + base -> salt + water

When an acid neutralises a base, the products are neutral (pH 7)

Sulfuric acid is used:

  • In car batteries
  • In making fertilisers
  • To clean and prepare metal surfaces
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Reactions of Acids

Metal oxides and metal hydroxides are bases that dissolve in water. They will react with acids to form a salt and water.

hydrochloric acid + copper oxide -> copper chloride + water

sulfuric acid + potassium hydroxide -> potassium sulfate + water

Acids and carbonates produce carbon dioxide as well as a salt and water:

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

Acids and ammonia produce ammonium salts:

hydrochloric acid + ammonia -> ammonium chloride

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Relative Formula and Atomic Mass

Relative Atomic Mass

This is the same as the mass number of an element on the periodic table. This is the bigger number of the two numbers.

Relative Formula Mass

This is all the relative formula masses in a compound added together

Relative Formula Mass of MgCl2:

24 + (35.5 x 2) = 95

If there are brackets in the compound, the number after the brackets applies to everything inside the brackets.

Relative Formula Mass of Ca(OH)2:

40    +     (16 + 1) x 2    =   74

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Calculating Masses In Reactions

What mass of magnesium oxide is produced when 60g of magnesium is burned?

1) Write out the balanced equation:

2Mg + O2 -> 2MgO

2) Work out the relative formula masses for the information needed: (You don't need to work out the oxygen because that is not relevant for the question)

2Mg        +       O2    ->     2MgO

2 x 24                        ->   2 x (24 + 16)

48                               ->         80

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Calculating Masses In Reactions

3) Use the following rule: Divide to get one, multiply to get all

48g of Mg reacts to give 80g of MgO

(Divide by 48 to get the answer for 1g)

1g of Mg reacts to give 1.67g of MgO

(Multiply by 60 to get the answer to 60g)

60g of Mg reacts to give 100g of MgO

Answer: 100g of magnesium oxide will be produced

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Percentage Yield

This compares the actual yield with the predicted yield.

More reactants = a higher actual yield.

The percentage yield doesn't depend on the amount of reactants.

Predicted yield can be worked out by calculating masses in reactions (on the previous card)

Percentage Yield = actual yield (grams) divided by predicted yield (grams) x 100

100% yield means that you got all the product you expected

0% yield means that no product was made at all

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Loss of Products/Reactants

Products and reactants get lost along the way:


Liquids will evaporate all the time


This can be due to evaporation. In reversible reactions, incresing the temperature moves the equilibrium position and may result in a lower yield


Some liquid will remain with the solid and the filter paper. Some solid will get left behing when it's scraped off the filter paper


When they are moved from one container to the other, some will get left behind on the surface of the old container.

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Three essential elements in fertilisers are nitrogen, phosphorus and potassium. If a plant does not get enough of these, their growth is affected.

Fertilisers replace missing elements or provide more of them to increase crop yield. Fertilisers must dissolve in water before they can be taken up by the roots.

Ammonia is a base and can be neutralised by acids to make ammonium salts.

Ammonium nitrate is a good fertiliser - it contains nitrogen from both the ammonia and the nitrc acid

Ammonium sulfate can be used as a fertiliser - sulfuric acid + ammonia

Ammonia can be used to make ammonium phospahate and urea for fertilisers.

Potassium nitrate can also be used as a fertiliser.

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Calculating Percentage Mass of Fertilisers

Percentage mass of an element in a compound =

Relative Atomic Mass of the element x no. of atoms of the element

divided by

Relative Formula Mass of the whole compound

x 100

Example: Find the percentage mass of nitrogen in ammonium sulfate (NH4)2SO4   H = 1 , N = 14 , O = 16 , S = 32

Relative Formula Mass of (NH4)2SO4 = 132

14 x 2  divided by  132 x 100 = 21.2

Percentage mass of ntirogen in ammonium sulfate = 21.2%

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If too much fertiliser is applied, it can get into river and streams. This results in eutrophication.

The process of eutrophication:

1) Excess nitrate washes into the river, causing rapid growth of plants and algae on the surface of the water

2) Plant at the bottom of the water start dying due to competition of light

3) Decomposers feed on dead plants and increase in population size

4) Decomposers use up all of the oxygen in the water causing other animals, such as fish to die due to lack of oxygen.

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Is this for the Higher paper of Foundation?

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