How much-Chemistry-AQA Additional Science

Information on mass numbers, masses of atom and moles, making ammonia, reversible reactions.

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Mass numbers

Protons and neutrons have equal masses. The relative masses of a proton and a neutron are both one unit. The mass of an electron is very small compared with a proton or neutron, and so the mass of an atom is made up almost entirely of its proton and neutrons. The total number of protons and neutrons in an atom is called its 'mass number'.

Atoms of the same element all have the same atomic number. The number of protons and electrons in an atom must always be the same, but there can be different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called 'isotopes'. The number of neutrons in an atom is equal to its mass number minus its atomic number.

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Masses of atoms and moles

Atoms are too small to weigh so we use 'relative' atomic masses. These are shown in the periodic table.

The relative formula mass (Mr) of a substance is found by adding the relative atomic masses of the atoms in its formula. E.g.

Calculate the Mr of CaCl2 - Ar of Ca = 40, Ar of Cl= 35.5 so 40+(35.5 x 2) = 111

The relative formula mass of a substance in grams is called 'one mole' of that substance. Using moles of substances allows us to calculate and weigh out in grams and masses of substances with the same number of particles. One mole of sodium atoms contains the same number of atoms as one mole of chlorine atoms. E.g.

What is the mass of one mole of NaOH? - Ar of Na = 23, Ar of O = 16, Ar oH=1

so 23g + 16g + g + 1g = 40g

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Percentages and formulae

We can calculate the percentage of any of the elements in a compound from the formula of the compound. Divide the relative atomic mass of the element by the relative formula mass of the compound and multiply the answer by 100 to convert it to a percentage. This can be useful when deciding if a compound is suitable for a particular purpose or to identify a compound.

E.g. Find the percentage of carbon in carbon dioxide (Ar of C = 12, Ar of O = 16) -

Mr of CO2 = 12 + (16 x 2) = 44

So percentage of carbon = (12 / 44) x 100 = 27.3%

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Making as much as we want

The yield of a chemical process compares how much you actually make with the maximum amount possible.

When you actually carry out chemical reactions it is not possible to collect the amounts calculated from the chemical equations. Reactions may not go to completion and some product may be lost in the process.

Atom economy - It is also important to consider the amount of the starting materials that ends up in the useful products. This is called the 'atom economy' of a process.

To avoid waste both percentage yield and atom economy should be as high as possible.

The yield of a reaction compares the amount of product actually made with the maximum amount that could be made.

Atom economy measures how much of the starting materials becomes useful products.

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Reversible reactions and Making ammonia - the Habe

If the products of a chemical reaction can react to produce the reactants the reaction can go in both directions. This type of reaction is called a reversible reaction.

The Haber Process - The Haber process is used to manufacture ammonia, which is used to make fertilisers and other chemicals.

Nitrogen from the air and hydrogen are purified and mixed in the correct proportions. The gases are passed over an iron catalyst at a temperature of about 450°C and a pressure of about 200 atmospheres.

The reaction is reversible and so some ammonia breaks down into nitrogen and hydrogen. The gases are cooled so the ammonia condenses. The liquid ammonia is removed from the unreacted gases and they are recycled.

The yield is less than 20%, but the ammonia is produced quickly and no gases are wasted.

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