Additional Science-Chemistry Chapter 3

GCSE AQA Additional Science-Chemistry Chapter 3

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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. 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.

Number of neutrons = Mass number - Atomic number

Sometimes the extra neutrons in the nucleus make it unstable so that it is radioactive.

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Masses of Atoms and Moles

Atoms are much too small to weigh and so we use 'relative' atomic masses (RAM).

The relative atomic mass of an element (RAM, Ar) is an average value that depends on the isotopes the element contains. However, when rounded to a whole number it is often the same as the mass number of the main isotope of the element.

The relative formular mass (RFM, Mr) of a substance is found by adding up the relative atomic masses of the atoms in its formula.

The relative formular 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 masses of substances with the same number of particles.

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Percentages and Formulae

The percentage mass of an element in a compound can be calculated from its Ar and the Mr 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.

The empirical formula is the simplest ratio of the atoms or ions in the compound. It is the formula used for ionic compounds, but for covalent compounds it is not always the same as the molecular formula. For example, the molecular formula of ethane is C2H6, but its empirical formula is CH3.

We can calculate the empirical formula of a compound from its percentage composition:

  • Divide the mass of each element in 100g of the compound by its Ar to give the ratio of atoms
  • Then convert this to the simplest whole number ratio
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Equations and Calculations

Chemical equations show the reactants and products of a reaction. When they are balanced they show the amounts of atoms, molecules or ions in the reaction.

2Mg + O2 --> 2MgO

shows that two atoms of magnesium react with one molecule of oxygen to form two magnesium ions and two oxide ions.

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Percentage Yield

The yield of a reaction compares the amount of product actually made with the maximum amount that could be made. It is not always possible to get the entire calculated amount of product. Reversible reactions may not go to completion and some product may be lost when it is removed from the reaction mixture

The yield of a reaction is the mass of product obtained. The theoretical yield is the maximum amount of product in a reaction possible. The actual yield is the amount of product collected.

Percentage yield is the ratio of actual mass of products obtained compared to the maximum theoretical mass. Percentage yield = 100 x actual mass divided by theoretical mass (http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_aqa_equa_percyield.gif)

For example, the maximum theoretical mass of product in a certain reaction is 20 g, but only 15 g is actually obtained.

Percentage yield = 15 ÷ 20 × 100 = 75%

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Atom Economy

Atom economy measures how much of the starting materials becomes useful products. Inefficient, wasteful processes have low atom economies. Efficient processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste.

The value of the atom economy expressed as a percentage is equal to 100 x mass of the desired product from the equation divded by the total mass of reactants from the equation (http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_aqa_equa_atom2.jpg)

E.g. What is the atom economy for making hydrogen by reacting coal with steam?

C(s) + 2H2O(g) → CO2(g) + 2H2(g)

  • total mass of products = 44 + 4 = 48g (same as the reactants: 12 + 36 = 48g)
  • mass of desired product (H2) = 4g

% atom economy = 448 × 100 = 8.3%

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Reversible Reactions

If the products of a chemical reaction can react to produce the reactants the reaction can go in both direction - reversible reaction Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif)

When there are no products the reaction can only go in the forward direction, but as products build up the reverse reaction can happen. In a closed system nothing can escape and the rates of both forward and backward reactions will become equal - equilibrium.

If the conditions of the system are changed the amounts of reacts and products may change. Increasing the concentration of a substance will increase the rate of the reaction away from that substance.

If the system if open and products can escape the forward reaction will continue to completion.

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Making Ammonia - Haber Process

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

Nitrogen (from air) and hydrogen (usually obtained from natural gas) are purified and mixed in the correct proportions. The gases are passed over an iron catalyst at the temperature of about 450ºC 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. Liquid ammonia is removed from the unreacted gases and are recycled.

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

N2(g) + 3H2(g) Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) 2NH3(g)

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Haber Process

Hydrogen is extracted from the reaction between methane and steam. Nitrogen is extracted from the combustion of hydrogen in air. Hydrogen and nitrogen are combined at a pressure of 200 atmospheres and a temperature of 450°C, with iron as a catalyst, to produce ammonia (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_41.gif)

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