# C5 atoms, masses and moles

Covers the OCR C5 checklist for higher and standard students

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## Bonds

An ionic bond is when one atom loses electrons which then join another atom. These atoms are then attracted to each other due to an opposite charge.

A covalent bond is where the atoms share electrons, allowing them to complete their outer shells.

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## Masses of an atom

The relative atomic mass of an atom is the average mass of the element compared to 1/12 of carbon-12. It is the number of protons and neutrons in the atom.

The atomic number is the number of protons in the atom. The number of electrons will be the same as this, unless the atom has a positive or negative charge.

Calculating the % mass of an element in a compound-

(Relative atomic mass x no. of atoms (of element)/relative atomic mass of entire compound) x100

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## Moles and Concentration

Formula for Moles-

No. of moles= mass of substance in grammes/relative mass (also called molar mass) of substance

Concentration is the number of solute particles in a given volume.

Formula for concentration in mol/dm3;

amount in moles/volume

The difference between concentration and strength is that strength is a measure of the H+ ions in an acid, concentration is a measure of the no. of moles of acid in one dm3

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## Limiting Factors

The amount of reactants used can limit the amount of product made.

This is because when the limiting reactant particles have all collided with the other reactant's particles then no more collisions can occur.

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## Reversible Reactions

A reversible reaction is one where the products can react to form the reactants.

If a chemical reaction occurs in a closed container where the reactants and products can't escape it will reach an equilibrium.

At an equilibrium the rate of the forward reaction will equal the rate of the backwards reaction. This means that the concentrations of the products and reactants won't change.

If you remove a product from a reversible reaction the equilibrium will shift to the right. This means more reactants are converted into products. This will also happen if you add more reactants.

Increasing the temperature will move the reaction in the direction of the endothermic reaction. Increasing the pressure will move the equilibrium to the side with the least number of moles of gas molecules.

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## The Haber Process

The Haber process is how we make ammonia from hydrogen and nitrogen.

Using a high pressure the two gasses are passed over an iron catalyst at about 450 degrees Celsius.

450 degrees is used because any higher would move the reaction to the left (making less ammonia) but any lower would decrease the rate of reaction.

A high pressure shifts the reaction to the right (making more ammonia) but it can't be too high because that would be dangerous and expensive.

The iron catalyst speeds up the rate of reaction but doesn't affect the yield.

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## The Contact Process

This is where sulphur dioxide is converted to sulphur trioxide.

2So2 + O2 = 2SO3

In the contact process a catalyst of vanadium oxide is used. This speeds up the reaction. Around 450 degrees Celsius is used becasue it makes the reaction fast enough without shifting it too far to the left. A low pressure is used because the equilibrium is already on the right so an expensive high pressure isn't needed.

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## Acids and Precipitation Reactions

H+ ions are produced when acid ionises in water. A strong acid will completely ionise in water but a weak acid won't fully ionise but will form an equilibrium mixture. The PH indicates the number of H+ ions in the acid.

Ethanoic acid will react slower than HCl acid of the same concentration because there are fewer H+ ions in it, meaning that there will be fewer collisions in ethanoic acid. The fact that there are less H+ ions in ethanoic acid also means it is less conductive than HCl.

In a precipitation reaction ions must collide with other ions. In the preparation of a dry sample of an insoluble compound by precipitation you must mix solutions of reactants, filter it and then wash and dry the residue.

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