This is a document that summarizes the CGP GCSE OCR Gateway Chemistry C3 notes. It contains everything you need to know within the C3 (elements, compounds and mixtures) unit in simple sentences and terms. It contains basic things about conservation of mass, chemical formulas, reactions and equation...
Conservation of Mass
The law of conservation of mass states that atoms cannot be created or destroyed by chemical reactions. They are just joined in a different
way, so the total mass stays the same during chemical reactions - forming products.
This means there are the same number and types of atoms on each side of a reaction equation.
We can use relative formula masses to show that mass is conserved in a reaction. However, in some reactions where a change of mass occurs
this means gas may be involved.
In a case of mass increase: it means that one of the reactants is a gas that’s found in air and the products are solids, liquids or aqueous. When
the gas reacts to form part of the product, it becomes contained inside the reaction vessel. So the total mass of the stuff inside the reaction
vessel increases.
E.g: The mass of the metal oxide produced equals the total mass of the metal and the oxygen that
reacted from the air.
Metal(s) + Oxygen(g) -> metal oxide(s)
In a case of mass decrease: it means some or all of the reactants are solids, liquids or aqueous and at least one of the products is a gas. If the
vessel isn’t enclosed, then the gas escapes from the reaction vessel as it’s formed. It’s no longer contained in the reaction vessel, so you can’t
account for its mass. So the total mass of the stuff inside the reaction vessel decreases.
E.g: When a metal carbonate thermally decomposes to form a metal oxide, the mass of the reaction vessel will appear to
decrease if it isn’t sealed as the carbon dioxide escapes. The mass of the metal oxide and carbon dioxide produced will equal
the metal carbonate that reacted
Metal carbonate(s) -> metal oxide(s)+ carbon dioxide(g)
Chemical Formulas
A chemical formula represents an element or compound in balance equations. The formula for most elements is just their symbols.
Some non-metals exist as simple molecules with two atoms joined together and these are called diatomic elements. They don’t exist by themselves
and often have a two with their element symbol.
Any positive ions can combine with any of the negative ions to form an ionic compound.
The overall charge of any compound is zero. So all the negative charges in the compound must balance all the positive charges. You can use the
charges on the individual ions present to work out the formula for the ionic compound.
You need to be able to write formulas using chemical symbols.
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, Chemical Equations
Chemical changes are shown using Chemical equations. One way to show this is through word equations.
E.g: Methane burns in oxygen giving carbon dioxide and water: Methane + oxygen -> carbon dioxide + water
Chemical changes can also be shown in symbol equations. It shows the atoms on both sides and shows the reactants and the products.
E.g: [Word equation] Magnesium +oxygen -> Magnesium oxide
[symbol equation] 2Mg +02 -> 2MgO (also a balanced equation)
Balancing Equations
Symbol equations need to be balanced; there must always be
the same number of atoms on both sides of the equation. It can
be balanced by putting numbers in front of the formulas where
needed. We need the same amount of atoms of one element on
both sides of the equation:
State Symbols
There are 4 different state symbols and these are placed in both
word equations and symbol equations.
(s) - solid
(l) - liquid (pure liquid)
(g) - gas
(aq) - aqueous (dissolved in water)
Ionic Equations
Ionic equations show the useful bits of reactions. You can also write an ionic equation for any reaction involving ions that happen in the solution.
In an ionic equation, only the reacting particles (and the products they form) are included.
To write an ionic equation, you’ve just got to look at the reactants and products. Anything that’s exactly the same on both sides of the equation
can be left out.
Half Equations
● Half equations show how electrons are transferred during reactions. In half equations, e- stands for one electron.
● You can’t write half equations for all chemical reactions - only ones where oxidation or reduction happens.
● You can combine half equations to create full ionic equations.
Moles
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● 1 mole is equivalent to 6. 02𝑥10 and this number is also known as Avogadro's constant.
● Moles measure the amount of substance in a chemical reaction.
● There is a formula related to mass and Mr or Ar and moles:
The number of moles = Mass in g / Mr
The molar mass of one substance is the mass of one mole of that substance it is equal to
● The relative atomic mass or Ar in grams for atoms and metals.
● The relative formula mass or Mr in grams for molecules and compounds.
Calculating Masses
The amount of product depends on the limiting reactant. A balanced chemical equation can be used to
work out the mass of a product formed.
1. Write out the balanced equation.
2. Work out the relative formula masses (Mr) of the reactant and product you want.
3. Find out how many moles there are of the substance you know the mass of.
4. Use the balanced equation to work out how many moles there’ll be of the other substance.
5. Use the number of moles to calculate the mass.
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