Chemical Reactions
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List 3 key signs of a chemical reaction
• A colour change. • A flame or explosion. • Reaction gets hot or cold by itself. • A gas is produced in the reaction without heating. • A solid is produced in the reaction without cooling.
What happens in a chemical reaction?
Some bonds are broken and others form to make new chemicals
What does not happen in a physical change?
No bonds are broken or formed
When balancing a chemical reaction, what must we not change?
The chemical formulae i.e. the “small” numbers cannot change
What do we mean by a balanced chemical reaction?
There are the same number of atoms of each element in the reactants and in the products e.g. if there are 2 oxygen atoms in the reactants, there must be 2 in the products
What is wrong with this balanced equation? 2Ca + O2 = 2CaO
An = symbol is used instead of an arrow (the 2 in O2 should also be a subscript)
How do you identify an exothermic reaction?
An increase in temperature as energy is transferred to the thermal store of the surroundings
How do you identify an endothermic reaction?
A decrease in temperature as energy is transferred from the thermal store of the surroundings.
What is oxidation?
When an element combines with oxygen to form a compound
What is combustion?
When a fuel rapidly reacts with oxygen transferring energy to the thermal store of the surroundings (high temperature)
Rusting would be an example of what type of chemical reaction?
Oxidation
Are metal oxides acidic or alkaline?
Alkaline e.g. sodium hydroxide is a drain cleaner
Are non-metal oxides acidic or alkaline?
Acidic
What happens when a compound combusts?
Each element in the compound forms an oxide
Methane is a hydrocarbon (it contains carbon and hydrogen), what products form when it combusts
Water (oxidation of hydrogen) and carbon dioxide (oxidation of carbon)
What is thermal decomposition?
A compound is broken down by heat
What is a test for carbon dioxide?
Bubble through limewater (the limewater turns cloudy)
What happens in displacement?
A more reactive metal takes the place of a less reactive metal in a compound
Why are alkali metals (group 1 elements) reactive?
They have 1 electron in their outer shell
Why are halogens (group 7 elements) reactive?
They have 7 electrons in their outer shell
Why are nobel gases not reactive?
They have a full outer shell
If a less reactive metal is added to a salt solution containing a more reactive metal, what will happen?
Nothing (no reaction will occur)
What do all acids have in common?
They contain hydrogen
An acid releases what into solution
Hydrogen ions (positive)