Chemical Reactions

Combination/Synthesis Reactions

• These reactions occur when 2 or more simpler substances react to form 1 more complex product 
• For example magnesiums (Mg) + oxygen (O) --> magnesium oxide (MgO)

Decomposition Reactions

• These reactions occur when one substance breaks down into 2 or more simpler substances 
• For example, copper carbonate --> copper oxide + carbon dioxide 
• If heat is used to break down the compound, it's thermal decomposition
• If an electric current is used to break down the compound, it's electrolysis 
• If a catalyst is used to break down a compound it's catalytic decomposition 

Combustion Reactions

• These reactions occur when a reactant is micedd with oxygen and the reaction releases light/heat energy

Complete Combustion

• Complete combustion is a reaction where there is lots of oxygen present 
• Carbon dioxide and water are produced from this reaction 
• Light and heat are produced as well

Incomplete Combustion

• This reaction occurs when there is a limited supply of oxygen 
• Carbon monoxide and water are the products of this type reaction
• Light and heat are produced as well 

Displacement Reactions

• These reactions happen when a more reactive element replaces a less reactive element from its compound 
• For example, zinc + copper sulphate --> zinc sulphate + copper 
  • Zinc is more reactive than copper, so it replaces copper 

Acid Reactions

• There are 3 types of acid reactions:
  • Acid + alkali
  • Acid + metal 
  • Acid + metal carbonate
• All acid reactions produce a metal salt and another product (either water, carbon dioxide or hydrogen gas)

Acid + Alkali

• This is also known as a neutralisation reaction 
• A salt and water are the products of this reaction 
• For example, nitric acid + magnesium oxide --> magnesium nitrate + water 

Acid + Metal 

• Salt and hydrogen gas are the products of this reaction 
• For example, magnesium + sulphuric acid --> magnesium sulphate + hydrogen (gas) 

Acid + Metal Carbonate 

• Salt, water and carbon dioxide are the products of this reaction
• For example, sodium carbonate + hydrochloric acid --> sodium chloride + carbon dioxide + water 

How to work out the salt formed:

• Hydrochloric acid - Chloride
• Sulfuric acid - Sulfate 
• Nitric acid - Nitrate 
• Ethanoic acid - Ethanoate 

Precipitate Reactions

• This reaction occurs when 2 soluble salt solutions react to form a precipitate (solid)
• The solid that is formed is insoluble (doesn't dissolve) 
• If there is an (aq) by an element, it means they break up into their ions
• If there's a (s) by an element, it means it's a solid and is insoluble 
• For example, potassium bromide + silver nitrate --> silver bromide(s) + potassium nitrate(aq)
• You can figure out which products are (aq) and (s) by using the solubility rules

Reaction Rates

• For a chemical reaction to occur, particles must collide in the right orientation
• The particles also need to collide with enough energy to overcome activation energy (an energy level that can break bonds)
• The reaction rate is how fast a chemical reaction occurs
• There are several ways to increase the reaction rate:

Concentration

• Refers to the amount of reacting particles per volume 
• Concentrated solutions have a higher number of reacting particles per volume, dilute solutions have less per volume
• When the concentration is higher the reaction rate is quicker because the particles collide more often per second, so the reaction happens faster

Temperature 

• When the temperature is higher the reaction rate is faster because the particles are moving quicker, therefore they have more kinetic energy to collide more
• The particles will collide more and will have more energy to overcome activation energy, this leads to a faster reaction rate

Surface Area 

• Breaking a solid into smaller pieces or grinding it into a powder will allow more surface area for the particle to react with, resulting in a faster reaction rate

Catalyst

• A substance that speeds up the reaction bu is not used in the reaction
• A catalyst lowers the activation energy needed for a reaction by putting reactants in the right orientation
• By lowering the activation energy more particles and collisions will have the energy needed to overcome activation energy and react