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Topic 5: Chemical Equation

What is a chemical reaction?

A chemical reaction is a chemical change. It takes place everytime, such as when irons rust, milk turns sour, leaves change color in the fall, digestion in the body occurs, etc. Energy can be absorbed or released in chemical reactions. When Hydrogen and Oxygen react to produce water, it releases energy (Hydrogen + Oxygen = Water + Energy). Atoms rearrange in chemical reactions, where bonds break and new bonds form. For example:

potassium chlorate + phosphorus -> potassium chloride + phosphorus(V) oxide + energy

In order to make a chemical reaction happen, particles must collide. For example, in order for a match to be lighted, one of the reactants, the match head, must strike the other reactant, the matchbook striking surface, or else a chemical reaction won't occur.

How are reactions written?

Reactions are described with chemical equations, which uses symbols rather than whole words. The reaction of hydrogen and oxygen bonding to form water can be written as:

2H2 + O2 -> 2H2O

If 2 hydrogen atoms and 2 oxygen atoms bond together to form H2O, then you would find only one oxygen atom in the products, and because of the law of conservation of mass, the other oxygen atom could not disappear. Actually, there should be 4 hydrogen atoms and 2 oxygen atoms in both the reactant and product, as stated in the above equation, which makes the equation balanced.

How can reactions be used?

There are 5 reaction types: synthesis, decomposition, single displacement, double displacement, and combustion. Synthesis is when two or more atoms bond together to form a compound. The above equation for hydrogen and oxygen bonding to form water is an example of a synthesis reaction. Decomposition is when a compound is separated into two or more atoms. This is a decomposition reaction:

2Ag2O -> 4Ag + O

A single displacement is when an atom replaces another atom in a compound. An activity series can be used to see if the equations work, because not all written reactions occur. This is an example of a single displacement reaction, where potassium replaces hydrogen to form potassium hydroxide with the hydroxide:

2K + 2HOH -> 2KOH + H2

A double displacement is when two elements in different compounds trade places. This is an example of a single displacement reaction, where lead(II) and potassium trade places to form lead(II) iodide and potassium nitrate:

2KI + Pb(NO3)2 -> PbI2 + 2KNO3

A combustion is an exothermic reaction where oxygen is used to form oxides with the reactant element. This is an example of a combustion, where hydrocarbon burns and carbon dioxide and water is produced:

CH4 + 2O2 -> CO2 + 2H2O + energy

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