Chemical Reactions (continued)

Reaction Speed

Chemical reactions do not occur at the same speed: some are faster, others are very slow. For example, the reaction between baking soda and vinegar is rapid. Just the reagents come in contact for it to occur. The reaction that occurs between iron, oxygen and water, forming the rust, is slow.

Some factors may change the speed of chemical reactions. In a reaction between an effervescent tablet and water, we can accelerate the speed of this reaction. Just divide the tablet into equal pieces. So the more crushed, the more divided, the faster the reaction. This factor is the contact surface, which increases and makes the reaction faster.

The same goes for temperature. If we put the effervescent tablet in cold water and another in hot water, we will observe that with hot water the reaction occurs faster. Then, increasing the temperature causes the speed of the chemical reaction to increase.

Types of Chemical Reactions

Chemical reactions are classified into four types:

- synthesis or addition
- analysis or decomposition
- simple change or move
- double exchange

- SYNTHESIS OR ADDITION - is the reaction where two or more substances react to become one. Examples:

C + O2 → CO2
Cao + H2O → Ca (OH)2

- ANALYSIS OR DECOMPOSITION - is the reaction where a substance is divided into two or more substances of simpler structure. Examples:

2AgBr → 2Ag + Br2
2Cu (NO3)2 → 2CuO + 4NO2 + O2 

- SIMPLE EXCHANGE OR DISPLACEMENT - is the reaction where a simple substance changes places with an element of a compound substance, transforming itself into a new simple substance. Examples:

Zn + H2ONLY4 → ZnSO4 + H2  
Fe + CuSO4 → FeSO4 + Cu

- DOUBLE EXCHANGE - is the reaction where two compound substances react and exchange their elements, turning into two compound substances. Examples:

HCl + NaOH → NaCl + H2O
FeS + 2HCl → FeCl2 + H2s