- Reactants form products.
- Reactions may be exothermic (releasing heat) or endothermic (absorbing heat).
Types of Reactions
- Synthesis: A + B → AB
- Decomposition: AB → A + B
- Single Replacement: AB + C → AC + B.
- Double Replacement: AB + CD → AD + CB.
- Precipitation - when two ionic compounds are dissolved in a solvent, they form four ions. Quite often, the positive ion from one compound will react with the negative ion from the other, forming an insoluble compound. This compound precipitates to the bottom as a solid.
- Neutralization - when an acid and base are mixed, water and a salt will form.
These are reactions where the oxidation numbers of particles change. The particle whose oxidation number decreases is reduced, and is called an oxidizing agent. The other particle is oxidized, and is called a reducing agent. If oxygen is the oxidizing agent, the reaction is called combustion.
Predicting Chemical Reactions
Types of Reactions
There are several guidelines that can help you predict what kind of chemical reaction will occur between a mixture of chemicals.
- Several pure elements mixed together may undergo a synthesis reaction.
- A single compound may undergo a decomposition reaction. It often forms water or hydrogen gas.
- A pure element mixed with an ionic compound may undergo a single replacement reaction.
- Two different ionic compounds are very likely to undergo a double replacement reaction.
- An organic compound (containing carbon and hydrogen) can usually react with oxygen in a combustion reaction.
However, not all elements will react with each other. To better predict a chemical reaction, knowledge of the reactivity series is needed.
When combining two chemicals, a single- or double-replacement reaction doesn't always happen. This can be explained by a list known as the reactivity series, which lists elements in order of reactivity. The higher on the list an element is, the more elements it can replace in a single- or double-replacement reaction. When deciding if a replacement reaction will occur, look up the two elements in question. The higher one will replace the lower one.
Elements at the very top of the series are so reactive that they can replace hydrogen from water. This explains the explosive reaction between sodium and water:
Elements in the middle of the list will react with acids (but not water) to produce a salt and hydrogen gas. Elements at the bottom of the list are mostly nonreactive.
Elements near the top of the list will corrode (rust, tarnish, etc.) in oxygen much faster than those at the bottom of the list.
The Reactivity Series
- Red: elements that react with water and acids to form hydrogen gas, and with oxygen.
- Orange: elements that react very slowly with water but strongly with acids.
- Yellow: elements that react with acid to form hydrogen gas, and with oxygen.
- Grey: elements that react with oxygen (tarnish).
- White: elements that are often found pure; relatively nonreactive.
- Rate constant - constant only for a specific reaction and temperature.
- Order of a reaction - add up all the exponents of the molarities.