As we remember how to predict products for the different types of reactions, we will typically be given the start of a word equation. We will then be responsible for finishing the word equation and providing a balanced equation.
Predicting Products – Synthesis Reactions
Remember that synthesis reactions involve the reaction of two (or more) substances to produce one product. Typically, this means that two elements will combine to form one compound.
- Oxidation: Metal or Non-metal + O2 → Oxide
calcium + oxygen → calcium oxide
2Ca + O2 → 2CaO*When finishing the word equation, think about the compound that would contain the combination of elements from the reactant side.
*When creating the balanced equation, be sure to think about whether the elemental reactants are regular monoatomics or if they are “special dudes" (H2, N2, O2, F2, Cl2, Br2, I2, P4, S8). Also, be sure to use the valences of the elements to create the formula of the product.
- Metal + Non-metal → Binary Compound
sodium + chlorine → sodium chloride
2Na + Cl2 → 2NaCl
- Metal Oxide + H2O → Base
magnesium oxide + water → magnesium hydroxide
MgO +
H2O → Mg(OH)2
*Notice that the product contains all of the atoms that are found in the reactants (Mg, O, H). Since we need to create a base (which must contain the hydroxide ion), it makes the most sense to combine the elements into magnesium hydroxide.
- Non-metal Oxide + H2O → Acid
carbon dioxide + water → carbonic acid
CO2 + H2O → H2CO3(aq)
* Notice that the product contains all of the atoms that are found in the reactants (H, C, O). Since we need to create an acid (which must start with a hydrogen ion), it makes the most sense to think about which polyatomic ion contains C & O. The first ion that comes to mind is carbonate. So, by combining H (valence +1) and carbonate (valence -2), we arrive at carbonic acid. We could have used percarbonate, carbonite or hypocarbonite instead of carbonate, but that would complicate the balancing. Make your life easy and always choose the polyatomic ion that creates the acid in an easy to balance equation.
*Note that for multivalent elements, more than one product is possible.
carbon + oxygen → carbon (IV) oxide
C + O2 → CO2
carbon + oxygen → carbon (II) oxide
2C + O2 → 2CO
Combustion Reactions
Complete combustion is the
reaction of a hydrocarbon with oxygen to form carbon dioxide and water. Incomplete combustion results in a different mixture of products. We will almost always be using complete combustion in our work (and hence, I will just say "combustion" - if I want you to use incomplete combustion, I will say "incomplete combustion").
- Complete Combustion of a Hydrocarbon
methane + oxygen → carbon dioxide + water
CH4 + 2O2 → CO2 + 2H2O- Incomplete Combustion
*occurs when the concentration of O2 is low
hydrocarbon + O2 → C + CO + H2O
Predicting Products – Decomposition Reactions
Remember
that decomposition reactions are the opposite of synthesis reactions. Thus, one reactant will break down into simpler components (usually just the elements found in the compound). Use
the valences of the elements to predict the reactants. Also, be sure to think about whether the
products are "special dudes" (H2, N2, O2, F2,
Cl2, Br2, I2, P4, S8).
- Binary Compound → Element + Element
hydrogen bromide → hydrogen + bromine
2HBr → H2 + Br2
Homework:
Finish the word equation and provide a balanced equation for each:
(a) sulfur + oxygen
(b) copper + chlorine
(c) sodium oxide + water
(d) nickel + fluorine
(e) carbon dioxide + water
(f) lithium oxide + water
(g) sulfur dioxide + water
(h) methane (CH4) + oxygen
(i) octane (C8H18) + oxygen
Answers:
