A chemical reaction is a process in which one or more substances are converted into new substances. The initial substances are the reactants, and the substances that result are called the products. As an equation this change is represented with reactants on the left with an arrow leading to products on the right. The phase of each substance is noted as solid, liquid, gas or aqueous, meaning dissolved in water. Special symbols over the arrow indicate when heat is added or a catalyst is used to speed up the reaction. The law of conservation of mass aids in balancing equations and creating a true picture of the reaction. When balancing equations it is useful to balance one element at a time, remembering that coefficients apply to whole molecules. It is also a good strategy to balance pure elements last.
Vocabulary words
chemical reaction, chemical equation, reactant, product, insoluble, soluble, aqueous (aq), law of conservation of matter, unbalanced chemical equation, balanced chemical equation
Key relationships
Chemical equation: Reactants → Products
“+” separates different reactants or products
Law of conservation of mass: mass of reactants = mass of products
Common Symbols
(g)
gas
(l)
liquid
(s)
solid
(aq)
aqueous (dissolved in H2O)
→
forms, produces, yields, etc.
reacts when heat is added
reacts with an added catalyst
Review problems and questions
Explain why it is impossible to balance the following equation: C3H8 → H2 + CO2
The reactants are missing oxygen atoms. Both sides of an equation must have the same atoms. This reaction is not only impossible to balance, it is impossible to occur.
Translate the following chemical equation to words: 2H2(g) + O2(g) → 2H2O(l)
Two molecules of hydrogen gas react with one mole of oxygen gas to produce two molecules of liquid water.
OR
Two moles of hydrogen gas plus a mole of oxygen gas yield two moles of liquid water.
There are a number of ways you could have written this answer. These are only two examples.
Balance the reaction: P4 + O2 → P2O5
P4 + 5O2 → 2P2O5
Balance the reaction: ClO2 + H2O → HClO2 + HClO3
2ClO2 + H2O → HClO2 + HClO3
Balance the reaction: Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4
Al2(SO4)3 + 3Ca(OH)2 → 2Al(OH)3 + 3CaSO4;
Remember, a number outside parenthesis gets distributed inside parenthesis by multiplication.
