Review Questions - Chemical Equations printable pdf download

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CHEMICAL EQUATIONS

A chemical equation is a simple and easy way to describe a chemical reaction. Chemical

equations use chemical formulas and other symbols instead of words to summarize a

reaction. A chemical equation tells you the substances you start with in a reaction and the substances

that are formed in the end. The chemical equation for the formation of water is shown below.

2 H

à

reactants

products

All chemical equations share a common structure. The substances you have at the beginning are

called the reactants. The new substances formed at the end of the reaction are called the products.

The formulas for the reactants are written on the left side of the equation, followed by an arrow ( à )

which means “ turns into ” or “ yields ”. The formulas for the products are written on the right side of the

equation. When there are two or more reactants or products, they are separated by plus signs.

The law of conservation of mass was first demonstrated by a scientist named Antoine

Lavoisier in the late 1700s. The law of conservation of mass states that in a chemical reaction, the total

mass of the reactants must equal the total mass of the products. In an open system, matter

can enter or escape from the surroundings. For example, a match burning in the open air is an open

system. You cannot measure the mass of all the reactants and products in an open system. In a closed

system, matter cannot enter or escape into the surroundings. A match burning inside a sealed glass

chamber is an example of a closed system. A closed system allows you to measure the mass of all the

reactants and products in a reaction.

Because of the law of conservation of mass, a chemical equation must be balanced. An

equation is balanced when the total number of atoms of the reactants equals the total number of atoms

in the products. To balance a chemical reaction, you may have to use coefficients. A coefficient is a

number placed in front of a chemical formula in an equation. The coefficient tells you how many

molecules of a reactant or product take part in a reaction.

+ O

à

unbalanced equation

balanced equation

C = 1

H = 4

H = 2

H = 4

O = 2

O = 3

O = 4

Review Questions - Chemical Equations