Chemical Equations

Honors Chemistry

Mr. Rivas

Chemical Equations

A chemical reaction is a process in which substances are changed into different substances.

The old substances are called reactants and the new substances are called products.

A chemical equation describes what happens in a chemical reaction.

Example of a chemical change

The reaction: When solid carbon reacts with oxygen gas, a

new substance, carbon dioxide gas is formed and energy is released

The equation:

C(s) + O2(g) CO2(g) Since this is an exothermic reaction we

can also write:

C(s) + O2(g) CO2(g) + heat

Chemical Equation

The chemical equation identifies: The original substances (reactants) and the

resulting substances (products). The phase of each

(s) Solid (l) Liquid (g) Gas (aq) Aqueous (dissolved in water)

The number of moles of each substance

Chemical Equation

The arrow reads yields or produces. It separates the reactants to the left from the products to the right.

Δ or ø means reactants must be heated.

N.R. means no reaction

indicates that the reaction is reversible.

Balancing Chemical Equations

In a chemical reaction the mass of the products must be the same as the mass of the reactants.

Steps: Write the unbalanced equation Balance the equation by making sure that the number

of atom of each element is the same in the products and the reactants.

Hint: Start by balancing an element that appears in only one reactant and one product

Example

Tin oxide is heated with hydrogen to form tin metal and water vapor.

Write the unbalanced equation SnO2(s) + H2(g) Δ Sn(s) + H2O(g)

Identify which elements are not balanced. Here we have two O atoms in the reactants and

only one in the products. We correct this by entering a coefficient of 2 for

water …

Example

This puts the hydrogen atoms out of balance.

SnO2(s) + H2(g) Δ Sn(s) + 2 H2O(g) To fix this we enter a coefficient of 2 for

hydrogen.

SnO2(s) + 2 H2(g) Δ Sn(s) + 2 H2O(g) Remember: coefficients are multipliers. When

we write 2H2O we mean 2x2 = 4 H atoms and 2x1 = 2 O atoms.

Remember…

When balancing a chemical equation you must never change the formulas of the reactants or products.

This means do not change the subscripts of the formulas.

The balancing must be achieved by writing the appropriate coefficient(s).

Balancing Equations Exercises

1) H2 + O2 H2O

2) NaOH + H2SO4 Na2SO4 + H2O

3) Na + O2 Na2O2

4) N2 + O2 N2O5

5) Cs + N2 Cs3N

6) NH3 + HCl NH4Cl

7) Li + H2O LiOH + H2

Answers to balancing equations

1) 2 H2 + O2 2 H2O

2) 2 NaOH + H2SO4 Na2SO4 + 2 H2O

3) 2 Na + O2 Na2O2

4) 2 N2 + 5 O2 2 N2O5

5) 6 Cs + N2 2 Cs3N

6) NH3 + HCl NH4Cl

7) 2 Li + 2 H2O 2 LiOH + H2

Worksheet

Complete Worksheet 9.1