CHEMISTRY 10

Writing and Balancing Equations

Writing and Balancing Equations

A chemical equation is a shorthand expression for a chemical change or reaction.

word - equation : states in words

mercury (II) oxide + heat mercury + oxygen

bulky and cumbersome

2 HgO 2Hg + O2

Writing and Balancing Equations

General Format for Writing Equations

1. The reactants are separated from the products by an arrow that indicates the direction of the reaction.

2. The reactants are placed to the left and the products to the right of the arrow. A plus sign (+) is placed between reactants and between products when needed.

Writing and Balancing Equations

3. Conditions required to carry out the reaction may be placed above or below the arrow. A delta indicates heat is applied.

4. Coefficients 2 H2O are placed in front to balance the equation. One is never placed there, it is just understood.

Writing and Balancing Equations

5. The physical state of a substance is indicated by the following symbols:

(s) solid (l) liquid (g) gaseous (aq) aqueous: means water solution

yields reversible

reaction

H2 gas evolved solid precipitate

Writing and Balancing Equations

Balancing Equations1. First you must have the equation in

symbol format, you cannot balance a word equation.

2. Balancing:a. Count and compare the number of

atoms of each element on each side of the equation and determine those that must be balanced.

Writing and Balancing Equations

Balancing Equationsb. Balance each element one at a time,

by placing whole numbers (coefficients) in front of the formulas containing the unbalanced element.

It is usually better to balance in this order: metals, nonmetals, hydrogen, oxygen.

Writing and Balancing Equations

A coefficient placed before a formula multiplies every atom in the formula by that number!

2H2SO4 = 4H’s 2S’s 8O’s

c. re-check each time you balance an element to see if anything else has become unbalanced. Make adjustments as needed.

Writing and Balancing Equations

d. Do a final check making sure that each element is balanced and that the smallest possible set of whole number coefficients has been used:

4HgO 4Hg + 2O2

incorrect

2HgO 2Hg + O2 correct

Writing and Balancing Equations

Equations for practice:Magnesium + oxygen Magnesium Oxide

Potassium Chlorate Potassium Choride + Oxygen

Aluminum hydroxide + sulfuric acid

Aluminum sulfate + water

C4H10 + O2 CO2 + H2O

Writing and Balancing Equations

Types of Chemical Reactions

1. Combination or Synthesis Reaction:

Two reactants combine to give one product.

A + B AB

2Mg + O2 2MgO

Writing and Balancing Equations

Types of Chemical Reactions

2. Decomposition:

A single substance is decomposed or broken down to give two or more different substances:

AB A + B

2PbO2 2PbO + O2

2 Na HCO3 Na2CO3 + H2O + CO2

Writing and Balancing Equations

Types of Chemical Reactions3. Single - Displacement Reaction:

One element reacts with a compound to take the place of one of the elements of that compound.

A + BC B + AC

Zn + 2HCl H2 + ZnCl2Note: Activity series table helps to predict which substances (elements) will be able to replace various other substances.

Writing and Balancing Equations

Types of Chemical Reactions4. Double - Displacement or Metathesis:

Two compounds exchange partners with each other to produce two different

compounds.

AB + CD AD + CB

NaCl + KNO3 NaNo3 + KCl

Writing and Balancing Equations

Heat in Chemical Reactions

Energy changes always accompany chemical reactions. One reason why reactions occur is that the products attain a lower, more stable energy state than the reactants.

Writing and Balancing Equations

Heat in Chemical Reactions

Reactions are either exothermic or endothermic.

Exothermic reactions liberate heat:

H2 + Cl2 2HCl + 185kJ (exothermic)

Endothermic reactions absorb heat:

N2 + O2 + 181kJ 2NO (endothermic)