SECTION 3 .2

Writing Chemical Equations

Objectives

At the end of this lesson, you will be able to:Translate chemical word equations into

formula equationsBalance simple chemical equations

Chemical Equations

Chemical equations represent the process of a chemical reaction

To write one, must know the reactants and products

Need to know if a chemical change has occurred Use our evidence of chemical changes- what are they?

Takes everything we have learned so far and puts it into a condensed equation i.e. compound names and formulas, states, ionic vs.

molecular etc

Writing Word Equations

A plus sign (+) groups the reactants together It does not matter which order the reactants are

written inAn arrow () separates the reactants from the

products and is read “produces”A plus sign (+) also joins the products

Word Equations

Taken from descriptions of a chemical reaction Ex. Solid magnesium metal reacts with aqueous

hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas

Evidence of a chemical change: gas, heat is given offHow do we write this as a chemical equation?

Using words? magnesium + hydrochloric acid magnesium chloride + hydrogen gas

Word Equations

Another example: aluminium foil reacts with blue copper II sulphate solution and produced solid copper and colourless (grey?) aluminium sulphate solution

As a word equation:

To Do:

Writing Formula Equations from Word Equations

Objectives

At the end of this lesson, you will be able to:Translate chemical word equations into

formula equationsBalance simple chemical equations

Formula equations

Uses chemical formulas of reactants and products in a chemical equations to represent a reaction Ex. hydrogen + oxygen water How do we write this as a formula equation? H(2)g + O(2)g H2O(l)

This is called a skeleton equation (just shows what is involved with the reaction) Doesn’t show correct proportions of reactants and

products

Formula Equations - example

Aluminium and copper II sulphate example:Aluminium (s) + copper II sulphate (aq) copper

(s) + aluminium sulphate (aq)As a formula equation:

Balancing Equations

We know what the correct proportions are through: Law of Conservation of Mass

Developed by Antoine LavoisierTotal mass of the reactants equals the

total mass of the productsUsing this, we can deduce that:

** total # of atoms present before a reaction equals the total # of atoms after a reaction

Balancing Equations

Let’s look at this example:Water decomposes (with electrical energy) to

produce oxygen gas and hydrogen gasAs a formula equation:

Does this equation follow the conservation of mass? Why or why not?

Balancing Equations

To get the number of atoms to stay the same, before and after the equation, more than one of a molecule may be involved in the reaction

The number of molecules is represented by a number in front of the formula called the coefficient

Balancing Equations

Look at the following reaction: 4 AlCl3 + 3 PbO2 2 Al2O3 + 3 PbCl4

An equation is balanced if the # of each type of atom on reactant side = # of each type of atom on product side – is it?

The numbers in front are coefficientsCannot change formulas of any substances;

you can only add coefficients to balanceHow is this related to the Law of

Conservation of Mass?

Practice

Keep a tally of the # atoms as you choose coefficients – you may change your mind, so change tally as you go

___Zn + ___HCl ___ZnCl2 + ___H2

___Li2O + ___Mg3P2 ___Li3P + ___MgO

___K + ___O2 ___K2O

___CH4 +___ O2 ___CO2 +___ H2O

Polyatomic Ions

When dealing with polyatomic ions (i.e. SO4

2-), treat them as single units to be balanced

Treat any number outside of the brackets as another coefficient

Fe(NO3)2(aq) + Na3PO4(aq) NaNO3(aq) + Fe3(PO4)2(s)

How do we approach this problem? Which atoms should we start with first?

Practice

Fe(NO3)2 + Na3PO4 NaNO3 + Fe3(PO4)2

H2SO4 + NaOH Na2SO4 + H2O

Practice

N2(g) + H2(g) NH3(g)

CaC2(s) + H2O(l) Ca(OH)2(s) + C2H2(g)

SiCl4(s) + H2O(l) SiO2(s) + HCl(aq)

H3PO4(aq) + CaSO4(s) Ca3(PO4)2(s) + H2SO4(aq)

Methane + oxygen carbon dioxide + water vapor

Sodium chloride sodium + chlorineCalcium nitrate + sodium sulfate sodium

nitrate + calcium sulfateSulfur + oxygen sulfur dioxide

Practice

Try the following for more practice:P. 90 # 7-9Chem Workbook:

“Balancing Formula Equations 1” “Balancing Formula Equations 2”