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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:
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
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
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