Balancing EquationsBalancing Equations

Chemical rxns occur when bonds (between Chemical rxns occur when bonds (between electrons of atoms) are formed or brokenelectrons of atoms) are formed or broken

Chemical rxns involve: Chemical rxns involve: changes in the chemical composition of matter changes in the chemical composition of matter

making new materials with new propertiesmaking new materials with new propertiesenergy changes:energy changes:

Bond breaking Bond breaking absorbsabsorbs Energy (endothermic) Energy (endothermic) Bond making Bond making releasesreleases Energy (exothermic) Energy (exothermic)

Note: Note: The overall change in energy for certain rxns can be The overall change in energy for certain rxns can be

found on Ref. Table Ifound on Ref. Table I

Chemical ReactionsChemical Reactions

Depict the kind of Depict the kind of reactantsreactants and and productsproducts and their relative amounts in a reaction.and their relative amounts in a reaction.

44 Al Al (s)(s) + + 33 OO2 (g)2 (g) → → 22 AlAl22OO3 (s)3 (s)

The numbers in the front are calledThe numbers in the front are calledstoichiometricstoichiometric coefficientscoefficients

Chemical EquationsChemical Equations

Symbols Used in EquationsSymbols Used in EquationsSolid (s), Liquid (l), Gas (g)Solid (s), Liquid (l), Gas (g)Aqueous solution (aq) Aqueous solution (aq) (dissolved in water)(dissolved in water)

Catalyst Catalyst HH22SOSO4 or Pt4 or Pt

Escaping gas (Escaping gas ())

Change of heat energy Change of heat energy (( or + 3kJ or – 3kJ) or + 3kJ or – 3kJ) *there is no subtraction…a negative sign means released/exothermic*there is no subtraction…a negative sign means released/exothermic

Express a chemical equation as follows:Express a chemical equation as follows:

Reactants Reactants Products Products The arrow is equivalent to an The arrow is equivalent to an ““==““ math. When math. When

describing the equation use the word describing the equation use the word ““yieldsyields”” or or ““producesproduces”” instead of equals instead of equals

Ex:Ex: C + OC + O22 CO CO22

This reads This reads ““carbon plus oxygen react to yield carbon dioxidecarbon plus oxygen react to yield carbon dioxide””

Because of the principle of the Because of the principle of the conservation of matter conservation of matter an an equation must be balanced.equation must be balanced.

matter can’t be created or matter can’t be created or destroyeddestroyed

Must have the Must have the same number of same number of each type of atom each type of atom on both sides.on both sides.

Must have same # grams of Must have same # grams of matter on both sides!! matter on both sides!!

Lavoisier, 1788Lavoisier, 1788

Coefficients not SubscriptsCoefficients not Subscripts

When balancing add When balancing add coefficientscoefficients in front of in front of compounds to balance the atoms in the reactioncompounds to balance the atoms in the reaction

DO NOTDO NOT change the subscripts of the change the subscripts of the formulas.formulas.

Changing subscripts changes the compound. Changing subscripts changes the compound.

SubscriptsSubscripts: : tell you how many atoms of a particular element are tell you how many atoms of a particular element are in a compound. in a compound.

Coefficient: Coefficient: tells you about the quantity, or number, of tells you about the quantity, or number, of molecules of the compound.molecules of the compound.

Balancing a chemical equation:Balancing a chemical equation:

DO NOT CHANGE THE FORMULAS!DO NOT CHANGE THE FORMULAS!

Find number of atoms for an element on left side. Find number of atoms for an element on left side. Compare those against number of the atoms of the same Compare those against number of the atoms of the same

element on right side.element on right side.

Place coefficients in front of formulas so that left side has Place coefficients in front of formulas so that left side has the same number of atoms as the right side for EACH the same number of atoms as the right side for EACH elementelement

Check your answer to see if:Check your answer to see if:The numbers of atoms on both sides of the equation The numbers of atoms on both sides of the equation

balanced.balanced.The coefficients are in the The coefficients are in the lowestlowest possible whole possible whole

number ratios. number ratios.

Helpful hints for balancing equations:Helpful hints for balancing equations:

Take one element at a time, working left to right. Take one element at a time, working left to right.

Save H for next to last, and O until last.Save H for next to last, and O until last.

IF IF everything balances except for O, and there is no way everything balances except for O, and there is no way to balance O with a whole number, to balance O with a whole number, double all the double all the coefficients and try again.coefficients and try again.

Useful in combustion reactions!Useful in combustion reactions!

(Shortcut) (Shortcut) Polyatomic ions that appear on both sides of Polyatomic ions that appear on both sides of the equation should be balanced as independent unitsthe equation should be balanced as independent units

Just circle them and think of them as one thingJust circle them and think of them as one thing

Balancing EquationsBalancing Equations

___ H___ H22(g) + ___ O(g) + ___ O22(g) → ___ H(g) → ___ H22O(l)O(l)

What happened to the other Oxygen atom?????

This equation is not balanced!

Balance EquationBalance Equation

Na + ClNa + Cl22 NaCl NaCl

Na = 1Na = 1 Na = 1Na = 1 Cl = 2Cl = 2 Cl = 1Cl = 1

The number of sodium atoms balance but The number of sodium atoms balance but the chlorine does not. Use coefficients in the chlorine does not. Use coefficients in order to balance this equation.order to balance this equation.

Inserting CoefficientsInserting Coefficients

Na + ClNa + Cl22 2 NaCl 2 NaCl

Na = 1Na = 1 Na = 2Na = 2 Cl = 2Cl = 2 Cl = 2Cl = 2

Now chlorine balances but the sodium does Now chlorine balances but the sodium does not! So we go back and balance the sodium.not! So we go back and balance the sodium.

Finally balanced!Finally balanced! 2Na + Cl2Na + Cl22 2 NaCl 2 NaCl

Na = 2Na = 2 Na = 2Na = 2 Cl = 2Cl = 2 Cl = 2Cl = 2

Since the number of each element on the Since the number of each element on the reactant side and the product side of the reactant side and the product side of the equation are equal, the equation is balanced.equation are equal, the equation is balanced.

Balance Practice Balance Practice (Page 2 in Chemical Equations Packet)(Page 2 in Chemical Equations Packet)

___Al___Al22OO33 + ___H + ___H22 → ___H → ___H22O + ___AlO + ___Al

___MgI___MgI22 + ___K → ___KI + ___Mg + ___K → ___KI + ___Mg

___AlF___AlF33 → ___Al + ___F → ___Al + ___F22

Balance Practice Balance Practice (Page 2 in Chemical Equations Packet)(Page 2 in Chemical Equations Packet)

___Na___Na22COCO33 + ___Mg → ___MgCO + ___Mg → ___MgCO33 + ___Na + ___Na

___ZnO → ___Zn + ___O___ZnO → ___Zn + ___O22

___Fe + ___O___Fe + ___O22 → ___Fe → ___Fe22OO33

Balancing PracticeBalancing Practice For more help go to: For more help go to:

http://richardbowles.tripod.com/chemistry/http://richardbowles.tripod.com/chemistry/balance.htm#part0balance.htm#part0

For some fun balancing equations go to:For some fun balancing equations go to: http://www.mpcfaculty.net/mark_bishop/http://www.mpcfaculty.net/mark_bishop/

balancing_equations_tutorial.htmbalancing_equations_tutorial.htm