Lesson 28 Writing & Lesson 28 Writing & Balancing EquationsBalancing Equations

Objectives:Objectives:

--                  The student will write chemical The student will write chemical equations representing reactions.equations representing reactions.

--                  The student will balance chemical The student will balance chemical equations.equations.

  PA Science and Technology Standards: 3.4.10.A; 3.4.12.A; PA Science and Technology Standards: 3.4.10.A; 3.4.12.A; 3.2.10.B3.2.10.B

PA Mathematics Standards: 2.2.11.A, 2.4.11.E, PA Mathematics Standards: 2.2.11.A, 2.4.11.E, 2.5.11.A2.5.11.A

I.I. Atoms and Mass are conserved in chemicalAtoms and Mass are conserved in chemical reactionsreactions

a.a.          There are word equations and chemical There are word equations and chemical equations for a reactionequations for a reaction

                                         i.i.            Hydrogen + Oxygen Hydrogen + Oxygen Water Water

                                                                         ii.ii.            HH22 + O + O22 H H22OO

b.b.            The only problem with the chemical The only problem with the chemical equation we have written is that there is more equation we have written is that there is more oxygen on the left than on the right – this is not oxygen on the left than on the right – this is not possible considering the law of conservation of possible considering the law of conservation of mass.mass.

c.c.            Also, when we measure the masses of the Also, when we measure the masses of the reactants and the products, there is no reactants and the products, there is no difference.difference.

d. d. We need to find some way to show that We need to find some way to show that the reaction has occurred, but following this the reaction has occurred, but following this informationinformation

II.II.      Coefficients indicate amounts of Coefficients indicate amounts of reactants and reactants and productsproducts

a.a.            We can place numbers in front of We can place numbers in front of each each of of the reactants and products to indicate the the reactants and products to indicate the amount of that reactant or product that is amount of that reactant or product that is present in the present in the reaction.reaction.

b.b.            We call these numbers We call these numbers coefficientscoefficients

c.c.            We We cannotcannot change the subscripts change the subscripts associated with any of the elements involved associated with any of the elements involved in in the reaction.the reaction.

d.d.            Coefficients can only go in front of Coefficients can only go in front of compounds or single elements present – compounds or single elements present – not in the not in the middle of a chemical formula.middle of a chemical formula.

e.e.            ExampleExample

         i.i.            Unbalanced --- HUnbalanced --- H22 + O + O22 H H22OO

                    ii.ii.            Balanced --- 2HBalanced --- 2H22 + O + O22 2H 2H22OO

f. The process of inserting coefficients to f. The process of inserting coefficients to make sure that the law of conservation of make sure that the law of conservation of mass is followed is called “mass is followed is called “balancing an balancing an equationequation”.”.

g.g.            Steps for balancing an equation (start where Steps for balancing an equation (start where you need to in order to solve the problem you are you need to in order to solve the problem you are given – you don’t always need to start at the given – you don’t always need to start at the beginning)beginning)

i. Read the problemi. Read the problem1.1.            In a gas hot water heater, In a gas hot water heater, methane gas is combined with methane gas is combined with oxygen, and in the process, oxygen, and in the process,

produces carbon dioxide and produces carbon dioxide and water vapor.water vapor.

                        ii.ii.            Write out the words in equation Write out the words in equation form. form.1.1.            Methane + Oxygen Methane + Oxygen Carbon Dioxide + Water Carbon Dioxide + Water

                        iii.iii.            Write the correct formulas for the Write the correct formulas for the different elements and compounds different elements and compounds

present.present.

1. CH1. CH44 + O + O22 CO CO22 + H + H22O O

          iv.iv.    Count the number of elements on Count the number of elements on each each side and make a chart. Make sure side and make a chart. Make sure the the elements are in the same order on elements are in the same order on both sides both sides to keep from making the to keep from making the problem any problem any more confusing.more confusing.

1.1.            Left side:Left side: Right side:Right side:

C atoms – 1C atoms – 1 C atoms – 1C atoms – 1

H atoms – 4H atoms – 4 H atoms – 2H atoms – 2

O atoms – 2O atoms – 2 O atoms – 3O atoms – 3

CHCH44 + O + O22 CO CO22 + H + H22OO

v.v.          Insert coefficients for atoms of one element at Insert coefficients for atoms of one element at a a time so that the law of conservation of mass time so that the law of conservation of mass is is satisfiedsatisfied

1.1.            Tips for balancing equationsTips for balancing equationsa.a.              First, balance the types of atoms First, balance the types of atoms that appear in only one reactant and one that appear in only one reactant and one

productproduct

b.b.            Balance the remaining types of Balance the remaining types of atoms one at a timeatoms one at a time

c.c.              Balance H atoms and O after most of Balance H atoms and O after most of the other elements have been balancedthe other elements have been balanced

d.d.            If the same polyatomic ions appear If the same polyatomic ions appear on both sides of the equation, treat them on both sides of the equation, treat them

as if they were single units, like as if they were single units, like monatomic ions.monatomic ions.

e.e.              If you get stuck – sometimes it is a If you get stuck – sometimes it is a good idea to start over from scratch.good idea to start over from scratch.

vi. Repeat steps iii. and iv. until the law of vi. Repeat steps iii. and iv. until the law of conservation of mass holds for all of the conservation of mass holds for all of the elements in the equation.elements in the equation.

vii.vii.  Go back and check your math – Go back and check your math – simple simple mistakes can cause large mistakes can cause large problems.problems.

          viii.Make sure that there are equal numbers viii.Make sure that there are equal numbers of atoms of each element on both sides of atoms of each element on both sides of the of the equation.equation.

          ix.ix.    Finally, check to see that all of the Finally, check to see that all of the coefficients are in lowest terms.coefficients are in lowest terms.

i.i.            Common PitfallsCommon Pitfalls

Many students forget one or more of the final three steps. Many students forget one or more of the final three steps. These are the places where mistakes made during the These are the places where mistakes made during the earlier stages of the problem are caught and corrected. earlier stages of the problem are caught and corrected. Since it is relatively easy to make these mistakes, and just Since it is relatively easy to make these mistakes, and just as easy to complete these three steps, always remember to as easy to complete these three steps, always remember to apply them to every problem you complete!apply them to every problem you complete!

1.        Check your math!1.        Check your math!

2.        Check for equal #’s of elements!2.        Check for equal #’s of elements!

3.        Check for lowest terms!3.        Check for lowest terms!

h.h.              Second exampleSecond example

                                I.    I.    Write the word equation, showing Write the word equation, showing all of the reactants and productsall of the reactants and products

Phosphorus + potassium chlorate Phosphorus + potassium chlorate potassium potassium chloride + phosphorus (V) oxidechloride + phosphorus (V) oxide

ii..              More examplesMore examples

                                    i.i.            CaSiCaSi22 + SbCl + SbCl33 Si + Sb Si + Sb + CaCl+ CaCl22

ii.ii.            CC22HH22 + O + O22 CO CO22 + H + H22OO

                                                                                                                  iii.iii.            Al + CHAl + CH33OH OH (CH (CH33O)O)33Al + HAl + H22

iv.iv.            Cellular respiration is the process that your Cellular respiration is the process that your body uses to get energy from the food you eat. body uses to get energy from the food you eat. In cellular respiration, sugars such as glucose, In cellular respiration, sugars such as glucose, CC66HH1212OO66, react with oxygen. The net result is an , react with oxygen. The net result is an

increase of energy and the production of carbon increase of energy and the production of carbon dioxide and water. Write the balanced equation dioxide and water. Write the balanced equation for cellular respiration.for cellular respiration.

v.v.            When calcium is added to water, calcium When calcium is added to water, calcium hydroxide and hydrogen gas are formed. When hydroxide and hydrogen gas are formed. When calcium hydroxide is heated, water and calcium calcium hydroxide is heated, water and calcium oxide are the products. Write balanced chemical oxide are the products. Write balanced chemical equations that describe this series of changes.equations that describe this series of changes.

 

Level 1 

1.Balance each of the following equations.

a.Zn + H2SO4 ZnSO4 + H2 b.Na + Br2 NaBr

c.H2O + energy H2 + O2 d.Cl2 + KI KCl + I2

e.HNO3 + LiOH H2O + LiNO3

f.N2 + O2 N2O

g.NH4Cl + NaOH NH4OH + NaCl

h.HCl + K2CO3 KCl + H2O + CO2 2. Balance each of the following equations.•Sodium + iodine sodium iodide•Zinc + hydrobromic acid zinc bromide + hydrogen •Potassium hydroxide + heat potassium oxide + water•Magnesium + water magnesium hydroxide + hydrogen

  Level 2Level 2  1.1. Balance each of the following equations.Balance each of the following equations.

a.a. KK33POPO44 + HCl + HCl KCl + H KCl + H33POPO44

b.b. Na + HNONa + HNO33 NaNO NaNO33 + H + H22

c.c. S + OS + O22 SO SO33

d.d. Ca(ClOCa(ClO33))22 + heat + heat CaCl CaCl22 + O + O22

2.2. Balance each of the following equations.Balance each of the following equations.a.a. Potassium iodide + lead (II) nitrate Potassium iodide + lead (II) nitrate potassium nitrate + lead (II) potassium nitrate + lead (II)

iodideiodideb.b. Iron (III) oxide + carbon Iron (III) oxide + carbon carbon monoxide carbon monoxide + iron + ironc.c. Mercury (II) oxide + heat Mercury (II) oxide + heat mercury + oxygen mercury + oxygen d.d. Calcium + aluminum chloride Calcium + aluminum chloride calcium chloride + aluminum calcium chloride + aluminume.e. Mercury (I) nitrate + sodium carbonate Mercury (I) nitrate + sodium carbonate sodium nitrate + mercury sodium nitrate + mercury

(I) carbonate.(I) carbonate.f.f. Potassium bromide + aluminum nitrate Potassium bromide + aluminum nitrate potassium nitrate + potassium nitrate +

aluminum bromidealuminum bromideg.g. Calcium phosphate + aluminum sulfate Calcium phosphate + aluminum sulfate calcium sulfate + calcium sulfate +

aluminum phosphatealuminum phosphateh.h. Rubidium + acetic acid Rubidium + acetic acid rubidium acetate + hydrogen rubidium acetate + hydrogen

Level 3Level 3  Write and balance chemical equations for each of the following Write and balance chemical equations for each of the following

reactions.reactions.  1.1. Nitrogen + hydrogen Nitrogen + hydrogen ammonia ammonia 2.2. butane (Cbutane (C44HH1010) + oxygen + heat ) + oxygen + heat carbon dioxide carbon dioxide + water + water 3.3. aluminum oxide aluminum oxide aluminum + oxygen aluminum + oxygen 4.4. ethyl alcohol (Cethyl alcohol (C22HH55OH) + oxygen + heat OH) + oxygen + heat carbon monoxide carbon monoxide

+ water + water 5.5. nitrogen + oxygen nitrogen + oxygen dinitrogen pentoxide dinitrogen pentoxide 6.6. octane (Coctane (C88HH1818) + oxygen + heat ) + oxygen + heat carbon dioxide carbon dioxide + water + water 7.7. aluminum sulfate + phosphoric acid aluminum sulfate + phosphoric acid aluminum phosphate + aluminum phosphate +

sulfuric acidsulfuric acid8.8. diphosphorus pentoxide + water diphosphorus pentoxide + water phosphoric acid phosphoric acid9.9. ammonia + nitric oxide ammonia + nitric oxide nitrogen nitrogen + water + water10.10. iron (III) oxide + carbon monoxide iron (III) oxide + carbon monoxide iron + carbon dioxide iron + carbon dioxide 11.11. copper + nitric acid copper + nitric acid copper (II) nitrate + nitric oxide copper (II) nitrate + nitric oxide + +

waterwater12.12. iron (II) sulfide + oxygen iron (II) sulfide + oxygen iron (III) oxide + sulfur dioxide iron (III) oxide + sulfur dioxide