Chemical Equations and Chemical Reactions cearley/ChemWrld/balance/balance.htm

Chemical Equations and Chemical Equations and Chemical ReactionsChemical Reactions

http://www.personal.kent.edu/~cearley/ChemWrld/balance/balance.htm

http://www.personal.kent.edu/~cearley/ChemWrld/balance/balance.htm

Writing A Chemical Equation

• The general format for chemical equations is:

• a + b c + d

• The substances on the left side of the equation (a and b in this case) are called the reactant(s).

• The + sign means “reacts with”.

• The means “produces or yields”.

• The substance(s) on the right side of the equation (c and d in this case) are called the product(s).

Lets work with the statement Lets work with the statement Hydrogen gas reacts with oxygen Hydrogen gas reacts with oxygen

gas to form liquid watergas to form liquid water..

• Word Equation:

• hydrogen + oxygen water

• Skeleton Equation:

• H2(g)+ O2(g) H2O(l)

• Balanced Equation:

• ___H2(g)+ ___O2(g) ___H2O(l)

• 2H2(g)+ O2(g) 2H2O(l)

Why must we balance equations?

• The Law of Conservation of Mass states that the total mass of reactants is always equal to the total mass of the products. (Lavoisier’s law)

Antoine-Laurent Lavoisier experimenting to determine the composition of water, French engraving, 19th century.

http://www.britannica.com/eb/art-15464/Antoine-Laurent-Lavoisier-experimenting-to-determine-the-composition-of-water




Why must we balance equations?

• Dalton’s atomic theory states that atoms can not be created or destroyed. Compounds are formed when atoms of different elements combine in fixed proportions.

John Dalton, F.R.S., engraved by Worthington after an 1814 painting by William Allen, published 25 June 1823 in Manchester and London. Note the charts with Dalton's atomic symbols lying on the table. Fisher Collection, Chemical Heritage Foundation.

To balance equations numbers called To balance equations numbers called coefficients coefficients are put before the are put before the

formulas. This changes the number of formulas. This changes the number of molecules. You cannot add or change molecules. You cannot add or change subscripts (small numbers). You do subscripts (small numbers). You do

not write coefficients of 1 (one).not write coefficients of 1 (one).

Sometimes you need to know the state Sometimes you need to know the state of the chemicals that are involved in a of the chemicals that are involved in a

chemical reaction.chemical reaction.

StateState AbbreviationAbbreviation

solidsolid

liquidliquid

gasgas

aqueous solutionaqueous solution


solidsolid (s)(s)

liquidliquid

gasgas



solidsolid (s)(s)

liquidliquid (l)(l)

gasgas



solidsolid (s)(s)

liquidliquid (l)(l)

gasgas (g)(g)



solidsolid (s)(s)

liquidliquid (l)(l)

gasgas (g)(g)

aqueous solutionaqueous solution (aq)(aq)

Final Balanced Chemical EquationFinal Balanced Chemical Equation

2H2H2(g) 2(g) + O+ O2(g)2(g) 2H 2H22OO((l)l)

Rules For Writing Balanced Rules For Writing Balanced Chemical EquationsChemical Equations

This requires a lot of practice and This requires a lot of practice and patience. These rules will not patience. These rules will not

apply in all equations.apply in all equations.

1. Write the skeleton equation.1. Write the skeleton equation.

2.2. Look for a Look for a polyatomic ion that polyatomic ion that appears once on each sideappears once on each side of the of the equation. equation. Balance these firstBalance these first..

3.3. PickPick single elements that appears single elements that appears on each sideon each side. . Choose coefficients Choose coefficients to balance this elementto balance this element. The terms . The terms you balanced in the previous step you balanced in the previous step have to remain balanced. Repeat have to remain balanced. Repeat for any other elements.for any other elements.

4. Check for any term you have not yet 4. Check for any term you have not yet looked at. Adjust the coefficient of looked at. Adjust the coefficient of the term so that the numbers of the the term so that the numbers of the elements appearing in that term are elements appearing in that term are balanced across the whole equation. balanced across the whole equation. If you have any fraction coefficients If you have any fraction coefficients clear them by multiplying. Also, clear them by multiplying. Also, make sure you have the smallest make sure you have the smallest possible coefficients by reducingpossible coefficients by reducing..

5. Check the equation!5. Check the equation!

Example 1• Word Equation:• sodium + water sodium hydroxide + hydrogen gas

• Skeletal Equation:

• Na + H2O NaOH + H2(g)


• ___Na + ___H2O ___NaOH +___H2(g)

• 2Na + 2H2O 2NaOH + H2(g)

Example 2• Balanced Chemical Equation• 2H2O__ 2Na__ + 2NaOH__ + H2___

• 2H2O(l) + 2 Na(s) 2NaOH(aq) + H2(g)

• Verification

Reactants Products

2 Na atoms 2 Na atoms

4 H atoms 4 H atoms

2 O atoms 2 O atoms

Example 2• Word Equation:• copper + silver nitrate copper(II) nitrate + silver


• ___ + ______ ______ + ___

• Cu + AgNO3 Cu(NO3)2 + Ag

Example 2…


• __Cu +__ AgNO3 __Cu(NO3)2 + __Ag

• Cu + 2AgNO3 Cu(NO3)2 + 2Ag

• Balanced Chemical Equation:

• Cu__ + 2AgNO3__ Cu(NO3)2__ + 2Ag__

• Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

ReactantsReactants ProductsProducts

Cu(s) + 2AgNO3(aq) Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s) Cu(NO3)2(aq) + 2Ag(s)

VerificationVerification


1 Cu atom1 Cu atom




1 Cu atom1 Cu atom 1 Cu atom1 Cu atom





2 Ag atoms2 Ag atoms





2 Ag atoms2 Ag atoms 2 Ag atoms2 Ag atoms






2 NO2 NO33-- ionsions






2 NO2 NO33-- ionsions 2 NO2 NO33

-- ionsions



Example 3• Word Equation:

• calcium nitrate + sodium hydroxide calcium hydroxide + sodium nitrate


• Ca(NO3)2 + NaOH Ca(OH)2 + NaNO3


• __Ca(NO3)2 + __NaOH __Ca(OH)2 + _NaNO3

• Ca(NO3)2 + 2NaOH Ca(OH)2 + 2NaNO3

Balanced Chemical Equation

• Ca(NO3)2__ + 2NaOH__ Ca(OH)2__ + NaNO3__

• Ca(NO3)2(aq) + 2NaOH(aq) Ca(OH)2(s) + NaNO3(aq)


Ca(NO3)2(aq) + 2NaOH(aq) Ca(NO3)2(aq) + 2NaOH(aq) Ca(OH)2(s) + 2NaNO3(aq) Ca(OH)2(s) + 2NaNO3(aq)


1 Ca atom1 Ca atom



1 Ca atom1 Ca atom 1 Ca atom1 Ca atom




2 NO2 NO33-- ionsions





-- ionsions





-- ionsions

2 Na atoms2 Na atoms





-- ionsions

2 Na atoms2 Na atoms 2 Na atoms2 Na atoms





-- ionsions


2 OH2 OH-- ions ions





-- ionsions


2 OH2 OH-- ions ions 2 OH2 OH-- ions ions


HomeworkHomework

p. 174 # 1 to 6p. 174 # 1 to 6

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Chemical Equations and Chemical Reactions cearley/ChemWrld/balance/balance.htm