CHEMICAL EQUATIONS:MORE WRITING AND

BALANCING PRACTICE

Go through this PowerPoint for extra tips and extra practice!

CHEMICAL EQUATIONS Two basic rules apply in writing chemical equations:

We cannot write an equation for a reaction unless we know how the substances react and what new substances they form

Every chemical equation must be balanced (the number of atoms of each element on the left side must be the same as the number of atoms of that element on the right side of the equation)

Chemical equations are balanced according to the law of conservation of mass

nothing is created from nowhere or destroyed to nowhere.

Mass of matter present after the reaction is the same as before the reaction

A rearrangement of entities at the molecular level has occurred

COMMUNICATING CHEMICAL REACTIONS

Chemical Equations from Word Equations

Hydrogen gas burns in the presence of oxygen gas to produce water vapour

H2(g) + O2(g) H2O(g) (unbalanced)

2H2(g) + O2(g) 2H2O(g) (balanced)

Sodium metal reacts with chlorine gas to form sodium chloride

Na(s) + Cl2(g) NaCl(s)(unbalanced)

2Na(s) + Cl2(g) 2NaCl(s) (balanced)

A solution of silver nitrate reacts with a solution of potassium chloride to form a precipitate of silver chloride and a solution of potassium nitrate

AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)

COMMUNICATING CHEMICAL REACTIONS

Word Equations from Chemical Equations

NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

A sodium hydroxide solution and a hydrochloric acid solution will react to produce a solution of sodium chloride and liquid water

2Na(s) + 2H2O(l) H2(g) + 2NaOH(aq)

Solid sodium metal and water will react to produce hydrogen gas and a solution of sodium hydroxide

C(s) + O2(g) CO2(g)

Solid carbon and oxygen gas will react (burn) to produce carbon dioxide gas

CaCO3(s) CaO(s) + CO2(g)

Solid calcium carbonate (when heated) will decompose to form solid calcium oxide and carbon dioxide gas

PRACTICE

Rewrite the following word equations as formula equation and balance them:

a) Solid potassium metal reacts with oxygen gas to produce solid potassium oxide

b) Methane gas reacts with oxygen gas to produce carbon dioxide gas and water vapour

c) Mercury(II) sulfide is decomposed into liquid mercury and solid sulfur

d) Aqueous cobalt(III) nitrate reacts with solid zinc to produce aqueous zinc nitrate and solid cobalt.

PRACTICE Answers: (remember: when there is no coefficient in front of a compound, there

is an invisible “1” there)

a) Solid potassium metal reacts with oxygen gas to produce solid potassium oxide

4K(s) + O2(g) 2K2O(s)

b) Methane gas reacts with oxygen gas to produce carbon dioxide gas and water vapour

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

c) Mercury(II) sulfide is decomposed into liquid mercury and solid sulfur

8HgS(s) 8Hg(l) + S8(s)

d) Aqueous cobalt(III) nitrate reacts with solid zinc to produce aqueous zinc nitrate and solid cobalt

2Co(NO3)3(aq) + 3Zn(s) 3Zn(NO3)2(aq) + 2Co(s)

CHEMICAL EQUATIONS A balanced chemical equation has the total atoms/ions on reactant side equal to the total number of the same kind of atom/ions on the product side

Coefficients are used as necessary in front of the chemical formulas of the skeletal equation to produce a balanced equation

States of matter are provide after each chemical formula

 

 

Summary and Practice on Pages 49 – 50 of Text

BALANCING CHEMICAL REACTION EQUATIONS

General steps for balancing chemical equations

Write an equation using the correct chemical formulas

Check the formulas of the reactants and products

Determine if the equation is balanced

Count the number of atoms of each element on the left side and the right side. Count polyatomic ion as a group if its formula is unchanged

Balance the equation one element at a time

Pick an element and balance the equation for that element by placing coefficients in front of the formulas to equalize the number of that element on both sides of the equation

A typical starting point is with the most complex formula

Balance diatomic elements last

Check to see if the equation is balanced

Sometimes balancing one element will put another element out of balance. If that happens, repeat steps 3 and 4 until the equation is balanced. Make sure the coefficients you use are the smallest possible whole numbers.

Never try to balance an equation by changing the subscripts in a formula – that would make the formula incorrect!

EXAMPLES

Balance the following chemical equations

Note: if polyatomic ions appear on both sides, keep them as a whole thing

Also it might be easier to write water as HOH instead of H2O if

hydroxide ions () are present on the other side of the equation

Note: fractions are not used for balancing equations – get rid of them by multiplying all coefficients in the equation by the denominator of the fraction

EXAMPLES

Answers:

1. 3Note: if polyatomic ions appear on both sides, keep them as a whole thing

Also it might be easier to write water as HOH instead of H2O if

hydroxide ions () are present on the other side of the equation

Note: fractions are not used for balancing equations – get rid of them by multiplying all coefficients in the equation by the denominator of the fraction

DO NOT

change correct chemical formulas and/or subscripts to make atoms of an element balance

add some real or imaginary chemical species to either side of the equation just to make atoms of an element balance

DO

perform a check to make sure all atoms/ions balance most chemical equations are balanced by what is called the inspection

method

This is somewhat a trial and error method which succeeds in most instances

Practice is necessary to become successful

Useful strategies develop as practice continues.

PRACTICE

Balance the following chemical equations

1. __Na(s) + __O2(g) __Na2O(s)

2. __HI(g) __H2(g) + __I2(g)

3. __SnF4(aq) + __Cr(s) __CrF3(aq) + __Sn(s)

4. __(NH4)3PO4(aq) + __CaBr2(aq) __Ca3(PO4)2(s) + __NH4Br(aq)

5. __C3H8(g) + __O2(g) __CO2(g) + __H2O(g)

6. __Al(NO3)3(aq) + __NaOH(aq) __NaNO3(aq) + __Al(OH)3(s)

PRACTICE

Answers: (Remember: it is not necessary to write down the coefficient when it is “1,” I put

it here to make it easier to understand. It is more conventionally correct to NOT put it.)

1. 4Na(s) + 1O2(g) 2Na2O(s)

2. 2HI(g) 1H2(g) + 1I2(g)

3. 3SnF4(aq) + 4Cr(s) 4CrF3(aq) + 3Sn(s)

4. 2(NH4)3PO4(aq) + 3CaBr2(aq) 1Ca3(PO4)2(s) + 6NH4Br(aq)

5. 1C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

6. 1Al(NO3)3(aq) + 3NaOH(aq) 3NaNO3(aq) + 1Al(OH)3(s)