Download Partial Pressures of Gases

SNC2D
1.7 Balancing Chemical Equations
Writing a balanced equation is almost always a useful skill for anyone working with
chemicals. Let’s use the method of balancing equations “by inspection,” which means
that we will simply count the number of atoms of each element before and after the
reaction and adjust coefficients to make sure the numbers are the same for each
element.
Let’s examine an equation for the reaction between copper metal and silver nitrate
solution. First, we observe metallic copper wire, Cu(s), being immersed in a colourless
solution of silver nitrate, AgNO3(aq). The solution gradually changes colour to blue, and
sparkling crystals form on the copper wire. Diagnostic tests confirm that the blue
solution is due to formation of copper(II) nitrate, Cu(NO3)2(aq), which goes into solution,
and the crystals are metallic silver, Ag(s). This reaction is used industrially to recover and
recycle silver by trickling waste solutions containing silver ions over scrap copper.
Step 1
Write a reaction equation, including states of matter:
? Cu(s) + ? AgNO3(aq) → ? Ag(s) + ? Cu(NO3)2(aq)
Step 2
First, balance atoms that are not in polyatomic ions, and are not O or H. (Oxygen and
hydrogen atoms may be constituents of many compounds in a reaction. Balancing O
and H early could be a waste of time, because they might need to be rebalanced again
at a later stage.) In this case, we can start with Cu and Ag, but as stated they are
already balanced (one atom of each on each side of the equation). Our equation
remains unchanged.
? Cu(s) + ? AgNO3(aq) → ? Ag(s) + ? Cu(NO3)2(aq)
Step 3
Next, balance any polyatomic ions. In this case, there are two (NO3)– ions in the
product, but only one in the reactant, so the quantity of the reactant compound needs to
be doubled:
? Cu(s)+ 2 AgNO3(aq) → ? Ag(s) + ? Cu(NO3)2(aq)
Doubling AgNO3 requires doubling of the Ag after reaction:
? Cu(s) + 2 AgNO3(aq) → 2 Ag(s) + ? Cu(NO3)2(aq)
Step 4
Next, balance O atoms and H atoms. We have none in this case. Then balance any
remaining elements, such as H2 or O2. Again, we have none, so we can state our final
equation:
Cu(s) + 2 AgNO3(aq) → 2 Ag(s) + Cu(NO3)2(aq)
SNC2D
Step 5
Final check:
Reactants: 1 Cu, 2 Ag, 2 (NO3)
Products: 1 Cu, 2 Ag, 2 (NO3)
The equation is balanced as stated.
This approach generally reduces the time spent in a random trial and error method.
There is one more tip on balancing some reaction equations that appear to require
fractional coefficients. For example, the following balanced equation requires 7 atoms of
oxygen contained in 7 molecules of O2.
2
2 NH3(g) + 7
2
O2(g) → 3 H2O + 2 NO2(g)
To avoid fractional coefficients, we double all coefficients, thereby obtaining a balanced
equation of whole number coefficients while retaining the same mole ratio for the
reaction.
4 NH3(g) + 7 O2(g) → 6 H2O + 4 NO2(g)
Steps for Balancing Chemical Equations by Inspection
1. Write the chemical formula for each reactant and product, including the state of
matter for each one.
2. Try balancing any atom that is not in a polyatomic ion and is not carbon, oxygen
or hydrogen.(ECHO)
3. If possible, balance polyatomic ions as a group.
4. Balance the remaining atoms and molecules, taking into account the oxygen and
hydrogen atoms and water; leave any elements in the equation, such as H 2 or O2
until the end.
5. Check the final reaction equation to ensure that there is the same number of
each type of atom before and after the reaction.
Sample Problem 1
Write a balanced equation for the reaction of aqueous sodium phosphate and aqueous
calcium chloride to produce sodium chloride in solution and a precipitate of calcium
phosphate.
? Na3PO4(aq) + ? CaCl2(aq) → ? Ca3(PO4)2(s) + ? NaCl(aq)
Solution : Using the steps outlined above you should be able to balance the equation to
get
2 Na3PO4(aq) + 3 CaCl2(aq) → Ca3(PO4)2(s) + 6 NaCl(aq)
SNC2D
Worksheet 1.7: Balancing Chemical Equations
Balance the following equations by inspection.
1. __Ni(s) + __HCl(aq)
2(aq)
+ __H2(g)
2. __Ca(OH)2(s) + __HCl(aq)
2(aq)
3. __Cl2(g) + __NaBr(aq)
4. __Cr2O3(s) |
(s)
2(l)
+ __NaCl(aq)
+ __O2(g)
5. __Fe(s) + __HCl(aq)
3(aq)
6. __C3H6(g) + __O2(g)
7. __P4(s) + __F2(g)
+ __HOH(l)
2(g)
+ __H2(g)
+ __H2O(g)
3(l)
8. __Ca(NO3)2(aq) + __KOH(aq)
9. __KHCO3(s)
2(s)
+ __KNO3(aq)
K2CO3(s) + __H2O(l) + __CO2(g)
10. __H3PO4(aq) + __NaOH(aq)
3PO4(aq)
11. __Ca(NO3)2(aq) + __Na3PO4(aq)
12. __Cu(s) + __HNO3(aq)
+ __HOH(l)
3(PO4)2(s)
3)2(aq)
+ __NaNO3(aq)
+ __NO2(g) + __H2O(l)
[this one is tricky!]