Lesson 12 Balancing Chemical Reactions
Predicting Products
If you know what the reactants are for a chemical reaction, can you predict the products?
B12.11 A student writing a chemical reaction equation on a chalk board
For all reactions so far, both the reactants and products have been identified for you. The next skill that you are going to develop is the ability to predict the products when you are given the identity of the reactants. You will look at
each of the five types of chemical reactions. Using the patterns for each reaction will aide you in predicting the products.
Each example has a video to go with it. To play the video, click on the play icon next to the example.
In simple formation reactions, the product formed will be the compound that results from the combination of the two elements reacting. You will need to recall from Section 2 how to balance charges to create neutral compounds.
For example, predict the product and write a balanced chemical reaction equation when copper on the roof of the parliament building oxidizes (reacts with oxygen) to create a green patina.
For example, predict the product and write a balanced chemical reaction equation when copper on the roof of the parliament building oxidizes (reacts with oxygen) to create a green patina.
Write a word equation. Keep in mind that copper is a multivalent metal; so the most common charge, which is written first on the periodic table, is the one that will be used.
copper + oxygen → copper(II) oxide
copper + oxygen → copper(II) oxide
Write the skeleton equation, remembering to write correct formulas for all compounds.
Cu(s) + O2(g) → CuO(s)
Cu(s) + O2(g) → CuO(s)
Balance the equation.
2Cu(s) + O2(g) → 2CuO(s)
2Cu(s) + O2(g) → 2CuO(s)
In simple decomposition reactions, a compound is broken down into its constituent elements. (Tip: You need to recall the seven diatomic elements.)
For example, predict the products and write a balanced chemical reaction equation when molten aluminum chloride is electrolyzed (decomposed).
For example, predict the products and write a balanced chemical reaction equation when molten aluminum chloride is electrolyzed (decomposed).
Word equation:
aluminium chloride → aluminium + chlorine
aluminium chloride → aluminium + chlorine
Skeleton equation:
AlCl3(l) → Al(s) + Cl2(g)
AlCl3(l) → Al(s) + Cl2(g)
Balanced equation:
2AlCl3(l) → 2Al(s) + 3Cl2(g)
Tip: Remember that states of elements are listed on the periodic table.
2AlCl3(l) → 2Al(s) + 3Cl2(g)
Tip: Remember that states of elements are listed on the periodic table.
The key to predicting products for a single replacement reaction is to carefully identify cations and anions and remember that ionic compounds need to be composed of a cation bonded to an anion. By first identifying entities as being able to form
a cation or an anion, you can then determine which entitles switch places.
Predict the products and write a balanced chemical reaction equation when lithium iodide solution is reacted with liquid bromine.
Write the word equation and identify the entities as being able to form a cation or an anion:
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Determine which entities switch places:
Iodine and bromine are both anions, so they will switch places.
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Iodine and bromine are both anions, so they will switch places.
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Skeleton equation:
LiI(aq) + Br2(l) → LiBr(aq) + I2(s)
LiI(aq) + Br2(l) → LiBr(aq) + I2(s)
Balanced equation:
2LiI(aq) + Br2(l) → 2LiBr(aq) + I2(s)
2LiI(aq) + Br2(l) → 2LiBr(aq) + I2(s)
Predict the products and write a balanced chemical reaction equation when magnesium metal is reacted with hydrofluoric acid.
Write the word equation and identify the entities as being able to form a cation or an anion:
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Determine which entities switch places:
Magnesium and hydrogen are both cations, so they will switch places.
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Magnesium and hydrogen are both cations, so they will switch places.
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Skeleton
equation:
Mg(s) + HF(aq) → MgF2(aq) + H2(g)
Mg(s) + HF(aq) → MgF2(aq) + H2(g)
Balanced equation:
Mg(s) + 2HF(aq) → MgF2(aq) + H2(g)
Mg(s) + 2HF(aq) → MgF2(aq) + H2(g)
The key concept to keep in mind for double replacement reactions is that the cations will switch places. Remember: In ionic compounds, a cation must be bonded to an anion. You will also need to check the solubility table to determine the states
of your products. Quite often, a precipitate (solid) will be produced during a double replacement reaction.
Predict the products and write a balanced chemical reaction equation when solutions of sodium chloride and lead(II) chlorate are mixed.
Word equation:
Skeleton equation:
NaCl(aq) + Pb(ClO3)2(aq) → PbCl2(s) + NaClO3(aq)
NaCl(aq) + Pb(ClO3)2(aq) → PbCl2(s) + NaClO3(aq)
Balanced equation:
2NaCl(aq) + Pb(ClO3)2(aq) → PbCl2(s) + 2NaClO3(aq)
2NaCl(aq) + Pb(ClO3)2(aq) → PbCl2(s) + 2NaClO3(aq)
Predict the products and write a balanced chemical reaction equation when solutions of iron(III) nitrate and potassium carbonate are mixed.
Word equation:
Skeleton equation:
Fe(NO3)3(aq) + K2CO3(aq) → KNO3(aq) + Fe2(CO3)3(s)
Fe(NO3)3(aq) + K2CO3(aq) → KNO3(aq) + Fe2(CO3)3(s)
Balanced equation:
2Fe(NO3)3(aq) + 3K 2CO3(aq) → 6KNO3(aq) + Fe2(CO3)3(s)
2Fe(NO3)3(aq) + 3K 2CO3(aq) → 6KNO3(aq) + Fe2(CO3)3(s)
Predicting products for hydrocarbon combustion is quite straightforward. When it is a complete combustion reaction, the hydrocarbon reacts with oxygen to produce carbon dioxide and water vapour.
For example, write the balanced chemical reaction equation for the combustion of butane, C4H10(g).
Notice that butane is a hydrocarbon and that this is a combustion reaction. Therefore, the other reactant will be oxygen gas, and the products will be carbon dioxide and water vapour.
For example, write the balanced chemical reaction equation for the combustion of butane, C4H10(g).
Notice that butane is a hydrocarbon and that this is a combustion reaction. Therefore, the other reactant will be oxygen gas, and the products will be carbon dioxide and water vapour.
Word equation:
butane + oxygen → carbon dioxide + water vapour
butane + oxygen → carbon dioxide + water vapour
Skeleton equation:
C4H10(g) + O2(g) → CO2(g) + H2O(g)
C4H10(g) + O2(g) → CO2(g) + H2O(g)
Balanced equation:
2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)
2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)
Read This
Please read pages 102 to 105 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on determining products and writing
balanced chemical reaction equations. Remember, if you have any questions or you do not understand something, ask your teacher! Practice Questions
Complete the following practice question to check your understanding of the concept you just learned. Make sure you write a complete answer to the practice
question in your notes. After you have checked your answer, make corrections to your response (where necessary) to study from.Balance the following skeleton reaction equations.
- Write a word equation and a balanced chemical reaction equation for the formation of zinc nitride from its elements.
zinc + nitrogen → zinc nitride
3Zn(s) + N2(g) → Zn3N2(s) - Write a word equation and a balanced chemical reaction equation for the complete combustion of pentane, C5H12(l).
pentane + oxygen → carbon dioxide + water vapour
C5H12(l) + 8O2(g) → 5CO2(g) + 6H2O(g) - Write a word equation and a balanced chemical reaction equation for the reaction of aqueous magnesium sulfate with aqueous strontium bromide.
MgSO4(aq) + SrBr2(aq) → SrSO4(s) + MgBr2(aq) - Write a word equation and a balanced chemical reaction equation for the reaction of zinc metal with a solution of tin(IV) chloride.
Note: When tin is written by itself as an element, no designation of charge is included (i.e., it is not written as tin(IV)).
2Zn(s) + SnCl4(aq) → Sn(s) + 2ZnCl2(aq) - Write a word equation and a balanced chemical reaction equation for the decomposition of iron(III) oxide into its elements.
iron(III) oxide → iron + oxygen
2Fe2O3(s) → 4Fe(s) + 3O2(g)