3.2 Predicting Reactions Study Guide

3.2 Predicting Reactions (Chem.3.2)

Explore this Phenomenon

Why is the silver dark? The pitcher shown above provides an example of tarnish, a chemical reaction caused when silver metal reacts with hydrogen sulfide gas produced by some industrial processes or as a result of decaying animal or plant materials. The tarnish can be removed using a number of polishes, but the process also removes a small amount of silver along with the tarnish.

This chemical reaction can be written like this:
2Ag + H₂S → Ag₂S + H₂

1. What is happening to the atoms in the silver cup and the hydrogen sulfide gas that is causing the tarnish?
2. What is different between the right and left side of the chemical reaction? What is the same?

 

Standard Chem.3.2
Analyze data to identify patterns that assist in making predictions of the outcomes of simple chemical reactions. Emphasize patterns based on the outermost electrons of atoms, trends in the periodic table, and knowledge of chemical properties. Examples could include reactions between main group elements, combustion reactions, or reactions between Arrhenius acids and bases. (PS1.B)

In this section, look for patterns that allow us to classify chemical reactions. How do you know that a chemical change has occurred? Pay attention to the evidence for chemical reactions. How can you use those patterns to predict the outcomes of chemical reactions?

 

Reactions
Many chemical reactions can be classified as one of five basic types. Having a thorough understanding of these types of reactions will be useful for predicting the products of an unknown reaction. The five basic types of chemical reactions are combination, decomposition, single-replacement, double-replacement and combustion. Analyzing the reactants and products of a given reaction will allow you to place it into one of these categories. Some reactions will fit into more than one category.

Combination Reactions
A combination reaction is a reaction in which two or more substances combine to form a single new substance. Combination reactions can also be called synthesis reactions.

The general form of a combination reaction is:

A + B → AB

One combination reaction is two elements combining to form a compound. Solid sodium metal reacts with chlorine gas to produce solid sodium chloride.

2Na( s )+ Cl₂(g)→2NaCl(s)

Notice that in order to write and balance the equation correctly, it is important to remember the seven elements that exist in nature as diatomic molecules (H₂ , N₂ , O₂ , F₂ , Cl₂ , Br₂ and I₂).

One sort of combination reaction that occurs frequently is the reaction of an element with oxygen to form an oxide. Metals and nonmetals both react readily with oxygen under most conditions. Magnesium reacts rapidly and dramatically when ignited, combining with oxygen from the air to produce a fine powder of magnesium oxide.

2Mg(s)+ O₂(g)→2MgO(s)

Sulfur reacts with oxygen to form sulfur dioxide.

S(s)+ O₂(g)→ SO₂(g)

When nonmetals react with one another, the product is a molecular compound. Often, the nonmetal reactants can combine in different ratios and produce different products. Sulfur can also combine with oxygen to produce sulfur trioxide.

2S(s)+3O₂(g)→2SO₃(g)

 

Sample Problem: Combination Reactions
Potassium is a very reactive alkali metal that must be stored under oil in order to
prevent it from reacting with air. Write the balanced chemical equation for the
combination reaction of potassium with oxygen.

• Step 1: Plan the problem.

Make sure formulas of all reactants and products are correct before balancing the equation. Oxygen gas is a diatomic molecule. Potassium oxide is an ionic compound and so its formula is constructed by the crisscross method. Potassium as an ion becomes K+ , while the oxide ion is O₂− .

• Step 2: Solve.

The skeleton (unbalanced) equation:
K(s)+ O₂(g)→ K₂O(s)

The equation is then easily balanced with coefficients.
4K(s)+ O₂(g)→2 K₂O(s)

• Step 3: Think about your result .
  The formulas are correct and the resulting combination reaction is balanced.

 

Practice Problems

Write a balanced equation for the combination reactions.

a) Al and O₂
b) Ca and Br₂

Decomposition Reactions
A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general form of a decomposition reaction is:

AB → A + B

Most decomposition reactions require an input of energy in the form of heat, light, or electricity. Binary compounds are compounds composed of just two elements. The simplest kind of decomposition reaction is when a binary compound decomposes into its elements. Mercury(II) oxide, a red solid, decomposes when heated to produce mercury and oxygen gas.

2HgO(s)→2Hg(l)+ O₂(g)

Sample Problem: Decomposition Reactions
When an electric current is passed through pure water, it decomposes into its elements. Write a balanced equation for the decomposition of water.

• Step 1: Plan the problem .

Water is a binary compound composed of hydrogen and oxygen. The hydrogen and oxygen gases produced in the reaction are both diatomic molecules.

• Step 2: Solve .

The skeleton (unbalanced) equation:
H₂O(l)→ H₂(g)+ O₂(g) via electrolysis

Balance the equation.
2H₂O(l)→2H₂(g)+ O₂(g) via electrolysis

• Step 3: Think about your result .

The products are elements and the equation is balanced.

Single-Replacement Reactions
A single-replacement reaction is a reaction in which one element replaces a similar element in a compound. The general form of a single-replacement (also called single-displacement) reaction is:

A + BC → AC + B

In this general reaction, element A is a metal and replaces element B , also a metal, in the compound. When the element that is doing the replacing is a nonmetal, it must replace another nonmetal in a compound, and the general equation becomes:

Y + XZ → XY + Z

Y is a nonmetal and replaces the nonmetal Z in the compound with X .

 

Metal Replacement
Magnesium is a more reactive metal than copper. When a strip of magnesium metal is placed in an aqueous solution of copper(II) nitrate, it replaces the copper. The products of the reaction are aqueous magnesium nitrate and solid copper metal.

Mg(s)+ Cu (NO₃)₂(aq)→ Mg(NO₃)₂(aq)+ Cu(s)

This subcategory of single-replacement reactions is called a metal replacement reaction because it is a metal that is being replaced (copper).

Hydrogen Replacement
Many metals react easily with acids and, when they do so, one of the products of the reaction is hydrogen gas. Zinc reacts with hydrochloric acid to produce aqueous zinc chloride and hydrogen (figure below).

Zn(s)+2HCl(aq)→ ZnCl₂(aq)+ H₂(g)

In a hydrogen replacement reaction, the hydrogen in the acid is replaced by an active metal.

Zinc metal reacts with hydrochloric acid to give off hydrogen gas in a single-replacement reaction.

Some metals are so reactive that they are capable of replacing the hydrogen in water. The products of such a reaction are the metal hydroxide and hydrogen gas. All group 1 metals undergo this type of reaction. Sodium reacts vigorously with water to produce aqueous sodium hydroxide and hydrogen (figure below).

2Na(s)+2H₂O(l)→2NaOH(aq)+ H₂(g)

Pictured here is about 3 pounds of sodium metal reacting with water. Sodium metal reacts vigorously when dropped into a container of water, giving off hydrogen gas. A large piece of sodium will often generate so much heat that the hydrogen will ignite.

 

Halogen Replacement
The element chlorine reacts with an aqueous solution of sodium bromide to produce aqueous sodium chloride and elemental bromine.

Cl₂(g)+2NaBr(aq)→2NaCl(aq)+ Br₂(l)

The reactivity of the halogen group (group 17) decreases from top to bottom within the group. Fluorine is the most reactive halogen, while iodine is the least. Since chlorine is above bromine, it is more reactive than bromine and can replace it in a halogen replacement reaction.

 

The Activity Series
Single-replacement reactions only occur when the element that is doing the replacing is more reactive than the element that is being replaced. Therefore, it is useful to have a list of elements in order of their relative reactivities. The activity series is a list of elements in decreasing order of their reactivity. Since metals replace other metals, while nonmetals replace other nonmetals, they each have a separate activity series. Listed below (table below) is an activity series of most common metals, and of the halogens.

Activity of Metals		Activity of Halogens
Li K Ba Sr Ca Na	React with cold water, replacing hydrogen.	F2 Cl2 Br2 I2
Mg Al Zn Cr Fe Cd	React with steam, but not cold water, replacing hydrogen.
Co Ni Sn Pb	Do not react with water, React with acids, replacing hydrogen,
H2
Cu Hg Ag Pt Au	Unreactive with water or acids.

For a single-replacement reaction, a given element is capable of replacing an element that is below it in the activity series. This can be used to predict if a reaction will occur. Suppose that small pieces of the metal nickel were placed into two separate aqueous solutions: one of iron(III) nitrate and one of lead(II) nitrate. Looking at the activity series, we see that nickel is below iron, but above lead. Therefore, the nickel metal will be capable of replacing the lead in a reaction, but will not be capable of replacing iron.

Ni(s)+ Pb(NO₃)₂(aq)→ Ni(NO₃)₂(aq)+ Pb(s)
Ni(s)+ Fe(NO₃)₃(aq)→ NR(no reaction)

In the descriptions that accompany the activity series of metals, a given metal is also capable of undergoing the reactions described below that section. For example, lithium will react with cold water, replacing hydrogen. It will also react with steam and with acids, since that requires a lower degree of reactivity.

Sample Problem: Single-Replacement Reactions
Use the activity series to predict if the following reactions will occur. If not, write NR. If the reaction does occur, write the products of the reaction and balance the equation.

1. Al ( s )+ Ba 3 N 2 ( aq )→
2. Sr ( s )+ HCl ( aq )→

• Step 1: Plan the problem .

For 1, compare the placements of aluminum and barium on the activity series. For 2, compare the placements of strontium and hydrogen.

• Step 2: Solve .

Since aluminum is below barium, it is not capable of replacing it and a reaction will not occur.
Al ( s )+ Ba 3 N 2 ( aq )→ NR

Since strontium is above hydrogen, it is capable of replacing hydrogen in a reaction with an acid. The strontium is now going to be bonded to chlorine. Take care to write the correct formulas for the products before balancing the equation. The balanced equation is:

Sr ( s )+2 HCl ( aq )→ SrCl 2 (aq) + H 2 (g)

• Step 3: Think about your result .

 

Double-Replacement Reactions
A double-replacement reaction is a reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds. The general form of a double-replacement (also called double-displacement) reaction is:

AB + CD → AD + CB

In this reaction, A and C are positively-charged cations, while B and D are negatively-charged anions. Double-replacement reactions generally occur between substances in aqueous solution. In order for a reaction to occur, one of the products is usually a solid precipitate, a gas, or a molecular compound such as water.

Examples:
2KI(aq)+ Pb(NO₃)₂(aq)→2KNO₃(aq)+ PbI₂(s)
Na₂S(aq)+2HCl(aq)→2NaCl(aq)+ H₂S(g)
HCl(aq)+ NaOH(aq)→ NaCl(aq)+ H₂O(l)

 

Sample Problem: Double-Replacement Reactions
Write a complete and balanced chemical equation for the following double-replacement reactions. One product is indicated as a guide.
1. NaCN(aq)+ HBr(aq)→(hydrogen cyanide gas is formed)
2. (NH₄)2SO₄(aq)+ Ba(NO₃)₂(aq)→(a precipitate of barium sulfate forms)

• Step 1: Plan the problem .

In 1, the production of a gas drives the reaction. In 2, the production of a precipitate drives the reaction. In both cases, use the ionic charges of both reactants to construct the correct formulas of the products.

• Step 2: Solve.

1. The cations of both reactants are 1+ charged ions, while the anions are 1− charged ions. After exchanging partners, the balanced equation is:
NaCN(aq)+ HBr(aq)→ NaBr(aq)+ HCN(g)

2. Ammonium ion and nitrate ion are 1+ and 1− respectively, while barium and sulfate are 2+ and 2−. This must be taken into account when exchanging partners and writing the new formulas. Then, the equation is balanced.
(NH₄)2SO₄(aq)+ Ba(NO₃)₂(aq)→2NH₄NO₃(aq)+ BaSO₄(s)

• Step 3: Think about your result .

Combustion Reactions
A combustion reaction is a reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat. Combustion reactions must involve O₂ as one reactant. Many combustion reactions occur with a hydrocarbon, a compound made up solely of carbon and hydrogen. The products of the combustion of hydrocarbons are carbon dioxide and water. Many hydrocarbons are used as fuel because their combustion releases very large amounts of heat energy. Propane (C 3 H 8 ) is a gaseous hydrocarbon that is commonly used as a fuel source in gas grills.

C₃H₈(g)+5O₂(g)→3CO₂(g)+4H₂O(g)

 

Practice Problem: Combustion Reactions
Ethanol can be used as a fuel source in an alcohol lamp. The formula for ethanol is C₂H₅OH. Write the balanced equation for the combustion of ethanol.

• Step 1: Plan the problem .

Ethanol and oxygen are the reactants. As with a hydrocarbon, the products of the combustion of an alcohol are carbon dioxide and water.

• Step 2: Solve .

Write the skeleton equation:
C₂H₅OH(l)+ O₂(g)→ CO₂(g)+ H₂O(g)
Balance the equation.
C₂H₅OH(l)+3O₂(g)→2CO₂(g)+3H₂O(g)

• Step 3: Think about your result .

Combustion reactions must have oxygen as a reactant. Note that the water that is produced is in a gas state rather than the liquid state because of the high temperatures that accompany a combustion reaction.

 

Putting It Together

Let us revisit this phenomenon:
Why is the silver dark? The pitcher shown above provides an example of tarnish, a chemical reaction caused when silver metal reacts with hydrogen sulfide gas produced by some industrial processes or as a result of decaying animal or plant materials. The tarnish can be removed using a number of polishes, but the process also removes a small amount of silver along with the tarnish.
This chemical reaction can be written like this:

2Ag + H₂S → Ag₂S + H₂

1. Which product is the tarnish?
2. What type of reaction is represented above?
3. Would you expect the same type of reaction for something made from gold (Au)? Why or why not?