4.4: Patterns of Chemical Reactions

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Learning Objectives

Define synthesis and decomposition reactions
Write the chemical equation for a synthesis or decomposition reaction, given the reactant(s)
Define replacement reaction
Write the chemical equation for a replacement reaction, given the reactants
Define combustion reaction
Write the chemical equation for a combustion reaction, given the reactant
Identify the reaction type based on the reactants

There is an underlying explanation of why a reaction takes place from an underlying chemical theory based on chemical reactivity. In this sections, we discussed various reactions to identify key chemical trends that allows chemists to predict the outcome of chemical reactions. By recognizing general patterns of chemical reactivity, you will be able to successfully predict the products formed by a given combination of reactants We can often predict a reaction if we have seen a similar reaction before. For example, sodium (\(\ce{Na}\)) reacts with water (\(\ce{H_2O}\)) to form sodium hydroxide (\(\ce{NaOH}\)) and \(\ce{H_2}\) gas:

\[\ce{ 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)} \nonumber \]

with \(\ce{(aq)}\) indicating aqueous liquid. As discussed later, potassium (\(\ce{K}\)) is in the same family (column) of elements as sodium and exhibits similar chemistry. Therefore, one might predict that the reaction of \(\ce{K}\) with \(\ce{H2O}\) would be similar to that of \(\ce{Na}\):

\[\ce{2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)} \nonumber \]

In fact, all alkali metals react with water to form their hydroxide compounds and hydrogen.

Synthesis and Decomposition Reactions

In a synthesis reaction, two or more molecules react to form a single, more complex compound. Many elements react with one another in this fashion to form compounds. The general chemical equation for a synthesis reaction is:

\[\ce{ A + B -> C} \nonumber \]

and an example includes the generation of ammonia (\(\ce{NH_3}\)) from nitrogen (\(\ce{N_2}\)), and hydrogen (\(\ce{H_2}\)):

\[ \ce{ N2(g) + 3H2(g) -> 2NH3(g)} \nonumber \]

In a decomposition reaction one substance undergoes a reaction to form two or more simpler products. Such reactions often occur when compounds are heated or electricity is added. The general chemical equation for a decomposition reaction is:

\[\ce{A -> B + C } \nonumber \]

For example, the thermal decomposition of limestone (\(\ce{CaCO3(s)}\)) generates quicklime (\(\ce{CaO(s)}\)) and carbon dioxide \(\ce{CO2(g)}\)

\[\ce{CaCO3(s) -> CaO(s) + CO2(g)} \nonumber \]

Example \(\PageIndex{1}\)

Write the balanced chemical equation for a synthesis reaction between nitrogen monoxide and oxygen to form nitrogen dioxide.

Solution

Step 1 Determine the chemical formulas for each reactant and product.

nitrogen monoxide: NO

oxygen: O₂

nitrogen dioxide: NO₂

Step 2 Write the reactant on the left side of the reaction arrow and the product on the right side.

\[\text{NO}+\text{O}_2 ⟶ \text{NO}_2\]

Step 3 Balance the chemical equation

\[\text{2NO}+\text{O}_2\ ⟶\ \text{2NO}_2\]

Exercise \(\PageIndex{1}\)

Write the balanced chemical equation for the synthesis of solid aluminum(III) fluoride from aluminum metal and fluorine gas.

Answer: \( \ce{ 2Al(s) + 3F2(g) -> 2AlF3(s)} \nonumber \)

Single-Replacement and Double-Replacement Reactions

A replacement reaction (also called a displacement reaction) has one part of a compound being replaced (or displaced). Displacement reactions usually occur between an ionic compound and a metal, two ionic compounds, an acid and a metal, or an acid and an ionic compound.

Single-Replacement

In a single-replacement reaction, an element replaces one part of a compound. The general chemical reaction for a single-replacement reaction is:

\[\ce{ A + BC -> AB + C} \nonumber \]

and an example includes the generation of hydrogen gas from hydrochloric acid ((\(\ce{HCl}\))) and zinc metal (\(\ce{Zn}\)):

\[ \ce{ 2HCl(aq) + Zn(s) -> H2(g) + ZnCl2} \nonumber \]

The hydrogen atoms in hydrochloric acid are replaced the zinc atoms. The reaction illustrates an important distinction: the replacement product may not have the same ratio between atoms as the reactant. You will need to use your skills with balancing charge and oxidation numbers if you are not told the product. The replacing reactant will be similar to the portion of the reactant being replaced. Hydrogen is more positive and zinc, as a transition metal, would also favor being positive.

Example \(\PageIndex{2}\)

Sulfuric acid reacts with copper. The copper in the copper-containing product has a 2+ charge.

Solution

Step 1 Determine the chemical formulas for the reactants.

Sulfuric acid: H₂SO₄

Copper: Cu

Step 2 Determine the products. The copper, as the lone reactant, must be replacing a portion of the sulfuric acid. Copper is going to end up positive, so it must be replacing the hydrogen. The product then contains Cu²⁺ and sulfate, SO₄²^-. Balancing the charges produces

Copper(II) sulfate: CuSO₄

The replaced hydrogen has to form a stable molecule and there is only one option when it is the only atom available, H₂.

Step 3 Write the reactant on the left side of the reaction arrow and the product on the right side.

\[\ce{ H2SO4 + Cu -> H2 + CuSO4} \nonumber \]

Step 4 Balance the chemical equation. In this case, it is already balanced.

\[\ce{ H2SO4 + Cu -> H2 + CuSO4} \nonumber \]

Exercise \(\PageIndex{2}\)

Write the balanced chemical equation for the reaction of zinc(II) chloride and lithium.

Answer: \[\ce{ ZnCl2 + 2Li -> 2LiCl + Zn} \nonumber \]

Double-Replacement

In a double-replacement reaction, one part of a compound replaces part of another compound. The general chemical reaction for a single-replacement reaction is:

\[\ce{ AB + CD -> AD + CB} \nonumber \]

and an example includes the reaction between potassium hydroxide ((\(\ce{KOH}\))) and copper sulfate (\(\ce{CuSO4}\)):

\[ \ce{ 2KOH(aq) + CuSO4(s) -> K2SO4(g) + Cu(OH)2} \nonumber \]

Double-displacement reactions are often described like atoms swapping dance partners.

Example \(\PageIndex{3}\)

Write the chemical equation for the reaction of iron(III) chloride and sodium hydroxide.

Solution

Step 1 Determine the chemical formulas for the reactants.

Iron(III) chloride: FeCl₃

Sodium hydroxide: NaOH

Step 2 Determine the products. The more positive element in iron(III) chloride will replace the more positive element in sodium hydroxide. The resulting combination are Fe³⁺ and OH^-, and Na⁺ and Cl^-. Balancing the charges produces

Iron(III) hydroxide: Fe(OH)₃

Sodium chloride: NaCl

Step 3 Write the reactant on the left side of the reaction arrow and the product on the right side.

\[\ce{ FeCl3 + NaOH -> Fe(OH)3 + NaCl} \nonumber \]

Step 4 Balance the chemical equation. In this case, it is already balanced.

\[\ce{ FeCl3 + 3NaOH -> Fe(OH)3 + 3NaCl} \nonumber \]

Exercise \(\PageIndex{3}\)

Write the chemical equation for the reaction of calcium chloride and sodium phosphate.

Answer: \[\ce{ 3CaCl2 + 2Na3PO4 -> Ca3(PO4)2 + 6NaCl} \nonumber \]

Combustion in Air

Combustion reactions are rapid reactions that produce a flame. Fire is such a dominant factor human history that it has its own reaction pattern, though many chemistry texts have begun to exclude it due to its less general nature. Common combustion reactions involve oxygen (O₂) from the air as a reactant. The other reactant is a hydrocarbon (CxH_y) or a carbohydrate (CxH_yO_z). The products of a combustion reaction are always carbon dioxide and water. For example, methane (CH₄) will combust in the presence of oxygen in that air (with an ignition event) to generate a fire.

The general chemical reaction for a combustion reaction is:

\[\ce{ hydrocarbon or carbohydrate + O2 -> CO2 + H2O} \nonumber \]

and an example includes the combustion of methane:

\[\ce{ CH4 + 2O2 -> CO2 + 2H2O} \nonumber \]

Soot

Most sources of hydrocarbons and carbohydrates in nature, like wood, do not provide ideal stoichiometric ratios of carbon and hydrogen. Excess form microscopic particles of soot. The soot emits blackbody radiation based on the size of the particles. Smaller particles rise on thermal currents and reach cooler air as they move farther from the source of the reaction. This is why there is a gradient from blue (the light released by the reaction) to yellow, orange, and red (the light from soot blackbody radiation).

Figure \(\PageIndex{1}\): The flames caused as a result of a fuel undergoing combustion (burning) (CC BY-SA 2.5; Et bål via Wikipedia)

The most common hydrocarbons fuel sources (reactants) used for combustion reactions are shown in Table \(\PageIndex{1}\).

Table \(\PageIndex{1}\): Examples of common hydrocarbons
Hydrocarbon	Molecular formula
methane	CH₄
propane	C₃H₈
butane	C₄H₁₀
octane	C₈H₁₈

Propane is the gas used in gas-grills for cooking:

\[ \ce{ C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(l)} \label{3.2.6} \]

Example \(\PageIndex{4}\)

Write the chemical reaction for the combustion of propane.

Solution

Step 1 Determine the chemical formulas for the reactants and products. Propane is the only reactant given, but because it is a combustion reaction the other reactant must by oxygen. The products are carbon dioxide and water, because it is a combustion reaction.

Propane: \(\ce{C3H8}\)

Oxygen: O₂

Carbon dioxide: \(\ce{CO2}\)

Water: \(\ce{H2O}\)

Step 2 Write the reactants on the left side of the reaction arrow and the products on the right side.

\[\ce{ C3H8 + O2 -> CO2 + H2O} \nonumber \]

Step 4 Balance the chemical equation.

\[\ce{ C3H8 + 5O2 -> 3CO2 + 4H2O} \nonumber \]

Exercise \(\PageIndex{4}\)

Write the chemical reaction for the combustion of butane.

Answer: \[\ce{ 2C4H10 + 13O2 -> 8CO2 + 10H2O} \nonumber \]

Summary

Chemical reactions often follow established patterns. The most common patterns are synthesis, decomposition, replacement, and combustion. Synthesis reactions create a single product from multiple reactants. Decomposition reactions create multiple products from a single reactant. Replacement reactions change one part of a compound with a new type of atom or polyatomic ion. Combustion reactions occur between a hydrocarbon or carbohydrate, and oxygen to produce carbon dioxide and water.

Glossary

carbohydrate: a molecule consisting of carbon, hydrogen, and oxygen atoms
combustion reaction: a reactant reacting to form more than one product, \(\ce{A -> B + C } \nonumber \)
decomposition reaction: a single reactant reacting to form more than one product, \(\ce{A -> B + C } \nonumber \)

double-replacement reaction: reaction involving the exchange of one part of a compound with a part of another compound, \(\ce{AB + CD -> AD + CB } \nonumber \)

hydrocarbon: a molecule consisting of carbon and hydrogen atoms
single-replacement reaction: reaction involving the exchange of one part of a compound with a similar element, ion, or polyatomic ion , \(\ce{AB + C -> AC + B } \nonumber \)
synthesis reaction: two or more reactants reacting to form one product, \(\ce{A + B -> AB } \nonumber \)

Exercise \(\PageIndex{5}\)

Identify the type of reaction which will occur given the reactant(s).
1. Hydrogen peroxide, H₂O₂
2. Dicarbon dihydride and oxygen
3. Calcium hydroxide and nitric acid
4. Nickel and carbon monoxide
5. Barium and magnesium chloride
Write the balanced chemical equation for each of the given descriptions.
1. Calcium sulfate decomposes into calcium oxide and sulfur trioxide.
2. The synthesis of solid sodium peroxide from sodium metal and oxygen gas.
3. Ethane, C₂H₄, is ignited.
4. Nitric acid reacts with iron to form an Fe³⁺ containing product.
5. Copper(II) chloride reacts with silver nitrate.

Answer

Identify the type of reaction which will occur given the reactant(s).
1. Decomposition
2. Combustion
3. Double-replacement
4. Synthesis
5. Single-replacement
Write the balanced chemical equation for each of the given descriptions.
1. \(\ce{CaSO4 -> CaO +SO3 } \nonumber \)
2. \(\ce{2Na(s) + O2(g) -> Na2O2(s) } \nonumber \)
3. \(\ce{C2H4 + 3O2 -> 2CO2 + 2H2O } \nonumber \)
4. \(\ce{6HNO3 + 2Fe -> 2Fe(NO3)3 + 3H2 } \nonumber \)
5. \(\ce{CuCl2 + 2AgNO3 -> Cu(NO3)2 + 2AgCl } \nonumber \)

Contributors and Attributions

Mike Blaber (Florida State University)