Main Types of Reactions
In this unit, the reaction types are:
| Reaction Type | Also Called | Basic Idea |
|---|---|---|
| Synthesis | Combination | Two or more substances combine to make one new substance |
| Decomposition | Breaking apart | One compound breaks into simpler substances |
| Single Replacement | Single Displacement | One element replaces another element in a compound |
| Double Replacement | Double Displacement | Two compounds switch parts |
| Combustion | Burning reaction | A compound reacts with oxygen and gives off energy |
1. Synthesis Reactions
Main idea
A synthesis reaction happens when two or more elements or compounds come together to form one new compound.
The word synthesis means "putting together" or "building."
Or from the slide pattern: E + E = Combination
Element + Element Compound
Example
2Na + Cl2 2NaCl
Sodium and chlorine combine to form sodium chloride.
Reactants
| Reactant | Type |
|---|---|
| Na | element |
| Cl2 | element |
Product
| Product | Type |
|---|---|
| NaCl | compound |
This is synthesis because two separate elements join together to make one compound.
Another example
2Mg + O2 2MgO
Magnesium reacts with oxygen to form magnesium oxide. This is synthesis because magnesium + oxygen magnesium oxide. Two substances join to make one compound.
How to recognize synthesis
A synthesis reaction usually looks like this:
Reactant + Reactant One product
The easiest clue is: Many things become one thing.
| Equation | Type | Why? |
|---|---|---|
| 2H2 + O2 2H2O | Synthesis | Hydrogen and oxygen combine to make water |
| N2 + 3H2 2NH3 | Synthesis | Nitrogen and hydrogen combine to make ammonia |
| CaO + CO2 CaCO3 | Synthesis | Two compounds combine into one compound |
2. Decomposition Reactions
Main idea
A decomposition reaction happens when a compound turns into less complex compounds or elements.
The word decomposition means "breaking down." It is the opposite of synthesis.
From the slide recognition pattern: C = Decomposition
Compound simpler substances
Example
2H2O 2H2 + O2
Water breaks apart into hydrogen gas and oxygen gas.
Reactant
| Reactant | Type |
|---|---|
| H2O | compound |
Products
| Product | Type |
|---|---|
| H2 | element |
| O2 | element |
This is decomposition because one compound breaks into simpler substances.
Another example
CaCO3 CaO + CO2
Calcium carbonate breaks down into calcium oxide and carbon dioxide. One compound becomes two products, so it is decomposition.
How to recognize decomposition
A decomposition reaction usually looks like this:
One reactant two or more products
The easiest clue is: One thing becomes many things.
| Equation | Type | Why? |
|---|---|---|
| 2H2O2 2H2O + O2 | Decomposition | Hydrogen peroxide breaks down |
| 2KClO3 2KCl + 3O2 | Decomposition | Potassium chlorate breaks down |
| CaCO3 CaO + CO2 | Decomposition | Calcium carbonate breaks apart |
3. Single Replacement Reactions
Main idea
A single replacement reaction happens when a compound and an element come together, and a new compound is formed while a different element is left alone.
In other words, one element replaces another element inside a compound.
From the slide recognition pattern: E + C = Single replacement
| Symbol | Meaning |
|---|---|
| E | element |
| C | compound |
Element + Compound New Compound + Different Element
Example
Fe + CuSO4 FeSO4 + Cu
Iron is an element by itself. Copper sulfate is a compound. Iron replaces copper, forming iron(II) sulfate, and copper is left by itself.
Before reaction
Fe + CuSO4
After reaction
FeSO4 + Cu
This is single replacement because iron replaces copper.
Another example
Zn + 2HCl ZnCl2 + H2
Zinc replaces hydrogen in hydrochloric acid. Products are zinc chloride and hydrogen gas. This is single replacement because: element + compound compound + element.
How to recognize single replacement
Look for this pattern:
Element + Compound Compound + Element
The easiest clue is: One element is alone on each side of the equation.
| Equation | Type | Why? |
|---|---|---|
| Zn + 2HCl ZnCl2 + H2 | Single replacement | Zinc replaces hydrogen |
| Fe + CuSO4 FeSO4 + Cu | Single replacement | Iron replaces copper |
| Cl2 + 2KBr 2KCl + Br2 | Single replacement | Chlorine replaces bromine |
4. Double Replacement Reactions
Main idea
A double replacement reaction happens when two compounds exchange parts and form two new compounds.
The reactants are usually two ionic compounds or acids. The reaction usually happens in an aqueous solution, which means the substances are dissolved in water.
What "aqueous solution" means
An aqueous solution is a solution where water is the solvent. In chemical equations, aqueous is written as (aq).
NaCl(aq) means sodium chloride dissolved in water.
Double replacement reactions often happen in water because ionic compounds can separate into ions when dissolved. For example, NaCl(aq) separates into Na+ and Cl-. This makes it possible for ions to switch partners.
Positive Ions Change Places
In a double replacement reaction, the positive ions change places. Positive ions are called cations. Negative ions are called anions.
In the example NaOH + FeCl3:
| Compound | Positive Ion | Negative Ion |
|---|---|---|
| NaOH | Na+ | OH- |
| FeCl3 | Fe3+ | Cl- |
The positive ions Na+ and Fe3+ switch places.
- Before switching: Na+ is with OH-; Fe3+ is with Cl-
- After switching: Fe3+ goes with OH-; Na+ goes with Cl-
So the products are: Fe(OH)3 + NaCl
Double Replacement Example: NaOH + FeCl3
Step 1: Write the reactants
NaOH + FeCl3 ->
Sodium hydroxide reacts with iron(III) chloride.
Step 2: Identify the ions
NaOH contains: Na+ and OH-
FeCl3 contains: Fe3+ and Cl-
Step 3: Switch the positive ions
The positive ions change places. So: Fe3+ joins with OH-; Na+ joins with Cl-
Step 4: Write the new compounds
Iron(III) joins with hydroxide: Fe3+ + OH- Fe(OH)3
Sodium joins with chloride: Na+ + Cl- NaCl
NaOH + FeCl3 Fe(OH)3 + NaCl
(unbalanced)
Step 5: Balance the equation
Because Fe(OH)3 has three hydroxide groups, we need 3 NaOH. Because FeCl3 has three chlorine atoms, we need 3 NaCl.
3NaOH + FeCl3 Fe(OH)3 + 3NaCl
(balanced)
Why Fe(OH)3 has parentheses
The formula is Fe(OH)3, not FeOH3. This is because OH- is a polyatomic ion called hydroxide.
Iron is Fe3+, so it needs three hydroxide ions to balance the charge.
| Ion | Charge |
|---|---|
| Fe3+ | +3 |
| OH- | -1 |
Three hydroxide ions give a total charge of -3. The parentheses show that there are three whole hydroxide groups.
How to Recognize Double Replacement
From the slide: C + C = Double replacement
Compound + Compound Compound + Compound
The easiest clue is: Two compounds switch partners.
| Equation | Type | Why? |
|---|---|---|
| AgNO3 + NaCl AgCl + NaNO3 | Double replacement | Silver and sodium switch partners |
| HCl + NaOH NaCl + H2O | Double replacement | Acid and base form salt and water |
| BaCl2 + Na2SO4 BaSO4 + 2NaCl | Double replacement | Barium and sodium switch partners |
| 3NaOH + FeCl3 Fe(OH)3 + 3NaCl | Double replacement | Sodium and iron switch partners |
5. Combustion Reactions
Main idea
A combustion reaction means "add oxygen." Combustion happens when a substance reacts with oxygen and gives off energy.
In everyday language, combustion is basically burning.
Combustion occurs when a compound with carbon, hydrogen, and maybe oxygen reacts with oxygen. These compounds are often called hydrocarbons if they contain only carbon and hydrogen.
General combustion pattern
A combustion reaction usually looks like this:
Fuel + O2 products + energy
For complete combustion of a compound containing carbon and hydrogen:
CxHy + O2 CO2 + H2O
If the compound also already contains oxygen:
CxHyOz + O2 CO2 + H2O
Complete Combustion
If combustion is complete, the products will be:
Complete combustion happens when there is enough oxygen. The carbon in the fuel becomes carbon dioxide. The hydrogen in the fuel becomes water.
Example: Methane combustion
Methane is CH4.
CH4 + 2O2 CO2 + 2H2O
This is combustion because methane reacts with oxygen. It is complete combustion because the products are carbon dioxide and water.
Example: Propane combustion
Propane is C3H8.
C3H8 + 5O2 3CO2 + 4H2O
This is complete combustion because the products are CO2 and H2O.
Incomplete Combustion
If combustion is incomplete, the products will include:
and possibly just C + H2O (soot).
Incomplete combustion happens when there is not enough oxygen.
Complete vs Incomplete Combustion
| Type | Oxygen Amount | Products |
|---|---|---|
| Complete combustion | Enough oxygen | CO2 + H2O |
| Incomplete combustion | Not enough oxygen | CO + H2O, or C + H2O |
Why incomplete combustion is important
Incomplete combustion can be dangerous because it may produce carbon monoxide, CO. Carbon monoxide is poisonous. It can also produce carbon, which appears as soot or black smoke. So if a fire is smoky, that can be a sign of incomplete combustion.
Combustion Example: Complete Combustion
C2H6 + O2 CO2 + H2O
Ethane reacts with oxygen. This is combustion because a compound containing carbon and hydrogen reacts with oxygen.
Balance it
Step 1: Balance carbon. There are 2 carbon atoms in C2H6, so put 2 in front of CO2.
C2H6 + O2 2CO2 + H2O
Step 2: Balance hydrogen. There are 6 hydrogen atoms in C2H6, so put 3 in front of H2O.
C2H6 + O2 2CO2 + 3H2O
Step 3: Balance oxygen. On the product side: 2CO2 has 4 oxygen atoms. 3H2O has 3 oxygen atoms. Total = 7. Oxygen gas is O2, so we need 7/2 O2.
C2H6 + 7⁄2O2 2CO2 + 3H2O
To avoid the fraction, multiply everything by 2:
2C2H6 + 7O2 4CO2 + 6H2O
Combustion Example: Incomplete Combustion
CH4 + O2 CO + H2O
This is incomplete combustion because the carbon becomes CO instead of CO2.
Balance it
Carbon is already balanced.
Hydrogen: CH4 has 4 H, so put 2 in front of H2O.
CH4 + O2 CO + 2H2O
Oxygen on product side: CO has 1 oxygen. 2H2O has 2 oxygen. Total = 3. So we need 3⁄2 O2.
CH4 + 3⁄2O2 CO + 2H2O
Multiply by 2:
2CH4 + 3O2 2CO + 4H2O
How to Recognize Each Reaction Type
The slide gives a quick pattern:
| Pattern | Reaction Type |
|---|---|
| E + E | Combination |
| C | Decomposition |
| E + C | Single replacement |
| C + C | Double replacement |
Where E means element and C means compound.
Pattern 1: E + E = Combination
If two elements combine, the reaction is usually combination.
2Na + Cl2 2NaCl
Sodium is an element. Chlorine is an element. They combine to make sodium chloride. So: E + E C. Reaction type: Combination.
Pattern 2: C = Decomposition
If one compound breaks apart, the reaction is usually decomposition.
2H2O2 2H2O + O2
Hydrogen peroxide is one compound. It breaks into water and oxygen. So: C C + E. Reaction type: Decomposition.
Pattern 3: E + C = Single Replacement
If an element reacts with a compound, the reaction is usually single replacement.
Zn + 2HCl ZnCl2 + H2
Zinc is an element. Hydrochloric acid is a compound. Zinc replaces hydrogen. So: E + C C + E. Reaction type: Single replacement.
Pattern 4: C + C = Double Replacement
If two compounds react, the reaction is usually double replacement.
AgNO3 + NaCl AgCl + NaNO3
Silver nitrate is a compound. Sodium chloride is a compound. They switch partners. So: C + C C + C. Reaction type: Double replacement.
Special Note About Combustion
Combustion does not fit as neatly into the E/C pattern because it has its own clue.
The biggest clue for combustion is: A carbon-hydrogen compound reacts with O2.
something with C and H + O2
| Equation | Reaction Type | Why? |
|---|---|---|
| CH4 + 2O2 CO2 + 2H2O | Combustion | CH4 reacts with oxygen |
| C3H8 + 5O2 3CO2 + 4H2O | Combustion | Propane reacts with oxygen |
| 2C2H6 + 7O2 4CO2 + 6H2O | Combustion | Ethane reacts with oxygen |
Full Comparison Chart
| Reaction Type | General Pattern | Reactants | Products | Main Clue |
|---|---|---|---|---|
| Synthesis / Combination | A + B AB | Two or more substances | One main product | Things join together |
| Decomposition | AB A + B | One compound | Two or more products | One thing breaks apart |
| Single Replacement | A + BC AB + C | Element + compound | Compound + element | One element replaces another |
| Double Replacement | AB + CD AD + CB | Compound + compound | Compound + compound | Ions switch partners |
| Combustion | CxHy + O2 CO2 + H2O | Fuel + oxygen | Carbon dioxide and water | Reacts with oxygen and gives energy |
Easy Way to Identify Reactions
When looking at a chemical equation, ask these questions in order.
Question 1: Is oxygen, O2, a reactant with a compound containing carbon and hydrogen?
If yes, it is probably combustion.
CH4 + 2O2 CO2 + 2H2O
Question 2: Is there only one reactant?
If yes, it is probably decomposition.
2KClO3 2KCl + 3O2
Question 3: Is there only one main product?
If yes, it is probably synthesis.
2Mg + O2 2MgO
Question 4: Is it element + compound?
If yes, it is probably single replacement.
Fe + CuSO4 FeSO4 + Cu
Question 5: Is it compound + compound?
If yes, it is probably double replacement.
3NaOH + FeCl3 Fe(OH)3 + 3NaCl
Practice Examples
Example 1
2Al + 3Br2 2AlBr3
This is synthesis. Why? Aluminum and bromine combine to make aluminum bromide. Pattern: E + E C.
Example 2
2H2O 2H2 + O2
This is decomposition. Why? Water breaks apart into hydrogen and oxygen. Pattern: C E + E.
Example 3
Mg + 2HCl MgCl2 + H2
This is single replacement. Why? Magnesium is an element. Hydrochloric acid is a compound. Magnesium replaces hydrogen. Pattern: E + C C + E.
Example 4
HCl + NaOH NaCl + H2O
This is double replacement. Why? Hydrochloric acid and sodium hydroxide are compounds. They switch partners and form sodium chloride and water. Pattern: C + C C + C.
Example 5
C3H8 + 5O2 3CO2 + 4H2O
This is combustion. Why? Propane contains carbon and hydrogen. It reacts with oxygen. The products are carbon dioxide and water.
Common Mistakes to Avoid
Mistake 1: Calling all oxygen reactions combustion
Not every reaction with oxygen is combustion. For example:
2Mg + O2 2MgO
This has oxygen, but it is better classified as synthesis because magnesium and oxygen combine to form one product. Combustion usually involves a compound containing carbon and hydrogen reacting with oxygen.
Mistake 2: Confusing synthesis and combustion
Both synthesis and combustion can involve oxygen. Compare:
| Equation | Type | Why? |
|---|---|---|
| 2Mg + O2 2MgO | Synthesis | Two substances form one product |
| CH4 + 2O2 CO2 + 2H2O | Combustion | Carbon-hydrogen compound burns in oxygen |
The key difference is that combustion usually produces CO2 and H2O if complete.
Mistake 3: Forgetting that double replacement starts with two compounds
Double replacement needs two compounds or acids.
NaOH + FeCl3 Fe(OH)3 + NaCl
Both reactants are compounds, so it is double replacement.
Mistake 4: Forgetting to balance double replacement equations
From the slide:
Unbalanced: NaOH + FeCl3 Fe(OH)3 + NaCl
Balanced: 3NaOH + FeCl3 Fe(OH)3 + 3NaCl
You must balance atoms after writing the correct products.
Mistake 5: Ignoring charges when writing double replacement products
When writing products, charges matter. For example: Fe3+ and OH-. Iron has a +3 charge. Hydroxide has a -1 charge. So the formula must be Fe(OH)3, not FeOH. The charges must balance.
Final Summary
Synthesis / Combination
Synthesis happens when two or more substances combine to form a new compound.
Pattern: A + B AB
Recognition clue: E + E = Combination
Example: 2Na + Cl2 2NaCl
Decomposition
Decomposition happens when a compound breaks down into less complex compounds or elements.
Pattern: AB A + B
Recognition clue: C = Decomposition
Example: 2H2O 2H2 + O2
Single Replacement
Single replacement happens when an element and a compound react, forming a new compound and leaving a different element alone.
Pattern: A + BC AB + C
Recognition clue: E + C = Single replacement
Example: Zn + 2HCl ZnCl2 + H2
Double Replacement
Double replacement happens when two compounds switch partners.
Pattern: AB + CD AD + CB
Recognition clue: C + C = Double replacement
Example: 3NaOH + FeCl3 Fe(OH)3 + 3NaCl
Combustion
Combustion happens when a compound containing carbon and hydrogen, and maybe oxygen, reacts with oxygen.
Complete combustion products: CO2 + H2O
Incomplete combustion products: CO + H2O or possibly C + H2O
Example: CH4 + 2O2 CO2 + 2H2O
Super Quick Memory Chart
| Reaction Type | Memory Trick |
|---|---|
| Synthesis | Things join together |
| Decomposition | One thing breaks apart |
| Single Replacement | One element replaces another |
| Double Replacement | Two compounds switch partners |
| Combustion | A fuel burns with oxygen |
The fastest way to remember them:
- Synthesis: many one
- Decomposition: one many
- Single replacement: element + compound
- Double replacement: compound + compound
- Combustion: carbon/hydrogen compound + oxygen carbon dioxide/water