Classifying Chemical Reactions
By Mike Isley
Writing and balancing chemical equations is an essential skill for chemistry students, who must learn to predict the products of a reaction when given only the reactants. This becomes much easier for students to do when they learn the pattern of 5 basic categories of chemical reactions: synthesis, decomposition, single replacement, double replacement, and combustion. This article outlines some simple protocols your class can follow so that student pairs can produce each of these types of reactions using common reagents.
- Synthesis reactions
Two or more reactants combine to make 1 new product.
Examples:C(s)+ O2(g) → CO2(g)
H2O(l) + SO3(g) → H2SO4(aq)
- Decomposition reactions
A single reactant breaks down to form 2 or more products.
Examples:H2CO3(aq) → H2O(l) + CO2(g)
CaCO3(s) → CaO(s) + CO2(g)
- Single-replacement reactions
A single element replaces a similar element of an adjacent reactant compound.
Examples:Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
- Double-replacement reactions
Two ionic compounds exchange ions, producing 2 new ionic compounds.
Examples:NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
- Combustion reactions
A single element or compound combines with oxygen gas releasing energy. This rapid oxidation is called burning.
Examples:C(s) + O2(g) → CO2(g) + energy
2Mg(s) + O2(g) → 2MgO(s) + energy
2C4H10(g) +13O2 → 8CO2(g) + 10H2O(g) + energy
National Science Education Standards
This activity is appropriate for high school students and addresses the following National Science Education Standards for grades 9–12:
- Science as Inquiry: Abilities Necessary to Do Scientific Inquiry; Understandings About Scientific Inquiry
- Physical Science: Structure and Properties of Matter; Chemical Reactions; Interactions of Energy and Matter
Note: Use safety glasses or goggles and be cautious with the burning splints and Bunsen burner. Follow all local and state recommendations for chemical disposal.
Each lab group requires the following items to perform all the activities. You may want to set up the activities at stations and have student teams rotate through them. If so, print the instructions onto a card for each station.
- Nail (3" or longer)
- ½ tsp Yeast (in a labeled test tube)
- 20 mL Hydrogen Peroxide, 3% (in a labeled 250-mL Erlenmeyer flask)
- ½ Steel Wool Pad
- 3 Wooden Splints
- Evaporating Dish
- 1 tsp Baking Soda
- Test Tube Rack
- 15 mL CuSO4 Solution, 0.5 M (in a labeled test tube)
- Bunsen Burner
- 30 mL Vinegar (in a labeled 250-mL beaker)
- 1 mL (20 drops) Ethanol (in a labeled, stoppered test tube)
- Laboratory Thermometer
- Rubber Band
- Paper Towels
- Measure room temperature and record: ____°C.
- Soak a steel wool pad in 30 mL of vinegar in a 250-mL beaker.
- Remove pad and squeeze the excess vinegar back into the beaker.The vinegar strips away the coating on the steel filaments, which are 96 to 98% iron.
- Wrap the pad around the thermometer bulb, secure it with a rubber band, and observe the temperature over 5 min.
- Record the final temperature of the steel wool: _____°C. Was there a temperature change?
- Describe any changes to the steel wool. ________________________________.
- Write a balanced equation for this reaction. Include heat as a reactant or product.
- Pour the yeast from the test tube into the flask containing the 20 mL of hydrogen peroxide (H2O2). The yeast contains the enzyme catalase that decomposes hydrogen peroxide. What gas or gases could be produced?
- Review the table below for confirming 3 common gases.
- Insert a flaming splint into the flask held at a 45° angle. Is the test positive for hydrogen (see Fig. 1)?
- Insert a glowing splint into the flask. Is the test positive for oxygen (see Fig. 1)?
Oxygen Gas Insert a glowing splint into the container. If it bursts back into flame, the test is positive. Hydrogen Gas Insert a flaming splint into the container held at a 45° angle. If there is a small explosion or barking sound, the test is positive. Carbon Dioxide Gas Insert a flaming splint into the container. If the flame is extinguished, the test is positive.
- Write a balanced equation for this decomposition reaction.
- Hold the test tube containing 0.5 M CuSO4 solution at a 45° angle and insert the nail.
- Place the test tube in the test tube rack. What happens?
- If you use a plain iron nail, here is the reaction:
Fe(s) + CuSO4(aq) → ______(aq) + ______(s)
A galvanized nail (coated with Zn), undergoes a different reaction:
Zn(s) + CuSO4(aq) → ________(aq) + _________(s)
- Complete and balance both equations.
- Pour the baking soda (sodium hydrogen carbonate, NaHCO3) from the spoon into the 250-mL beaker containing the vinegar (acetic acid, HC2H3O2).
- Describe what happens.
- Complete and balance the equation below for this reaction:
NaHCO3 + HC2H3O2 → _______(aq) + _______(aq)
- One of the products, carbonic acid (H2CO3), immediately decomposes into water and a gas. Complete and balance this equation, and identify the gas with a flaming or glowing splint:
H2CO3 → H2O + _____(g)
- Pour the 1 mL of ethanol from the stoppered test tube into the evaporating dish.
- With a flaming splint, carefully ignite the ethanol in the dish. (This reaction occurs in your car’s engine if you use gasoline that contains ethanol.) Do not touch the dish until 5 min after the flame extinguishes.
- Complete and balance the equation for this reaction:
C2H5OH(l) + O2(g) → _______(g) + ________(g) + energy
- The combustion reaction for a Bunsen burner uses methane (CH4) or propane (C3H8). Complete and balance these 2 reactions:
CH4(g) + O2(g) → _______(g) + _______(g) + energy
C3H8(g) + O2(g) → ______(g) + _______(g) + energy
- Have students use the metal activity series to predict whether a free metal will replace another metal ion in an ionic compound for single-replacement reactions.
- Have students use a digital camera to document examples of the 5 types of chemical reactions in their local communities. Then, they can share these examples with the class in a PowerPoint® presentation.
- As a way to illustrate the relevance of synthesis and single-replacement reactions, have students research corrosion reactions (e.g., the verdigris on the Statue of Liberty) and some of the ways of preventing corrosion on ships and pipelines.