Laboratory Activity: Teacher Notes

Activity 1: Types of Reactions

Major Chemical Concept

Many chemical reactions can be classified into four types-Synthesis, Decomposition, Single Replacement, Double Replacement.

Level

General or basic student

Expected Student Background

Students should be able to use a balance and burner correctly.

Time

45 min

Safety

Read the Safety Considerations in the Student Version.

Materials (For 24 students working in pairs)

Nonconsumables

Consumables (see Advance Preparation)

Advance Preparation

30 min to prepare solutions. Concentrations are not critical. Other concentrations may be substituted if available.

Pre-Laboratory Discussion

Review the evidence for chemical change. You may wish to demonstrate reactions that exhibit the types of evidence that students can use to decide whether a chemical reaction has occurred. Project Seraphim "Periodic Table" videodisc or ACS "Closeup on Chemistry" videodisc present excellent reactions.

Teacher-Student Interaction

As students are doing the activity, circulate through the laboratory and make sure they are attentive to all changes. Students may focus on one type of change (sparks given off when copper burns) and entirely miss another type of change (the copper turned black).

Anticipated Student Results Part I

The steel wool shows a small increase in mass. The copper wool often does not because small fragments of copper escape.

Part II

Both hydrates lose mass. Both show steam and condensation. The copper(II) sulfate pentahydrate changes from blue to white. There is no color change with the calcium chloride hexahydrate.

Part III

The magnesium metal is replaced by a "coppery" colored solid and the blue color of the solution fades. Hydrogen bubbles may be noticed on the surface of the magnesium at the beginning of the activity. They are formed because the pH of 0.10 M CuSO 4 is about 4. Bubbles are formed on the surface of the zinc. If the piece of zinc is small enough it may disappear.

Part IV

A precipitate is immediately formed when each pair of solutions is mixed.

Answers to Data Analysis and Concept Development

Part I

  1. The color changed.
  2. The mass increased, so some substance must have been added.
  3. In both reactions the substance changed color and increased in mass.

  4. Copper + oxygen ---> Copper oxide*

    Iron + oxygen ---> Iron oxide

    *This would be a good time to teach students the use of Roman numerals to identify the oxidation states of metals.

  5. See equations in Post-Laboratory Discussion.
  6. Two elements are combined to form a single substance or a single substance is synthesized from the elements.

    NOTE: Although these reactions started with elements, synthesis or combination also applies to using two or more compounds to synthesize another compound.

    Part II

    1. Color change, water formed
    2. The water was removed.
    3. Water was formed in each reaction.
    4. Copper(II) sulfate pentahydrate ---> Copper(II) sulfate + Water
      Calcium chloride hexahydrate ---> Calcium chloride + Water
    5. See equations in Post-Laboratory Discussion.
    6. Two substances were made from a single substance.

    Part III

    1. In one reaction a color change occurred and a new solid was formed. In the other, gas bubbles appeared.
    2. Not applicable
    3. One substance disappeared and another with different properties appeared.
    4. Copper(II) sulfate + Magnesium ---> Copper + Magnesium sulfate
      Hydrochloric acid + Zinc ---> Hydrogen + Zinc chloride
    5. See equations in Post-Laboratory Discussion.
    6. When looking at the equation one can see that one element replaced another in a compound.

    Part III

    1. A precipitate was formed.
    2. Not applicable
    3. Solutions of two compounds were mixed and a precipitate was formed.
    4. Barium chloride + Sodium sulfate ---> Barium sulfate + Sodium chloride
      Cobalt(II) nitrate + Sodium phosphate ---> Sodium nitrate + Cobalt(II) phosphate
    5. See equations in Post-Laboratory Discussion.
    6. Looking at the equations and formulas one can see that the positive ions were exchanged. One of the products was insoluble and precipitated.

Answers to Implications and Applications

  1. a. Double replacement
    b. Synthesis
    c. Decomposition
    d. Single replacement
  2. Most often it is not. One can only observe whether a chemical change has occurred.
  3. Names and formulas of starting materials would be very helpful.

Post-Laboratory Discussion

Part I

2Cu(s) + O 2 (g) ---> 2CuO(s)

4Fe(s) + 3O 2 (g)---> 2Fe 2 O 3 (s)

Use logic to show how a decrease in mass or no change in mass would not support the conclusion that a chemical reaction has occurred.

Example: If mass stays the same then the only possibility is copper (reactant) --->copper (product), implying no chemical change.

Example: If mass decreases then the only possibility is that copper (reactant)---> something less than copper (product); logically that is not possible because copper is an element and is not decomposed in normal chemical reactions. Emphasize the observations of a new product being formed on the surface of the copper wool.

Part II

CuSO 4 . 5H 2 O(s)---> CuSO 4 (s) + 5H 2 O(l)

CaCl 2 . 6H 2 O(s) --->CaCl 2 (s) + 6H 2 O(l)

(NH 4 ) 2 CO 3 (s) --->2NH 3 (g) + CO 2 (g) + H 2 O(l)

The apparent loss of mass needs to be examined since it would contradict the Law of Conservation of Mass. Make a drawing of the copper(II) sulfate (simple is fine) and use the color change from bright blue to near-white as evidence for change. The "loss" of water molecules can be verified by the steam and condensation seen at the mouth of the test-tube while heating. Calcium chloride will not show a color change, but will show condensation as evidence for change.

Part III

CuSO 4 (aq) + Mg(s) ---> Cu(s) + MgSO 4 (aq)

Zn(s) + 2HCl(aq)---> H 2 (g) + ZnCl 2 (aq)

The loss of the blue color of the copper(II) sulfate solution shows that copper ions have changed to copper metal. If students use only 1.0 cm of magnesium metal this will be more evident. Be sure that the students understand that all materials are present originally and are not created in the laboratory activity. Have them predict the identity of products that form. It should be noted that a "single replacement" reaction is a redox reaction (see Oxidation-Reduction module).

Part IV

BaCl 2 (aq) + Na 2 SO 4 (aq) ---> BaSO 4 (s) + 2NaCl(aq)

3Co(NO 3 ) 2 (aq) + 2Na 3 PO 4 (aq) &---> Co 3 (PO 4 ) 2 (s) + 6NaNO 3 (aq)

H 2 SO 4 (aq) + NaOH(aq) --->NaHSO 4 (aq) + H 2 O(l)

Use these reactions to introduce exchange reactions. Students can practice predicting products. Emphasize using correct formulas. It should be noted that a "double replacement" reaction involves precipitation reaction and does not involve nonreacting spectator ions.

Possible Extensions

These reactions can be used again when students are introduced to moles and quantitative results of reactions. For example, water of hydration laboratory activities can be done quantitatively and total moles of water of hydration can be calculated.

Assessing Laboratory Learning

These reactions can be demonstrated and explanations given verbally. Formulas for the reactants may be helpful. You may wish to choose reactions from those given in the activity or demonstrations section or others from your repertoire.

  1. Identify the type of reaction.
    a. A metal is heated in air. The product is a black powder. The mass of the powder is greater than the mass of the metal. [synthesis]
    b. Two clear colorless solutions are mixed. A precipitate forms immediately. [double replacement]
    c. A red powder is heated. A silvery liquid and a clear colorless gas forms. [decomposition]
    d. A metal and a clear liquid are mixed. A dark colored solid and a greenish solution result. [single replacement]
  2. Draw "pictures in the mind" to illustrate each reaction studied in this laboratory activity.

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