Experiment # 5

Chemical Reactions

Objective: To observe and discover different ways of detecting a chemical reaction. To experience the five basic types of chemical reactions and to express these reactions using balanced chemical equations.

Discussion: Chemical change is always associated with a chemical reaction. Atoms, molecules or ions rearrange to form new substances. The substances doing the rearranging are called reactants. The new substances are called products. Keep in mind; the Law of Conservation of Matter and Energy is always obeyed.

Therefore, all atoms present in the reactants must be accounted for in the products.

There are a few basic observations that indicate a chemical reaction as occurred. These are as follows:

1. A precipitate forms. That is, insoluble solid particles appear in solution.

2. A gas evolves. Bubbles are seen in the reaction solution. The gasses we will test for are, oxygen, hydrogen and carbon dioxide.

a. If the gas evolved is oxygen, the splint will glow brighter.

b. If the gas evolved is hydrogen, you will hear a characteristic “bark” as the hydrogen ignites when an ignited splint is brought to the mouth of the test tube.

c. Finally, if the gas is carbon dioxide, the ignited splint will be extinguished.

3. A color change. If, after mixing the reactants you see a color change, a reaction took place.

4. Heat is given off (exothermic) or absorbed (endothermic). The reaction vessel becomes warm or cold, or light is given off.

5. Change in pH. Litmus paper can be used to determine a change in acidity. Remember, blue litmus paper turns red in acidic solution and red litmus turns blue in basic solutions.

There are five basic types of chemical reactions. They are tabulated as follows:

Reaction Type	Description	Example
Combination	Two or more substances combine to form one substance.	C(s) + O2(g) à CO2(g)
Decomposition	One substance breaks down into two or more.	2KClO3(s) à 2KCl(s) + 3O2(g)
Double Replacement	Cations of both reactants swap places.	AgNO3(aq) + NaCl(aq) à AgCl(s) + NaNO3(aq)
Acid/base Neutralization	A type of DR rxn, where H^+ions are transferred between reactants.	HNO3(aq) + NaOH(aq) à H2O(l) + NaNO3(aq)
Redox Rxns (Sinlgle Replacement or Combustion reactions)	These are electron transfer reactions. In (1) Single Replacement reactions, one element reacts with a compound to replace one of the elements of that compound. In (2) combustion, oxygen combines with a substance, heat and light are evolved.	1) Cl2(g) + 2KI(aq) à I2(s) + 2KCl(aq) 2) 2Mg(s) + O2(g) à 2MgO(s)

Procedure:

Wear safety goggles!!

Make sure to dispose of all chemicals according to your instructor’s direction!

Note: Observations are descriptions the changes you see occurring. Written notes of observations are considered data. Therefore, record observations in ink on lab report.

Read the instructions for each reaction carefully and record your observations in ink in the space provided. Think about what species are present before and after mixing and record those in the appropriate spaces. Finally write the equation for the reaction

.

Prelab Questions

1. Just by looking at its formula, which two of the four gases would you want to test for upon heating CuCO₃? ________________ and __________________

2. An acid turns the indicator litmus to a ____________ color.

3. we can sometimes tell when a chemical reaction has taken place by:

a. a change in color of the substance

b. the evolution of an identifiable gas

c. either or both of the above

4. You have produced a gas which you suspect to be hydrogen. Describe how you might confirm your suspicion. State exactly what you would do and what would you expect to observe?

5. Is there any part of this experiment where the top loader balance is used?

P art I. Combustion:

1. Cut an approximately 1-inch piece of magnesium ribbon. Hold the piece of Mg ribbon with your crucible tongs and ignite it with your Bunsen burner. Caution!! Do not look directly at the burning ribbon. Is the reaction exothermic or endothermic?

Observations
Reactants	Products
Balanced Chemical equation

2. Pour approximately 1 mL of heptane into an evaporating dish. (Measure the heptane in a graduated cylinder). Carefully ignite the heptane by holding a burning splint over the heptane until it combusts. Use a test tube holder to invert a test tube over the flame produced by the heptane. Observe the test tube. What do you see? What is this product? Again, hold a test tube over the burning heptane. Light a splint from the heptane flame. Quickly insert the burning splint into the inverted test tube. What happened to the burning splint? Carefully note your observations. What can you conclude about the gas produced from the reaction?

(See next page for data sheet.)

Observations
Reactants	Products
Balanced Chemical equation

Part II. Single Replacement:

1. Fill a test tube half full with DI water and place it behind the safety shield in the hood. Sodium metal is stored under a non-reactive solvent such as hexane. Take a small piece of sodium from the stock bottle and place it on a piece of filter paper, being careful not to touch it with your hands. Blot the sodium with the filter paper. As you blot the sodium, apply pressure. Is the sodium malleable? Using tongs or tweezers pick up the piece of sodium and drop it into the test tube of water. What happens? Take a burning splint and bring it to the mouth of the test tube as the sodium is reacting. What happens? Carefully note your observations. Phenolphthalein is an acid/base indicator, which is colorless in acid solution (contains H⁺ ions) bright pink in base solution (contains OH^- ions). Take a few drops of phenolphthalein and place them into test tube. What color is the resulting solution?

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

2. The reactivity of an element is related to its tendency to lose or gain electrons; that is, to be oxidized or reduced. It’s possible to arrange nearly all elements into a single series in order of their reactivity. This is known as an activity series. A more active element will replace an element from a compound. Generally speaking: A + BC à B + AC; where A is the more active element and replaces B in the compound. With the following reactions, evidence of a reaction may not be immediately apparent. Before making a decision be sure to let the reaction stand fro approximately 10 minutes.

Obtain 3 pieces of zinc, 2 pieces of copper and 1 piece of lead. Clean the metal pieces with sandpaper to expose the surface. Place 6 test tubes in a rack and label each appropriately. Add the following reactants:

Observe the contents of each of the test tubes for evidence of a reaction. Record your observations.

Tube 1: Copper strip and approximately 4 mL silver nitrate

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Tube 2: Lead strip and about 4 mL copper(II) nitrate

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Tube 3: Zinc strip and about 4 mL of lead(II) nitrate

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Tube 4: Zinc strip and about 4 mL of magnesium sulfate

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Tube 5: Copper strip and about 4 mL 3M sulfuric acid

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Tube 6 : Zinc strip and about 4 mL of 3M sulfuric acid

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Part III. Decomposition

Into a test tube, place about 2 mL of 3% hydrogen peroxide along with a pea size amount of MnO₂. Manganese dioxide is used as a catalyst. Remember, that a catalyst is used to lower the activation energy of a reaction, thereby speeding up the reaction. It does not get consumed in the reaction and therefore is not part of your balanced chemical equation. Note any evidence of the reaction. Has a gas been evolved? Light a wood splint and blow out the flame. Put the glowing splint into the test tube. What happens? Remember, when writing your balanced equation, hydrogen gas was not produced.

Observations
Reactants	Products
Balanced Chemical equation

Part IV. Double Replacement

For the following reaction be sure to include a total ionic equation and a net ionic equation.

1. In a small test tube, place approximately 3 mL of 0.1 M BaCl₂ and add 3 mL of 0.1 M K₂SO₄. Carefully note your observations. One way to determine which substance precipitated out is to do a little bit of investigation. Place 3 mL each of DI water into two test tubes. Add a pea size amount of potassium chloride into one test tube and the same amount of barium sulfate into the other. Note your observations. What can this tell you about the precipitate that formed?

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

2. In a clean, dry test tube, place a small amount of calcium carbonate. Carefully and slowly add about 3 mL of 6M HCl solution. Allow the reaction to proceed for 60 seconds, the insert a lit splint into the test tube. What did you observe? From your observation, determine what gas is evolved.

Observations
Reactants	Products
Balanced Chemical equation
Total Ionic Equation
Net Ionic Equation

Post Lab Questions.

1. Arrange Cu, Ag, Pb, Zn, and Mg in order of their activities, listing the most active first. Explain how you determined this order.

2. Where would H fit in the activity series you developed in question 1? Can you conclusively determine its position? Why or why not? If not, what experiment could you perform to determine its position?

3. Would magnesium react with dilute sulfuric acid? Why or why not?