Lab Practice: Chemistry

Date: November 19,2011

By: Swati Kulkarni

Lab Practice: Chemistry

Analysis of a Chemical Reaction:

To determine the correct chemical equation from the given four possibilities by determining the mole ratio for a reactant and a product in a chemical reaction and using the mole ratio information along with qualitative evidence to identify the reaction from among four possibilities.

Background Knowledge:

·  Naming chemical compounds and their formulas

·  To know the types of chemical reactions

·  Stoichiometry of Chemical Reactions

·  Mass relationships

·  Mole Ratios

·  Handling Chemical Equipments and know what are they called.

Safety Instructions:

ü  Behave responsibly in the lab all the times

ü  Tie long hair back

ü  Wear Safety goggles and lab coat

ü  Know the location of the fire equipment; review use of that equipment before

beginning work.

ü  Read all the procedure thoroughly

ü  Be very careful while handling hot objects

ü  Do not place hot objects on the balance pan

ü  Be precise and accurate in measurements to obtain accurate results

ANALYSIS OF A CHEMICAL REACTION

Introduction: During this lab you will be determining the correct chemical equation from these four possibilities:

A] NaHCO3 (s) → NaOH (s) + CO2 (g)

B] 2 NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (g)

C] 2 NaHCO3 (s) → Na2O (s) + 2CO2 (g) + H2O (g)

D] 4 NaHCO3 (s) → 2Na2C2O4 + O2 (g) + 2H2O (g)

Purpose:

·  To determine the mole ratio for a reactant and a product in a chemical reaction.

·  To use the mole ratio information along with qualitative evidence to identify the reaction from among four possibilities

Pre-Lab: /2 K/U /8 I /5 C

1)  What is the formula of sodium bicarbonate? (1 K/U)

The formula of sodium bicarbonate is NaHCO3

2)  What is the test for the presence of carbon dioxide? Why does it work? (1 K/U, 1 I)

To test for the presence of carbon dioxide, collect a sample of the gas produced during the reaction in test tube, add a small amount (about 2 to 3 ml) of lime water to the test tube and shake well. If the solution turns milky white the gas is carbon dioxide. This works because lime water and carbon dioxide react and white precipitate is formed which makes the solution milky.

3) How might you be able to demonstrate that water is being produced? (1 I)

To demonstrate that water is being produced, cobalt chloride test can be done. If we touch the sample with a piece of cobalt chloride paper and it turns blue to pink, we will be able to demonstrate the presence of water.

4)  You will be deciding which reaction is correct based on both qualitative and quantitative observations.

a)  Explain how the qualitative observations will help you determine which reaction(s) are potentially correct for this experiment. (2 I)

The qualitative observations demonstrate the presence of certain chemical compounds that are formed during the chemical reaction. For example, in the above reaction, if we determine the presence of both carbon dioxide and water (moisture), it is likely that equation B or C is the possible equation we are looking for.

b)  How will quantitative observations be used to determine which reaction is correct? (2 I)

If we check the Ratio of Mass of the reactant: Mass of the product, it will help us to determine which reaction is correct. Using stoichiometry we can figure out that if the above mentioned ratio is approximately 2, then the expected product is sodium carbonate and not sodium oxide. (comparing molar masses of sodium carbonate and sodium oxide, the ratio of mass of reactant to the mass of product will be more than 2) so we can determine that equation B is correct.

5)  Does the mass in step 1 of the procedure need to be exact, why or why not? (2 I)

The mass in step 1 does not need to be exact. It is to be measured as accurately as measuring device allows and is about 2.00 grams. Depending on the ratio of mass of reactant to the mass of product we can determine what product is formed

6)  Draw a flow chart for this experiment. (5c)

Materials:

·  test tubes

·  test tube clamp

·  Bunsen burner

·  Retort stand

·  Clay triangle

·  Crucible and cover

·  Heating pad (to protect lab bench)

·  ring clamp

·  Test tube tongs

·  Crucible tongs

·  Sodium bicarbonate

Procedure:

1)  Add about 0.5 g of sodium hydrogen carbonate to a test tube. Clamp this test tube on a right angle to the retort stand so that the sodium hydrogen carbonate is spread horizontally along the side of the tube. Heat the sodium hydrogen carbonate GENTLY for 2 minutes. Be sure not to heat the rubber on the test tube clamp – it will burn. Hold another test tube half-filled with limewater under the mouth of the test tube that is being heated (use test tube tongs). After the 2 minutes of heating shake the test tube containing limewater. What do you observe? What do you notice on the sides of the test tube that contains sodium hydrogen carbonate?

Record both observations:

2)  The observations from step one will help you narrow your choice of possible equations for the reaction. To determine which of the remaining possible reactions is the correct one you must obtain quantitative data. You will be completing the table below: (2 marks)

3)  Measure the mass of a clean, dry crucible and cover. Add about 2.0 g of sodium hydrogen carbonate. The mass does not need to be exactly 2.0 g but should be measured as accurately as your measuring device allows. Determine and record the exact mass of the crucible, cover and sodium hydrogen carbonate.

4)  Place the crucible, with its cover slightly agar, on a clay triangle supported by an iron ring on a ring stand. Heat gently for 2 minutes and then heat more strongly for 8 minutes. Place the crucible and cover on a heating pad and allow them to cool. Determine the mass of the crucible, cover and product.

Analysis

1.  What do you observe in the test tube that was heated in step # 1 (the one that contains the sodium hydrogen carbonate)? What does this indicate? (1 A)

We observe fumes of gas coming from the test tube and water vapor, It indicates some gas and water vapors are formed in the reaction

2.  After you shook the test tube with limewater, what did you observe? What does this indicate?(1 A)

After we shook the test tube with limewater, we observe that the solution turns milky. This indicates the gas produced in the reaction is carbon dioxide.

3.  On the basis of the observations after step #1, which of the four choices can be ruled out? Why? (1 A)

On the basis of the observations after step #1, equation A and D of the four choices can be ruled out because only equation B and C demonstrate formation of carbon dioxide and water.

4.  Theoretically, what is the value for the ratio of the mass of sodium hydrogen carbonate: mass of solid product, for each of the possible reactions not ruled out by qualitative observations? Remember that the masses of the gases will not be observed in your experiment and so you are only calculating the ratio of the solid masses in the equations. Clearly show all of your calculations. (3 A)

Theoretically, the value for the ratio of the mass of sodium hydrogen carbonate: mass of solid product is 2, for each of the possible reactions not ruled out by qualitative observations.

For Reaction B; 2 moles of NaHCO3 produce 1 mole of Na2CO3

2.00 g NaHCO3 x (Na2CO3/2 NaHCO3)

= 2.00 g NaHCO3 x (105.99 g Na2CO3 /168.02 g NaHCO3)

= 1.26 g Na2CO3

The Ratio of the mass of sodium hydrogen carbonate: mass of solid product, Na2CO3 = 2.00g

1.26g

This is 1.59 approximately.

For Reaction C; 2 moles of NaHCO3 produce 1 mole of Na2O

2.00 g NaHCO3 x (Na2O/2 NaHCO3)

= 2.00 g NaHCO3 x (61.98 g Na2CO3 /168.02 g NaHCO3)

= 0.74 g Na2O

The Ratio of the mass of sodium hydrogen carbonate: mass of solid product, Na2O = 2.00g

0.74g

This is 2.70 approximately.

5. What is the experimentally determined mass ratio? Which of these ratios agrees with the experimentally determined ratio? Write the correct chemical equation of the decomposition of sodium hydrogen carbonate (2 A)

The experimentally determined mass ratio is = Mass of sodium bicarbonate = 1.980 g

Mass of product 1.185 g

This is 1.67 approximately.

From the above mentioned two reactions B & C, The Ratio of the mass of sodium hydrogen carbonate: mass of solid product, Na2CO3 which is from reaction B agrees with the experimentally determined mass ratio

2 NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (g)

The experimental ratio of moles of sodium hydrogen carbonate to moles of solid product = 2 (0.0235 moles) / 0.0191 moles = 2.46

7. How does your answer to #6 compare to the mole ratio of the balanced chemical equation you have

chosen? (1 A)

From the balanced chemical equation the NaHCO3: Na2CO3 mole ratio is 2:1 which is close to

the experimental ratio of moles of sodium hydrogen carbonate to moles of solid product =2.46

Since the NaHCO3: Na2CO3 mole ratio is 2:1, then 0.0235 moles of NaHCO3 would form 0.0117 moles of Na2CO3. The mass of this amount of Na2CO3 would be:

grams of Na2CO3 = n X mm = 0.0117 mol X 105.99 g/mol = 1.24 grams of Na2CO3 which is very close to the experimental mass of the solid product obtained = 1.185 g

Marking Scheme for Lab:

10 marks Communication based on communication rubric and proper lab format

20 marks Application (2 data table, 11 analysis, 2 error analysis , 2 conclusion, 3 lab performance)

/20 A /10 C