CHEM Experiment #6 - chemistry lab report PDF

Preview

Summary

chemistry lab report...

Description

CHEM CH116 Experiment #6 Fall 2016

Experiment 6 Analysis of a Copper(II) Compound by a Single Displacement Reaction with Zinc Metal Purpose To analyze a sample of copper(II) sulfate pentahydrate (CuSO4∙5H2O) for its copper content by a single displacement reaction with an excess amount of zinc metal in aqueous acid solution. Introduction Gravimetric Analysis Techniques Gravimetric Analysis is a technique that uses weighing as the method to determine or isolate an unknown analyte, which is a chemical substance that is being measured and quantified in an experiment. Although gravimetric analysis can be a time-consuming technique, it is considered a relevant method and at times the chosen method for analysis of some compounds. This experiment involves the precipitation (the production of a solid from a solution) of copper followed by its isolation and weighing to determine the percent by mass of copper in the initial sample. Note that the procedure has been simplified from a true gravimetric analysis.

Gravimetric 1Elemental Analysis for Copper Determination of the percentage by mass of the different elements present in a pure compound or a mixture is a common analytical task. Every specific method of analysis takes advantage of different chemical and physical properties of the elements present. An analysis method that uses measurements of mass only is called gravimetric analysis. In this experiment, a sample of the soluble salt copper(II) sulfate pentahydrate will be analyzed for its copper content. The details are provided below. Step 1 A sample of solid copper(II) sulfate pentahydrate (CuSO4∙5H2O) will be weighed by difference and dissolved in distilled water. This releases the copper(II) ions into solution. A small amount of HCl will also be added to the solution. This addition helps to dissolve the CuSO4∙5H2O salt and also ensures that any excess zinc metal is removed before recovering the copper solid by filtration.

Page 1 of 12

CHEM CH116 Experiment #6 Fall 2016

CuSO4∙5H2O (s)  CuSO4 (aq) + 5 H2O CuSO4 (aq)  Cu2+ (aq) + SO42- (aq)

(Equation 1: Dissolving in Water/HCl) (Equation 2: Ions Separating)

Step 2 Zinc metal granules, which have a large amount of surface area for reaction, will be added to the solution. Zinc metal is more active than copper metal and will therefore react with the dissolved copper(II) ions. The zinc metal will be oxidized to zinc(II) ions by the copper(II) ions, and the copper(II) ions will be reduced to copper metal. The copper metal will precipitate from the solution. This is an example of a single displacement reaction, which is a type of oxidationreduction (redox) reaction: Zn (s) + Cu2+  Cu (s) + Zn2+ (aq)

(Equation 3: Singe Displacement Reaction)

To ensure the quantitative recovery of copper metal (meaning that all the copper is precipitated), an excess of zinc metal must be added. The copper ions are therefore the limiting reactant, and the zinc metal is the excess reactant in this reaction.

Step 3 When the reaction to form copper is complete, the excess zinc metal must be removed before recovering the copper solid by filtration. The removal takes advantage of the greater activity of zinc metal. Zinc reacts with hydrochloric acid, but copper metal is unreactive: Zn (s) + 2HCl (aq)  ZnCl2 (aq) + H2 (g)

(Equation 4: Removal of excess zinc metal)

Cu (s) + HCl (aq)  No Reaction At the end of the reaction, the solution must be clear and colourless, and there should be no zinc metal present. The only visible precipitate should be the solid copper metal at the bottom of the reaction vessel.

Step 4 The product copper solid will be filtered from the solution by gravity, using a pre-weighed filter paper. The solid on the filter paper will be washed free of impurities with water.

Step 5 The copper metal will be dried to constant weight by evaporation in the drying oven at 110 120 ºC.

Step 6 Page 2 of 12

CHEM CH116 Experiment #6 Fall 2016

The mass of the recovered copper metal and the percent recovery of the copper metal will be calculated. Hydrates Many metal salts form stable crystalline solids containing molecules of water. To distinguish a solid salt without water in the crystals (anhydrous salt) from a salt with water in the crystals, the latter are referred to as hydrates. For example, CuSO4 is named copper(II) sulfate, and CuSO4∙5H2O is named copper(II) sulfate pentahydrate. Often, the hydrated salt is a different colour than the anhydrous salt. There is a special naming system and formula writing system for hydrates. Some hydrates are useful in industry or in the chemical laboratory. Some salts have more than one hydrate. Anhydrous Salt

Name

Hydrated Salt

CuSO4

Copper(II) Sulfate

CuSO4∙5H2O

CaCl2

Calcium Chloride

CaCl2∙2H2O

MgSO4

Magnesium Sulfate

MgSO4∙H2O

MgSO4

Magnesium Sulfate

MgSO4∙7H2O (Epsom Salt)

CaSO4

Calcium Sulfate Calcium Sulfate

CaSO4

Name

Uses

Copper(II) Sulfate Hydrate used as a Pentahydrate fungicide Anhydrous salt Calcium Chloride used as a drying Dihydrate agent Magnesium Anhydrous salt Sulfate used as a drying Monohydrate agent

CaSO4∙½H2O (Plaster-of-Paris)

Magnesium Sulfate Heptahydrate Calcium Sulfate Hemihydrate

Hydrate used in the dyestuff industry Intermediate in making drywall

CaSO4∙2H2O (Gypsum)

Calcium Sulfate Dihydrate

Drywall construction material

Some anhydrous salts have the ability to remove water molecules from the air or from liquids in which the water is dissolved, forming hydrated solids. These substances are called drying agents or desiccants. Examples are anhydrous calcium chloride and anhydrous magnesium sulfate. CaCl2 (aq) + 2 H2O  CaCl2∙2H2O(s)

calcium chloride dihydrate

MgSO4 (aq) + H2O  MgSO4∙H2O(s)

magnesium sulfate monohydrate

MgSO4 (aq) + 7 H2O  MgSO4∙7H2O(s)

magnesium sulfate heptahydrate

Page 3 of 12

CHEM CH116 Experiment #6 Fall 2016

Anhydrous calcium chloride is used as both a commercial and laboratory desiccant. In laboratory work, a sealed apparatus called a desiccator is used to dry solids, or to keep them dry after heating, or to store them away from moisture for long periods. The drying agent is placed inside the desiccator, which may be as small as a lunch box or as big as a room. Anhydrous magnesium sulfate is used to remove traces of water from solvents such as ether during chemical synthesis of many substances.

Percent Recovery At the end of a chemical analysis, the percent recovery (or percent yield) is often calculated because it is a useful check of an analysis. In theory, percent recovery should be 100%, but there are a number of reasons why most percent recovery values are less than 100% and include the limitations of the method, product loss, etc. Sometimes, percent recovery values can be above 100%; this could occur if the solid product is not dried properly to constant weight (it is still wet with solvent) or is contaminated.1 The formula to calculate percent recovery is given below. percent recovery =

experimental yield x 100% theoretical yield

The experimental yield is the recovered mass of the analyte at the end of an experiment (in this experiment, the solid copper metal recovered and dried to constant weight), and the theoretical yield is the calculated value based on the reaction stoichiometry.

Sample Calculations A 2.0860-g sample of copper(II) sulphate pentahydrate was reacted with excess of zinc metal by the method of this experiment. The experimental yield of copper metal was 0.5020 g. Calculate the following quantities: 1. The amount of zinc metal required for the reaction with copper(II) sulphate pentahydrate; 2. The theoretical yield of copper metal; and 3. The percent recovery of copper metal. Answer to Question 1 The reaction is 1 In this experiment, for example, any excess Zn metal not removed by the addition of HCl(aq) would contaminate the Cu solid because it would be filtered along with the Cu. This would result in a percent recovery value greater than 100%. Page 4 of 12

CHEM CH116 Experiment #6 Fall 2016

Zn (s) + Cu2+  Cu (s) + Zn2+ (aq) The reaction stoichiometry between Zn and Cu is therefore 1:1. The source of copper is copper(II) sulphate pentahydrate. The amount of zinc metal required for the reaction is calculated as follows: Mass of Zn = 2.0860 g CuSO 4 ∙5 H2 O ×

×

mol CuSO 4 ∙5 H 2 O mol Cu × mol CuSO 4 ∙5 H2 O 249.69 g CuSO 4 ∙5 H 2 O

mol Zn 65.38 g Zn × mol Zn mol Cu = 0.5463 g Zn

Remember: to ensure the quantitative recovery of copper metal, a slight excess of zinc metal must be added, and any excess Zn metal must be removed by the addition of HCl(aq).

Answer to Question 2 Each mole of CuSO4∙5H2O can theoretically produce one mole of Cu metal by a singledisplacement reaction with zinc (refer to Equations 2 and 3). The theoretical yield of copper metal is calculated by assuming that all of the CuSO4∙5H2O reacts and is calculated as follows: Mass of Cu = 2.0860 g CuSO 4 ∙5 H2 O ×

mol CuSO4 ∙5 H 2 O 63.55 g Cu mol Cu × × mol CuSO 4 ∙5 H 2 O mol Cu 249.69 g CuSO 4 ∙5 H 2 O

= 0.5309 g Cu

Answer to Question 3 percent recovery =

experimental yield x 100% theoretical yield =

0.5020 g x 100% 0.5309 g = 94.56 %

Page 5 of 12

CHEM CH116 Experiment #6 Fall 2016

Page 6 of 12

CHEM CH116 Experiment #6 Fall 2016

Prelab 

  

Review the following: o The phrase “weigh accurately approximately” (Experiment 2); o Weighing by difference by using a weigh boat technique (video posted under Experiment 2). Watch the video How to Perform Gravity Filtration posted on eLearn posted under Experiment 6. Review all the calculations shown in the preceding pages. Data book preparation: o Amend the Table of Contents; o Provide a date, title, purpose, and a list of materials; o Provide a flow chart and prepare all data tables. Note: you may prepare data tables in a manner similar to the data tables shown under the Lab Report section.

Notes:  Your instructor will not demonstrate the gravity filtration technique that will be used for this experiment, because it is demonstrated in a video posted on eLearn. It is your responsibility to ensure that you watch the video before the lab.  You must complete all your calculations before leaving the lab. So, manage your time accordingly.  All observations must be written clearly and descriptively in Table 3.

Procedure Ensure that the fume hood fans are switched ON and are operating.  

Any excess of hydrochloric acid may be diluted with tap water and then disposed of into the sinks. All additional waste and any excess material other than hydrochloric acid must be collected for disposal as hazardous waste. A labelled collection bottle will be available in a fume hood.

Procedure A-1: Weighing and Dissolving Copper(II) Sulfate Pentahydrate 

Weigh accurately approximately 2 g of CuSO4∙5H2O. Use a clean and dry weigh boat. Transfer the sample quantitatively into a clean 250-mL beaker. Record the results in the data book. Also record the appearance of the solid.

Page 7 of 12

CHEM CH116 Experiment #6 Fall 2016



  



Place a rubber policeman on a clean stirring rod, and place the rod into the beaker, policeman-end up. The policeman may be used later to remove solid out of the beaker when filtering. The stirring rod must remain in the beaker at all times. If it is taken out of the beaker for any reason, then wash it into the beaker with distilled water from a wash bottle to avoid any loss of sample. Obtain about 50 mL of distilled water (use a clean 100-mL graduated cylinder). Pour the water down the stirring rod into the beaker to avoid splashing and possible loss of sample. Several acid dispensing stations will be set up in the fume hoods. In the fume hood, add 2 mL of 6 M HCl to the sample by using a clean 10-mL graduated cylinder. Use the stirring rod to stir the solution and dissolve the solid. Gently break up any large crystals with the stirring rod to speed up the dissolving process. If any solid remains 5 minutes after the crystals are all crushed, then add another 1 mL of the 6 M HCl to the solution. If the solid still does not dissolve completely, then consult with the instructor. Describe the colour of the solution.

Procedure B-1: Reaction of Copper Ions with Zinc Metal       

Based on the actual value of the copper salt weighed, recalculate the mass of zinc metal required for the reaction. Check the calculation before proceeding. Weigh the calculated amount of zinc on a top-loading balance (there is no need to use an analytical balance to weigh out the zinc metal). Carefully add the zinc into the copper solution. Stir the mixture gently and carefully for about 5 minutes without splashing. Record the observations. Continue stirring if necessary until all the zinc is dissolved. If the solution is still blue, then more zinc needs to be added. Only add a few granules at a time until the blue colour disappears. If the solution turns cloudy, then too much zinc has been added, and solid Zn(OH) 2 has formed); add a few drops of 6M HCl until the solution clears. This Procedure is complete when the solution is clear, colourless and all the zinc has been removed. Only the Cu precipitate should be visible at the bottom of the beaker.

Procedure C-1: Recovery of Copper Metal by Gravity Filtration     

Set up a retort stand and a funnel holder as demonstrated in the video posted on eLearn. Obtain a piece of fast filter paper. Label the filter paper in pencil with your initials to avoid losing it later when drying it in the oven. Weigh the filter paper on an analytical balance, and record its mass in the data book. Fold the filter paper, and fit it to the long-stem funnel as demonstrated in the video posted on eLearn. More about folding a filter paper and fitting it to a funnel is illustrated on the next page.

Page 8 of 12

CHEM CH116 Experiment #6 Fall 2016

   



Wet the filter paper with gentle streams of distilled water from a wash bottle and check for a complete fit. There should be no gap between the filter paper and the funnel. Place the tip of the funnel well into a 250-mL Erlenmeyer receiving flask. Pour the liquid from the beaker carefully and slowly down the stirring rod into the filter paper. Use the technique demonstrated in the video posted on eLearn. Once all the liquid has been filter through, rinse the inside of the beaker and the stirring rod with a small portion of distilled water from a wash bottle, and filter these rinse solutions through the filter paper. Repeat this procedure several times until all the solid inside the beaker has been rinsed and filtered. Ask a lab mate to help with this process as recommended in the eLearn video. If necessary, turn the beaker over above the funnel, and use the rubber policeman and a stream of distilled water from the wash bottle to transfer any residual solid from the beaker to the filter paper. Rinse the rubber policeman, and filter the rinses through filter paper. How to Fold and Fit a Filter Paper

Fold the filter paper circle twice into a quarter shape as shown above. Do not crease the paper sharply. This will weaken the fibers and cause tearing to occur.

Open the filter paper quarter shape into a cone shape as shown above.

Fit the filter paper cone into the funnel. Use distilled water from the wash bottle to make the paper stick to the glass.

Page 9 of 12

CHEM CH116 Experiment #6 Fall 2016

Procedure D-1: Drying the Copper Metal Precipitate to Constant Weight     

 

 

Remove the filter paper from the funnel. Be careful not to tear it. A pair of tweezers may be helpful. Open the filter paper carefully and place it flat on a clean watch glass. Place the watch glass and the filter paper with the product solid in a 110 - 120 ºC drying oven for at least 15 minutes. Do not put the watch glass and paper at the very bottom of the oven. The heating element is at the bottom of the oven. The paper may char if it becomes too hot. Caution: high temperature. Use tongs. After at least 15 minutes in the oven, remove the watch glass and filter paper from the oven onto a heatproof pad. Allow the filter paper and solid to cool completely to room temperature (this should take about 5 minutes). Weigh the filter paper and solid, and record the mass value in the data book. Repeat the heating, cooling and weighing cycle until the difference between the last two weighings is no more than 0.0100 g. This should take no more than 2 – 3 cycles. Check with the Instructor at this point as to whether any additional cycles are required (time permitting). Describe the colour and appearance of the dried product. Calculate the mass of copper metal recovered. Use the last value of the mass of the filter paper plus copper. Calculate the theoretical yield and percent recovery of copper metal.

Page 10 of 12

CHEM CH116 Experiment #6 Fall 2016

Lab Report Name

Student ID

Table 1: Mass of CuSO4∙5H2O Initial Mass (g)

Final Mass (g)

Mass of Sample (g)

Mass of Zinc Required for the Reaction In the space below, show how the mass of the zinc metal required for the reaction was calculated.

Table 2: Drying the Copper Metal to Constant Weight Mass of Filter Paper (g)

Mass of Filter Paper + Cu (g) Cycle 1 Cycle 2 Cycle 3

In the space below, show how the mass of Cu was calculated.

Page 11 of 12

Mass of Cu (g)

CHEM CH116 Experiment #6 Fall 2016

Table 3: Observations Step

Observations (Colours, Gas Bubbles, Heat Produced, etc.)

Copper Sulfate Pentahydrate Sample Sample Solution Reaction of Sample Solution with Zinc Metal and HCl Removal of Excess Zinc Metal with HCl Product Solid

Calculations and Discussion 1. Theoretical Yield. In the space below, calculate the theoretical yield (g) of copper metal. Provide the answer to the correct number of significant figures.

2. Percent Recovery. In the space below, calculate the percent recovery of the copper metal. Provide the answer to the correct number of significant figures.

3. In the space below, discuss the possible sources of error which may explain why the percent recovery was not 100%.

Page 12 of 12...