Periodic Trends

Learning objectives

In a group you will:

  1. Prepare salt solutions
  2. Observe and report the colour of each solution
  3. Look for trends in the data to find relationships between:
    1. the colour of the solutions and the identity of the metal ion
    2. the relationship between the colour and the absorbance spectrum of the solution
  4. Make a predictive model based on the trends above

On your own you will:

  1. Write a report summarizing your results

Required reading

M.S. Silberberg, S. Amateis P. Lavieri, and R. Venkateswaran. Chemistry: the molecular nature of matter and change. McGraw-Hill Higher Education, 2016. Sections 3.5, 7.1 and 7.2 and page 249 (241 in the first edition).

Scenario

A food manufacturer has approached your group of consultants to discuss a new product they are working on: coloured table salt. They want to mix a small amount of different salts with their NaCl table salt in order to liven up their product. They have come to you with two main questions:

  1. Can we predict the colour of a solution of salt based on the identity of the salt’s metal ion?
  2. Can we predict the colour of a solution of salt based on the solutions absorption of visible light?

Pre-Lab Question-Hypothesis

Hypothesize your answer for the questions above and briefly explain your reasoning.

Your group will test a series of salt solutions to determine the answers to these questions and provide a report that summarizes your findings.

Apparatus and Chemicals

Apparatus

Chemicals

Al(NO3)3 Cu(NO3)2
Ca(NO3)2 NaNO3
SnCl2 in HCl Fe(NO3)3 in HNO3

Chemical Safety

Safety glasses must be worn at all times.

Aluminum(III) nitrate is hazardous in case of skin contact (irritant, permeator), eye contact (irritant), or inhalation and ingestion. Prolonged exposure may result in skin burns and ulcerations. Over-exposure by inhalation may cause respiratory irritation.

Copper(II) nitrate is hazardous in case of skin contact (irritant, permeator, corrosive), eye contact (irritant), or inhalation and ingestion. Strong oxidizer, keep away from combustible material. Very toxic to aquatic life, never rinse down the sink.

Calicium nitrate is hazardous in case of skin contact (irritant, permeator), eye contact (irritant), or inhalation and ingestion. Prolonged exposure may result in skin burns and ulcerations. Over-exposure by inhalation may cause respiratory irritation.

Sodium nitrate is hazardous in case of skin contact (irritant, permeator), eye contact (irritant), or inhalation and ingestion. Prolonged exposure may result in skin burns and ulcerations. Over-exposure by inhalation may cause respiratory irritation.

Tin(II) chloride is very hazardous in case of skin contact (irritant, corrosive), eye contact (irritant, corrosive), or inhalation and ingestion. Eye contact can result in corneal damage or blindness. Skin contact can produce inflammation and blistering. Inhalation of dust will produce irritation to gastro-intestinal or respiratory tract, characterized by burning, sneezing and coughing. Severe overexposure can produce lung damage, choking, unconsciousness or death. Inflammation of the eye is characterized by redness, watering, and itching. Skin inflammation is characterized by itching, scaling, reddening, or, occasionally, blistering.

Iron(III) nitrate is hazardous in case of skin contact (irritant, permeator), eye contact (irritant), of inhalation and ingestion. Prolonged exposure may result in skin burns and ulcerations. Over-exposure by inhalation may cause respiratory irritation.

Nitric acid is corrosive and a powerful oxidizing agent. It will hydrolyse skin, so it is of utmost importance to be wearing gloves, lab coat and safety glasses when handling nitric acid. Nitric acid on the skin can also stain the skin yellow. If you get nitric acid on yourself you should rinse the affected area with cold water for 15 minutes. Any clothing contaminated with nitric acid (including gloves) should be removed immediately as the acid will do damage to the skin underneath. Do not mix nitric acid with other chemicals unless instructed to do so. Nitric acid is also a skin irritant, (permeator) an eye irritant and an irritant due to ingestion and inhalation.

HCl is very hazardous in case of skin contact (corrosive, irritant, permeator), eye contact (irritant, corrosive) or ingestion. It is also slightly hazardous in case of inhalation (lung sensitizer). Wear gloves when handling HCl.

Procedure Part I

  1. Each group of two will be assigned one salt. Mark down the reagent formula for your salt (in the data sheet). Different salts have differing waters of hydration, which is import when determining the amount in mol of the salt you are using. You will need to know the reagent formula to prepare your solution.
  2. Determine the amount of salt necessary to make 25 mL of a 0.10 M salt solution.

    3. Pre-Lab Question

    Determine the amount in grams of salt necessary to make 25 mL of a 0.10 M solution of Cu(NO3)2 • 3 H2O

  3. On an electronic balance, weigh your salt. You do not need to weigh out exactly the amount you calculated, but you should be within 0.1 g of this amount. Mark down the exact amount to three decimal places in your data sheet and calculate the exact molarity of your solution based on the weight that you used.
  4. Add the salt to a 25 mL Erlenmeyer flask. Add water to reach the 25 mL mark on the flask. For tin chloride you will use dilute HCl from the labelled squeeze bottle instead of water and for iron nitrate you will use dilute HNO3.

    5. Note

    For this lab we are mainly concerned with qualitative results, so we are not worried about using glassware with high precision and accuracy.

  5. In your data sheet record the colour of the solution. With the help of the rest of your team, fill out the colours of all the solutions. (If you are having trouble determining the colour of a solution, do no be afraid to ask your peers for help.)
  6. As a team, on the Smartboard screen, highlight the colour of each solution with the appropriate colour, in the position of the salt's metal ion element on the periodic table. Mark down these colours on your data sheet.
  7. Inspect the pre-made solutions prepared for you. Add these salts to the periodic table, both on the Smartboard and on your data sheet.
  8. In your group discuss the following questions. Make sure to take notes as you will need to summarize the discussion for your lab report.
    1. Is there a relationship between the colour of the solution and the position of the metal in the periodic table?
    2. Can you predict what colour a solution will be based on your answer above? For example, would you expect a solution of palladium nitrate to be coloured or colourless?
    3. Look at the two periodic tables in your data sheet. One has the electron configurations of the metal ions tested, while the other has the periodic table with the ionic radii of common metal ions. Record any observable pattern(s) in the colour of the salt solutions as they relate to either the ionic radii or electron configuration.

Part II

In the second portion of the lab we will be investigation how the solutions made in part I interact with visible light. In order to determine how your salt solutions interact with visible light, you will use a spectrophotometer. See page 249 (241 in the first edition) of your textbook for background on spectrophotometers. You will be taking the absorption spectrum of a solution of your ionic salt.

Figure 1: Wavelength of the visible region of the electromagnetic spectrum

You will take the samples you prepared earlier and place them in the spectrophotometer. The spectrophotometer will irradiate the sample with light in the wavelength range of 400 nm to 750 nm. The spectrophotometer then detects the intensity of the light after it has passed through the sample. On the resulting spectrum, if a wavelength of light is absorbed by the sample, it will show up as a peak on the spectrum. Below is a spectrum of chlorophyll a. You can see two peaks that correspond to the blue (425 nm) and red (~660 nm) light being absorbed by the leaf. Our eyes perceive the colour of leaves to be green because the chlorophyll in the leaf absorbs the blue and red light while reflecting the green light back to your eyes.

Pre-Lab Question

If chlorophyll had absorption peaks at 550 nm and 650 nm rather than 425 nm and 660 nm what colour would the leaf appear to your eyes?

Figure 2: Visible absorption spectrum of chlorophyll a

Procedure Part II

  1. On the instrument, set the wavelength display to the shortest wavelength required (400nm) by pressing the following keys:
    1. Standard Methods
    2. Spectrum:
  2. Open the cell compartment of the spectrophotometer and insert the cuvette containing a reference solvent (distilled water) into the sample holder. Fill the cuvette roughly 2/3 full with the reference. Wipe off the sides and bottom of the cuvette with a Kimwipe. Close the cover. Press the 0A/%T button to set the reference.
  3. Remove the cuvette, replace the reference solution with your solution, and return the cuvette to the spectrophotometer. Press the (green ►) button. Make sure to fill the cuvette roughtly 2/3 full of the sample and to wipe off the sides and bottom of the cuvette with a Kimwipe.
  4. After the scan, the display should show a spectrum. Mark down the wavelength of the two biggest peaks higher than 0.50 on the y-axis (If you cannot see the peak because its maximum is outside the 400-750 nm range, as long as the portion of the curve visible in the spectra is above 0.50 nm you should list the curve as a peak in the table with the λmax being either 400 nm or 750 nm since we cannot be sure exactly where the peak lies.) If there are no peaks you may leave the space blank in both the datasheet and on the excel results file.
  5. Compile all the team results on the Smartboard and be sure to copy down the results from the rest of the class for yourself.

Checklist

To do before coming to the lab:

To do during the lab:

To do after the lab:

Periodic trends data sheet

First Name: _________________________________
Last Name: _________________________________
Student ID Number: _________________________________
Demonstrator: _________________________________
Lab Section: _________________________________
Date: _________________________________
Assigned Salt
Reagent Formula
Molar Mass
Mass of Salt needed by calculation
Actual Mass Measured
Actual Concentration
Salt Reagent Formula Solvent Solution colour Largest peak

λ max (nm)

 2nd largest peak

λ max (nm)
Aluminum nitrate Al(NO3)3 • 9H2O H2O
Calcium nitrate Ca(NO3)2 • 4H2O H2O
Tin(II) chloride SnCl2 • 2H2O HCl
Sodium nitrate NaNO3 H2O
Copper(II) nitrate Cu(NO3)2 • 3H2O H2O
Nickel(II) nitrate Ni(NO3)2 • 6H6O NH3
Iron(III) nitrate Fe(NO3)3 • 9H2O HNO3
Chromium(III) nitrate Cr(NO3)3 • 9H2O H2O
Cobalt(II) nitrate Co(NO3)2 • 6H2O H2O
Mark down the colour for each tested metal ion on at least one of these periodic tables, and hand in with your data sheet.

Pre-Lab Questions

  1. Hypothesize your answer for the questions below and briefly explain your reasoning.
    1. Can we predict the colour of a solution of salt based on the identity of the salt’s metal ion?
    2. Can we predict the colour of a solution of salt based on the solutions absorption of visible light?
  2. Determine the amount in grams of salt necessary to make 25 mL of a 0.10 M solution of Cu(NO3)2 • 3 H2O
  3. If chlorophyll had absorption peaks at 550 nm and 650 nm rather than 425 nm and 660 nm what colour would the leaf appear to your eyes?