experiment 5 - anions and cations.docx - napa valley … labs... · web viewexperiment 5 –...

Experiment 5 – Periodic Nature Name __________________Lab Section __________________

Experiment 5 – Periodic Nature

Introduction

The periodic table, in its most basic form, was developed only recently in history. It was in 1869 that Dmitri Mendeleev, a Russian chemist, devised the ordering of elements by increasing mass (subatomic particles had yet to be discovered). In doing so, he noticed that a repeating pattern, a periodic nature, was observed. This table was improved upon in 1913 by Henry Moseley, an English physicist. Owing to the discoveries of the electron, proton, and neutron, the periodic table was ordered by increasing number of protons.

In addition to this ordering, certain elements were put into groups, based on their chemical (reactivity) and physical properties. These groupings resulted in the seven rows on the periodic table and the eighteen columns. The rows are called periods, and the columns are called groups. There are four groups in particular that are given specific names, they are: alkali metals (group 1), alkali earth metals (group 2), halogens (group 17), and noble gases (group 18). Each of these groups, referred to some as “families”, are grouped based on their reactivity. For example, each element in the alkali metal family will react with oxygen in a 2:1 ratio (e.g. Na2O or K2O).

In this experiment, we will use each element’s unique color, as well as its reactivity to determine the composition of an unknown compound. When a salt, a metal combined with a nonmetal, is placed in water, it can dissociate into the ions that comprise it. The positively charged metal ion portion is called the cation. This experiment will look at the cations Li+, Na+, K+, Ca2+, Sr2+, and Ba2+. The negatively charged nonmetal portion is called the anion. The anions Cl-, Br-, and I-

will be investigated.

Each cation will have its own distinct color when excited (remember experiment 4). In this case, a flame test will be used to excite the electrons. Here, a sample of the dissolved salt will be burned in order to cause the electrons to become excited, and the resulting color will be seen. In addition, the similar chemical reactivity of each of the groups will be studied. Each anion will also be determined by its color, though in a different fashion to the cations.

In this experiment keen observations and careful recording of data will allow us to determine the composition of an unknown compound by comparing it to the behavior of known compounds.


Procedure

Identifying Cations

Flame Test Colors

Place six test tubes into a test tube rack and label them for each of the six solutions being put inside of them. Refer to Figure 5.1.

Put 20 drops of each known solution into its test tube.

Clean a flame-test wire by dipping it in hydrochloric acid (HCl) and burning off the solution in the flame. Do this three times, or until no color is seen in the flame.

Test each of the solutions by lightly dipping the flame-test wire into the solution and then placing the wire into the hottest portion of the flame. Note the color that it produces. It may take a few times to see the color.

Be sure to thoroughly clean the flame-test wire between solutions.

Figure 5.1: The six cations


Solubility of Solutions

Place six test tubes into a test tube rack and label them for each of the six solutions being put inside of them.

Put 20 drops of each known solution into its test tube.

Add 2-3 drops of ammonium carbonate, (NH4)2CO3, to each test tube. Note whether or not a precipitate forms.

Rinse the test tubes into a waste beaker for later disposal.

Repeat this procedure using ammonium sulfate, (NH4)2SO4, and ammonium phosphate, (NH4)3PO4.

Put all waste into a waste beaker for later disposal as indicated by your instructor.

Identifying Anions

Place three test tubes into a test tube rack and label them for each of the three solutions being put inside of them (e.g. Cl2, Br2, I2).

Put 20 drops of each solution into its test tube.

Add 20 drops of hexane, C6H14, to the test tube containing the chlorine solution.

Add 2 drops of nitric acid, HNO3, then add 2-3 drops of chlorine water (bleach, NaClO) to this test tube

Gently shake this test tube as demonstrated by your instructor. Note the colors of each layer in the test tube. The top layer contains your anion.

Pour the waste into your waste beaker and repeat for each other solution.

Figure 5.2: The three anions


Identifying the Unknown Solution – Cation and Anion

Obtain an unknown solution from your instructor and record its number.

Cation Test

Perform the flame test on your sample as before

Place 20 drops of your unknown solution into three test tubes and check for its solubility with each of the ammonium carbonate, (NH4)2CO3, ammonium sulfate, (NH4)2SO4, and ammonium phosphate, (NH4)3PO4 solutions.

Anion Test

Put 20 drops of each solution into its test tube.

Add 20 drops of hexane, C6H14, to the test tube containing the chlorine solution.

Add 2 drops of nitric acid, HNO3, then add 2-3 drops of chlorine water (bleach, NaClO) to this test tube.

Gently shake this test tube. Note the colors of each layer in the test tube. The top layer contains your anion.

Compare these results for the unknown sample with those of the known solutions to determine the composition of the unknown solution


Prelaboratory Questions

1) What are the names of the three groups on the periodic table that will be investigated in this lab. Which elements belong to each of those groups?

Group Name:

Group 1 __________ Group 2 ___________ Group 17 __________

Example Elements:

Group 1 __________ Group 2 ___________ Group 17 __________

2) Look up the flame test colors for the six cations being investigated. What are the expected colors for each of these elements?

Li __________ Na __________ K __________

Ca __________ Ba __________ Sr __________

3) Look up the colors of the three anions being investigated. What are the expected colors for each of these elements?

Cl2 __________ Br2 __________ I2 __________


Data Table

Cation Tests for Known Solutions

Flame Test - Color of Flame

Known Solution

Lithium (Li)

Sodium (Na)

Potassium (K)

Calcium (Ca)

Strontium (Sr)

Barium (Ba)

Color Observed

______________________________

______________________________

______________________________

______________________________

______________________________

______________________________

Solubility Test – PPT or No Reaction

Known Solution

Lithium (Li)

Sodium (Na)

Potassium (K)

Calcium (Ca)

Strontium (Sr)

Barium (Ba)

Ammonium Carbonate(NH4)2CO3

__________

__________

__________

__________

__________

__________

Ammonium Sulfate(NH4)2SO4

__________

__________

__________

__________

__________

__________

Ammonium Phosphate(NH4)2PO4

__________

__________

__________

__________

__________

__________

Anion Test – Color of Layers

Chloride

____________________

Bromide

____________________

Iodide

____________________


Unknown Solution – ___________

Cation Test

Flame Test - Color of Flame

Unknown Solution

Color Observed

______________________________

Solubility Test – PPT or No Reaction

Unknown Solution

Ammonium Carbonate(NH4)2CO3

__________

Ammonium Sulfate(NH4)2SO4

__________

Ammonium Phosphate(NH4)2PO4

__________

Anion Test – Color of Layers

Unknown Solution

Color Observed

______________________________

Composition of Unknown Solution

_______________


Postlaboratory Questions

1) To which group does each of the following elements belong? (family name)

Na ___________ F ____________ Mg ____________ Xe ____________

2) Which element is similar to the one listed below? (circle the appropriate answers)

Element Similar to:

Ca Fe Be Na Cd

Cu Co Cr Ag Zn

O S N Ar Li

K Na Br Li Kr

3) An unknown solution is subjected to the same flame-test, solubility test, and anion test that you just did in lab. It gave a light green color in the flame test and a precipitate with each of the ammonium salts. In addition, it gave an orange color in the hexane layer. What was this compound?

4) An unknown solution is subjected to the same flame-test, solubility test, and anion test that you just did in lab. It gave a red color in the flame test and a precipitate with each of the ammonium salts except ammonium sulfate. In addition, it gave no color in the hexane layer. What was this compound?

5) Use arrows on the following blank periodic tables to indicate the directions in which each of the following properties increases:

Ionic Radius Electronegativity Metallic Nature

experiment 5 - anions and cations.docx - napa valley … labs... · web viewexperiment 5 –...

Documents