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Testing for Cations and Anions Determine the presence of a cation or anion by a chemical reaction Determine the identity of cations and anions in an unknown.

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Presentation on theme: "Testing for Cations and Anions Determine the presence of a cation or anion by a chemical reaction Determine the identity of cations and anions in an unknown."— Presentation transcript:

1 Testing for Cations and Anions Determine the presence of a cation or anion by a chemical reaction Determine the identity of cations and anions in an unknown salt

2 What are Cations? Sometimes certain metals (Na, K, Ca, Mg …) are more stable if they lose their valence electrons Na becomes Na +, K becomes K +, Ca becomes Ca 2+, Mg becomes Mg 2+ Na +, K +, Ca 2+, Mg 2+ are positive ions, or cations Sometimes a metal only loses some of its ions to make it more stable like Fe 3+ Sometimes a molecule is more stable if it loses electrons, like the ammonium ion NH 4 +

3 What are Anions? When the metals lose their valence electrons they give them to some non-metal elements are more stable with those electrons Cl becomes Cl -, F becomes F -, Br becomes Br - In these examples the electrons gain fill up the valence shell Some molecules are also more stable if they have extra electrons, examples are sulfate, SO 4 2-, phosphate, PO 4 3-, carbonate CO When atoms or molecules have extra electrons and a negative charge we call them anions

4 Cations and Anions in Nature In nature you always find cations and anions together Rock salt is made up of Na + and Cl - (sodium chloride or NaCl) A liter of Sea water has about 0.03 g of NaCl Ions in water allow it to have a current flow through it Nerves use ions to pass nerve impulses Na + and Cl - are found outside cells in the body. They help regulate and control the level of body fluids K + are the principal cations found inside cells in the body. Potassium ions help regulate cellular functions, including nerve impulses and heartbeats, and the level of body fluids Ca 2+ and Mg 2+ are found mainly in the bones, but they are also vital components of many enzymes, which are substances our bodies need in order to release energy from the food we eat Fe 3+ in hemoglobin is used to bring O 2 into the body In the body the amount of ions is tightly regulated for correct functioning

5 Tests for Positive Ions (Cations) In this experiment you will observe tests for the following cations Na +, K + and Ca 2+ can be determined with a flame test Ca 2+ reacts with ammonium oxalate (NH 4 ) 2 C 2 O 4 to give a white solid (precipitate) Converting NH 4 +  NH 3 gives a gas with a distinctive odor and turns red litmus paper blue Iron Fe 3+ is detected by the distinctive red color it gives when it reacts with potassium thiocyanate (KSCN) CationsSolution sodium (Na + )0.1M NaCl potassium (K + )0.1M KCl calcium (Ca 2+ )0.1M CaCl 2 iron (Fe 3+ )0.1M FeCl 3 ammonium (NH 4 + )0.1M NH 4 Cl

6 Tests for Negative Ions (Anions) In this experiment you will observe tests for the following anions When AgNO 3 is added to certain anions several insoluble salts form When nitric acid (HNO 3 ) is added all these solids dissolve except AgCl Phosphate PO 4 3- reacts with ammonium molybdate and forms a yellow solid When BaCl 2 is added to a solution containing SO 4 2- a solid (precipitate) forms of BaSO 4 that doesn’t dissolve in HNO 3 Carbonate anion CO 3 2- is identified by adding HCl which produces bubbles of carbon dioxide CO 2 AnionSolution chloride (Cl - )0.1M NaCl phosphate (PO 4 3- )0.1M Na 3 PO 4 sulfate (SO 4 2- )0.1M Na 2 SO 4 carbonate (CO 3 2- )0.1M Na 2 CO 3

7 Testing Your Unknown You and your partner will be given an unknown compound to identify the cation and the anion Use the previous tests to find the cation and the anion Write the name and formula for the compound

8 Writing the formula An ionic compound contains a cation (positive) and an anion (negative ion) The compound is charge neutral which means that when the total number of positive charges = total number of negative charges We always write cation first and then the anion (Na 1+ ) (PO 4 3- ) (Na 1+ ) 3 (PO 4 3- ) 1 Sodium Phosphate Na 3 PO 4

9 What you will need 7 test tubes (small or large) Spot plate (for flame test) Flame test wire Bunsen burner thermometer Hot plate with a 250 ml half filled with tap water heated to 60 o C Red litmus paper glass stirring rod


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