General Chemistry Experiment 3Lecture

Transcription

1 General Chemistry Experiment 3Lecture Part 1 Ionic Bonds Ionic Compounds Ions are positively and negatively charged atoms or groups of atoms that are each formed by the loss or gain of an electron..... Na + :Cl: [Na] + + [:Cl:] - NaCl... Chemical compounds composed of ions are called ionic compounds. Positive and negative ions combine in ionic compounds so that the net charge of the compound is zero. The equation above shows that one mole of Na + combines with one mole of Cl - to form 1 mole of NaCl Solid ionic compounds are held by together by the mutual attraction of the positive and negative ions. Show picture here Most ionic compounds are crystalline in structure. Most ionic solids are poor conductor of electricity because the ions are rigidly fixed in their positions. The ions become free to move and conduct electricity when the ionic compounds melt or dissolve in water. When ionic compounds are dissolved in water the compounds dissociate to its positive and negative ions. Ionic bonds tend to form atoms or elements have a big difference in electronegativity. Ionic Equations A precipitation reaction is a reaction in which an insoluble product is formed when two solutions are mixed. Example When sodium chloride solid is dissolved water, the solution consists of Na + and Cl -. Similarly aqueous silver nitrate contains Ag + and NO 3 -. When the two aqueous solutions are mixed, a white precipitate is immediately observed. The precipitate is AgCl. The molecular reaction is AgNO 3(aq) + NaCl (aq) AgCl (s) + NaNO 3 The ionic equation for the reaction is Ag + + NO Na + + Cl - AgCl (s) + Na + + NO 3 - Because Na + and NO 3 - appear in both sides of the equation, they do not play a direct role in the equation. Na + and NO 3 - in the above reaction are called spectator ions. By cancelling the spectators ions, the net ionic equation is obtained. Ag + + Cl - AgCl (s)

2 Experiment You will use precipitation reactions to separate mixture of cation solutions of Ba +2, Ca +2 and Mg +2. Ba SO 4 BaSO 4 (solid) Ca C 2 O 4 CaC 2 O 4 (solid) 3Mg HPO 4 + 2OH - Mg 3 (PO 4 ) (solid) + 2H 2 O Each step that a cation is precipitated, it is separated from the remaining solution. Part A Known Analysis Test Tube #1 Mixture of Ba +2, Ca +2, Mg +2 (NH 4 ) 2 SO 4 (NH 4 )C 2 O 4 Supernate Test Tube #2 Precipitate: Yes No Color Cation Present? Supernate Test Tube #3 Precipitate: Yes No Color Cation Present? Na 2 HPO 4 NaOH Precipitate Yes No Color Cation Present?

3 Procedure 1. Test for completeness of precipitation. If not done well, some cation that is being separated will remain in solution. This could react with the reagent will be used in the next precipitation experiment. 2. Make sure the centrifuge is balanced. Balancing the centrifuge means that there is a test tube with the same volume of liquid directly across the tube that you want to centrifuge. Breakage that could fly out of the centrifuge could occur if not balanced 3. Use the pasteur pipet with a bulb to separate the solution (supernatant or centrifugate) from the precipitate. 4. Always mix the solutions when adding the reagent to the cation solution. 5. All waste go into the waste jar. Part 2 Covalent Bonds Atoms that are held together as result of sharing of the valence electron are bonded by covalent bonds. Example Cl 2 Single covalent bond is a bond where the atoms are held together by sharing two electorns. Covalent bonds form between two atoms when ionic or metallic is unlikely because of the loss of or gain of electrons requires large amount of energy. The number of valence electron and the octet rule govern the formation of most covalent bonds. C (Group IV) N (Group V) O (Group VI) 4 valence electrons 5 valence electrons 6 valence electrons 4 shared pairs 3 shared pairs 2 shared pairs Understanding the Three-Dimentional Models Molecules will arrange themselves to the most stable structure. Structures are often characterized by their bond length and bond angles. Bond length (A) is the distance between nuclei of two bonded atoms.

4 Bond angle is the angle of the arc between any two bonds that are joined to the same atom. The three-dimensional models are represented on as shown: How are three dimensional structures demonstrated? a. ball and stick models b. space filling models Write the structural formula and electron dot formula for molecules after building a model. (a) Draw the structural formula (b) Draw the electron dot formula corresponding to the model (c) verify the electron dot formula by checking the total number of electron dots against the sum of all valence electrons. Hydrogen shares two electrons and is complete. Carbon shares a total of eight electrons and satisfies the octet rule. However, each chlorine also requires an octet, which we will complete as follows: (c) To verify the above electron dot formula we will find the sum of all valence electrons. 1H (1 x 1 e - ) = 1 e - 1C (1 x 4 e - ) = 4 e - 3Cl (3 x 7e - ) = 21 e - sum of valence electrons = 26 e -

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