Chapter 7 Chemistry Notes - What do you observe Chapter 7A...

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9/27/2013 1 Chapter 7A - Chemical Reactions Evidence of a chemical reaction Evidence of a reaction Chemical equations Writing/balancing a chemical equation Ionic compounds in aqueous solution Solubility rules for ionic compounds What do you observe? An orange solid, ammonium dichromate, is heated. __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ Two white solids, lead(II) nitrate and potassium iodide are added to opposite ends of a Petri dish containing water._____________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ http://www.youtube.com/watch?v=n_pOYGM5zgM&feature=related http://www.youtube.com/watch?v=blKd3VhZpbY&feature=related http://www.youtube.com/watch?v=3xx4AUcT-D4 Each Reaction Involves a Chemical Change What are some common visual clues for chemical change? 1. ___________________ 2. ___________________ 3. ___________________ 4. ___________________ 5. ___________________ 6. _and others_________ Observations: when heated the sample produces a gas. Does this prove a chemical reaction has occurred? ___________________ Conclusion: macroscopic observations (& measurements) provide important evidence, but they do not provide a complete picture of what is going on. The only proof is at a sub-microscopic level. You know where I’m headed, right? A complete picture has many levels: Macroscopic Sub-Microscopic (molecular) Symbolically Models/ Word/ Math equations Macroscopic Symbolic Molecular A chemical reaction is a symbolic summary. Think about the first reaction of the ammonium dichromate. Macroscopic- heat (q), gases, and green solid, produced from an orange solid. Sub-Microscopic- Start with reactant: (NH 4 ) 2 Cr 2 O 7 and end with products: N 2 , H 2 O, and Cr 2 O 3 (consistent with macroscopic data). Symbolically- we write the reaction as an chemical equation: (NH 4 ) 2 Cr 2 O 7 (s) N 2 (g) + 4 H 2 O(g) + Cr 2 O 3 (s) + q Notice how this efficiently summarizes what is known. !→ ! Δ This is a molecular style chemical equation: (NH 4 ) 2 Cr 2 O 7 (s) N 2 (g) + 4 H 2 O (g) + Cr 2 O 3 (s) + q In a molecular style equation the compounds are written by their neutral chemical formula (formula units or molecular formula). To start, all reactants are added and the physical state is specified (i.e. phase): (s)-solid, ( )-liquid, (g)-gas, or (aq)-aqueous (i.e. dissolved in water). An arrow is used to signify the direction of chemical change. Sometimes additional information such as sample heating( Δ ), light (h ν ), the solvent, or a catalyst is written above the arrow. The products are added after the arrow (including physical state ) To conserve mass, the number of atoms of each type on the reactant side must exactly equal the atoms of each type on the product side. This is called balancing the chemical reaction. Writing a Chemical Equation !→ ! Δ Balance the following: CoCl 2 + Na 3 PO 4 Co 3 (PO 4 ) 2 + NaCl Balancing a Chemical Equation http://www.youtube.com/watch?v=vcsgc3hh1Ro 1. Write the unbalanced equation with all chemical formulas written correctly.
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