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Cornell Notes Topic: _____________________________________ Questions Notes Writing & Balancing Chemical Equations What are chemical reactions? Chemical Reactions •when substances undergo chemical changes to form new substances •bonds break, atoms are rearranged and new bonds form What are the signs of a chemical reaction? Signs of Chemical Reactions 1. energy released or absorbed as heat or light 2. production of a gas 3. formation of a precipitate. •A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate. 4. color change How are chemical reactions represented? Chemical Reactions A chemical equation represents the identities and amounts of the reactants and products in a chemical reaction. word equation: the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide and water vapor formula equation: (NH4)2Cr2O7(s) → N2(g) + Cr2O3(s) + 4H2O(g) Chemical Reactions Reactants – substances that go through the chemical change (starting parts) Product – a substance that is the result of a chemical change (ending parts) Symbols Used in Chemical Equations How are chemical equations written? Writing Chemical Equations 1. Identify reactants & products 2. Translate the compound names into formulas 3. Place the reactants’ formulas on the left separated with + 4. Add the → or ↔ after the reactants Writing Chemical Equations 5. Place the products’ formulas on the right separated with + 6. Add the symbols for state of matter (s) (l) (aq) (g) 7. Add the ↑ for gas production or ↓ for precipitate products Watch this video lesson on writing compound formulas from names. Writing Chemical Formulas http://www.youtube.com/watch?v=i3M-DXJkCVQ Watch this video lesson on writing chemical equations. Reaction Equations http://www.youtube.com/watch?v=bRmnrAwQztY How are chemical equations balanced? Balancing Equations Because of the law of conservation of mass the number of reactant atoms must equal the number of product atoms. H2 + O2 → H2O •This equation must be balanced by inserting coefficients in front of molecules. 2H2 + O2 → 2H2O Balancing Chemical Equations 1. Write the formula equation for a reaction. 2. Draw boxes around each compound. 3. Make an element inventory. 4. Add coefficients in front of the boxes until the elements balance. Watch this video lesson on balancing chemical equations. Balancing Equations http://www.youtube.com/watch?v=bWQu1xSPJ74 Chemical Equations Review Key Ideas • Chemical reactions bond & unbond atoms to form new compounds. • Reactions can be described with word or formula reactions. • All reaction equations must be balanced by placing coefficients in front of formulas. Key Terms reactant product aqueous precipitate coefficient catalyst electrolysis