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17-2 Describing Chemical Reactions

17-2 Describing Chemical Reactions. a shorter, easier way to show chemical reactions, using symbols, not words, for the reactants and the products. CHEMICAL EQUATION . h ydrogen molecules react with oxygen molecules to form water molecules. H2 + O2 H2O.

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17-2 Describing Chemical Reactions

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  1. 17-2 Describing Chemical Reactions

  2. a shorter, easier way to show chemical reactions, using symbols, not words, for the reactants and the products CHEMICAL EQUATION hydrogen molecules react with oxygen molecules to form water molecules H2 + O2 H2O reactant + reactant “YIELDS” products

  3. Law of Conservation of Mass total mass of reactants must equal total mass of products matter is NOT destroyed or created in a chemical reaction =

  4. ELEMENTS – represented by a one or two-letter symbol (letter) C Na H Chemical Formulas & Counting Atoms COMPOUNDS– represented by a chemical formula(word) which uses subscripts to show the ratio of elements in the compound H2O2 CO2 C3H8O HOW MANY? coefficient in front of the chemical formula tells “how many” atoms or molecules 3H2O2 4CO2 2C3H8O

  5. Balancing Chemical Equations • Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) • (note: only time a subscript can & must be changed) H+ O     H2O • Count the number of atoms that are present in the basic equation and record • Pick an element that is not balanced on both sides of the equation ( H & O last) • Add a coefficient in front of the chemical formula to adjust the count and record the new values • Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation • (change coefficient, then change counts)

  6. Balancing Chemical Equations • Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) • (note: only time a subscript can & must be changed) Mg+ O     MgO • Count the number of atoms that are present in the basic equation and record • Pick an element that is not balanced on both sides of the equation ( H & O last) C + Cl CCl4 • Add a coefficient in front of the chemical formula to adjust the count and record the new values • Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation • (change coefficient, then change counts)

  7. Building Equations (basic equation is given) Mg + O2 ------> MgO C + Cl2 ------> CCl4

  8. Building w/Marshmallows Al + O2 Al2O3 Note: add like elements on the same side of the equation SO2 + O2 SO3

  9. Balancing Chemical Equations • Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) • (note: only time a subscript can & must be changed) Al + O Al2O3 • Count the number of atoms that are present in the basic equation and record • Pick an element that is not balanced on both sides of the equation ( H & O last) SO2 + O SO3 • Add a coefficient in front of the chemical formula to adjust the count and record the new values • Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation • (change coefficient, then change counts)

  10. Classifying Chemical Equations SYNTHESIS – two or more substances combine to make a more complex substance AB + C ABC (synthesize means to put together) DECOMPOSTION - a complex substance breaks down into simpler substances ABC AB + C (decompose means to break down) REPLACEMENT – two elements in two different compounds trade places AB + CD AC + BD (replace means to find a new partner) 2 SO2 + O2 + 2 H2O 2 H2SO4 Synthesis of Acid Rain 2 H2O2 2 H2O + O2 Decomposition of Hydrogen Peroxide 2 CuO+ C 2 Cu + CO2 Isolating Copper by Replacement

  11. Synthesis, Decomposition, or Replacement

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