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Chemical Reactions

Chemical Reactions. Chemical Equations. The starting substances are called reactants and the substances formed are called products . Na + Cl 2  NaCl. A+B  AB. PRODUCTS. REACTANTS. Describing Chemical Reactions. Writing Equations. 2H 2 ( g ) + O 2 ( g )  2H 2 O( g ).

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Chemical Reactions

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  1. Chemical Reactions Chemical Equations

  2. The starting substances are called reactantsand the substances formed are called products. Na + Cl2 NaCl A+B AB PRODUCTS REACTANTS

  3. Describing Chemical Reactions

  4. Writing Equations 2H2(g) + O2(g)  2H2O(g) • Identify the substances involved. • Use symbols to show: • How many? - coefficient • Of what? - chemical formula • In what state? – (physical state) • Remember the diatomic elements.

  5. Law of Conservation of Mass • The Law of Conservation of Mass states that the mass of all substances present before a chemical change is equal to the mass of all substances after the change. 10 g 10 g Before Reaction After Reaction

  6. Law of Conservation of Mass Sometimes the mass of the reactants and the mass of the products appears to beunequal. This is usually because some of the mass is lost as a gas. Wood + Fuel ash + released gas 99g + 1 g = 99.5g + 0.5g Total Mass = 100g Total Mass = 100g Although it appears mass is lost, 0.5g of mass is released into the air. The sum on both sides of the reaction is equal.

  7. Balancing Equations

  8. Balancing Steps 1. Write the unbalanced equation. 2. “Take Inventory”- Count atoms on each side. 3. Add coefficients to make #s equal. Coefficient  subscript = # of atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

  9. Helpful Tips Balance one element at a time. Update “inventory” after adding a coefficient. If an element appears more than once per side, balance it last. Balance polyatomic ions as single units. “1 SO4” instead of “1 S” and “4 O”

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