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Chemical Reactions

Chemical Reactions. CH 4. Reactions involve chemical changes in matter that result in new substances. Reactions involve rearrangement and exchange of atoms to produce new molecules. Reactants  Products CH 4 + 2O 2 → CO 2 + 2H 2 O. O 2. CO 2. H 2 O. Evidence of Chemical Reactions.

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Chemical Reactions

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  1. Chemical Reactions CH4 • Reactions involve chemical changes in matter that result in new substances. • Reactions involve rearrangement and exchange of atoms to produce new molecules. • Reactants  Products CH4 + 2O2→ CO2 + 2H2O O2 CO2 H2O

  2. Evidence of Chemical Reactions • A chemical change occurs when new substances are made. • Visual clues (permanent): • Color change, precipitate formation, gas bubbles, flames, heat release, cooling, light • Other clues: • New odor, permanent new state

  3. Evidence of Chemical Reactions (cont.)

  4. Chemical Equations • Shorthand way of describing a reaction • Provides information about the reaction: • Formulas of reactants and products • States of reactants and products • Relative numbers of reactant and product molecules that are required • CH4(g)+ 2O2(g) → CO2(g) + 2H2O(l) • Can be used to determine weights of reactants used and of products that can be made

  5. Symbols Used in Equations • Symbols used after chemical formula to indicate state: • (g) = gas; (l) = liquid; (s) = solid • (aq) = aqueous, dissolved in water • e. g. NH3(aq) indicates ammonia dissolved in water

  6. Conservation of Mass • Matter cannot be created or destroyed. • In a chemical reaction, all the atoms present at the beginning are still present at the end. • Therefore, the total mass cannot change.

  7. O H H O + + C O O C H H H H O 1 C + 4 H + 2 O 1 C + 2 O + 2 H + O 1 C + 2 H + 3 O Combustion of Methane • Methane gas burns to produce carbon dioxide gas and liquid water CH4(g) + O2(g)  CO2(g) + H2O(l)

  8. O O O O H H H H + + + C C + H H O O O O H H 1 C + 4 H + 4 O 1 C + 4 H + 4 O Combustion of Methane Balanced • To show a reaction obeys the Law of Conservation of Mass, it must be balanced. CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)

  9. Writing Equations • Use proper formulas for each reactant and product. • Proper equation should be balanced. • Obey Law of Conservation of Mass. • All elements on reactants side also on product side. • Equal numbers of atoms of each element on reactant side as on product side. • Balanced equations show the relationship between the relative numbers of molecules of reactants and products. • Can be used to determine mass relationships

  10. Balancing Chemical Equations • If the equation is in words, write the formulas for the reactants and products (include physical states). • Balance the equation by counting the atoms on both sides of the equation and changing the coefficients as needed. Never change the subscripts! • This is done by trial and error. Start with the most complicated compound first. • The best balanced equation is the one with the smallest integer coefficients (not fractions).

  11. Balance the following equations: • Solid potassium reacts with gaseous nitrogen to form solid potassium nitride. • C3H8(g) + O2(g) → CO2(g) + H2O(l) • CuSO4(aq) + KI(s) → CuI(s) + I2(s) + K2SO4(aq)

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