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Chapter 21

Chapter 21. Chemical Reactions (Rxns). Sec. 1 Chemical Changes. Chemical Reaction —a change in which 1 or more substances are converted into new substances. Reactants —substances that react or the starting substances. Products —new substances produced.

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Chapter 21

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  1. Chapter 21 Chemical Reactions (Rxns)

  2. Sec. 1 Chemical Changes • Chemical Reaction—a change in which 1 or more substances are converted into new substances. • Reactants—substances that react or the starting substances. • Products—new substances produced. • Ways to tell if a chemical reaction has occurred: • Odor, color change, change in heat, bubbles

  3. Conservation of Mass • Law of Conservation of Mass—mass cannot be created nor destroyed. • The mass of reactants is always equal to the mass of products. • Ex. 0.7g O2 + 9.3g Hg  10.0g HgO2

  4. Chemical Equations • Chemical equation—a way to describe a chemical rxn using chemical formulas and other symbols. • Reactants are written on the left, products on the right. • Reactants  Products • The  is read as “yields” or “produces”

  5. States of Matter & Coefficients • The physical state of the reactants and products is written after the formula in parentheses. • Solid (s) • Liquid (l) • Gas (g) • Aqueous (aq): a substance dissolved in water. • Coefficients (#s to the left of formulas) tell the # of units of each substance in the rxn.

  6. Writing Equations • A chemical reaction can be written in words • Ex. One unit of solid zinc plus 2 units of hydrochloric acid dissolved in water produce one unit of hydrogen gas plus one unit of zinc chloride. • You can use chemical formulas & symbols in the chemical equation. • Ex. Zn(s) + 2HCl(aq)  H2(g) + ZnCl2(aq)

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  8. Balancing Chemical Equations • Determine the formulas for all reactants and products. (this may be given) • Write reactants on the left and products on the right of the arrow. • Count the # of each element in reactants & products Ex. CH4 + O2 CO2 + H2O C = 1 C = 1 H = 4 H = 2 O = 2 O = 3

  9. Balancing Chemical Equations 4. Balance each element by using coefficients—number placed in front of the symbol. 5. Make sure all elements are balanced. 6. Make sure all coefficients are in the lowest ratio. Ex. CH4 + O2 CO2 + H2O C = 1 C = 1 H = 4 H = 2 O = 2 O = 3 ____ CH4 + ____ O2 ____ CO2 + ____ H2O 2 2

  10. Types of Chemical Rxns

  11. Combination/Synthesis Rxn • 2 or more reactants combine to form a single product. • Examples: • 2K(s) + Cl2(g) 2KCl(s) • CaO(s) + H2O(l) Ca(OH)2(aq)

  12. Decomposition Rxn • A single compound is broken down into 2 or more products. • Examples: • PbO2(s) Pb(l) + O2(g) • CaCO3(s)  CaO(s) + CO2(g)

  13. Single-Replacement Rxn • One element replaces a second element in a compound. • Example: • Mg(s)+2AgNO3(aq)Mg(NO3)2(aq)+2Ag(s)

  14. Double-Replacement Rxn • Involve an exchange of positive ions between 2 ionic compounds. • Examples: • Na2S + Cd(NO3)2 CdS + 2NaNO3 • 2NaCN + H2SO4 2HCN + Na2SO4

  15. Combustion Rxn • An element or compound reacts with oxygen (O2) often producing CO2 & H2O • Heat&lightare also often given off • Example: • 2CH3OH(l) + 3O2(g)2CO2(g) + 4H2O(g)

  16. Completing Chem Rxns • Some steps for doing reactions • Identify the type of reaction • Predict the product(s) • Balance the equation using COEFFICIENTS • Remember: some elements are diatomic (exist as 2 atoms) • Hydrogen: H2, Nitrogen: N2, Oxygen: O2, Fluorine: F2, Chlorine: Cl2, Bromine: Br2, Iodine: I2

  17. When Predicting Products • When predicting products, remember to write the formulas correctly—DON’T FORGET TO BALANCE OUT THE CHARGES! • Ex.Li + CuSO4  • Type of Rxn:Single Replacement • Li replaces Cu & bonds with SO4 • Look at charges of ions: Li+1SO4-2 • Products are… Li2SO4 + Cu

  18. Activity Series for Metals • In Single-Replacement, not all metals elements can replace other metals. • The single metal element must be more reactive than the one in the compound. • Look on P. 748 in textbook for the activity series WRITE THIS DOWN!!!

  19. State the type of reaction, Predict the products & Balance the equation. Type of Rxn Reactants Products 1. _____________ __BaCl2 + __H2SO4 2. _____________ __C6H12 + __O2  3. _____________ __Zn + __CuSO4  4. _____________ __Cs + __Br2  5. _____________ __FeCO3  __BaSO4 + __HCl 2 Double-Replacement __CO2 + __H2O 6 6 Combustion 9 __ZnSO4 + __Cu Single-Replacement Balanced!!! __CsBr Combination 2 2 __FeO + __CO2 Decomposition Balanced!!!

  20. Romance analogy for the four types of reaction • CombinationFalling in love • DecompositionThe Break-up • Single ReplacementThe player • Double Replacement The swingers

  21. Sec. 4 Chem Rxns & Energy • Some chemical rxns require heat in order to happen • Some rxns give off heat. • Thermochemistry—heat changes that occur during chemical reactions.

  22. Exo & Endo • Exothermic—a rxn where energy (heat) is given off. • Ex: burning wood, exploding dynamite. • Exothermic rxns feel warm. • Endothermic—a rxn where energy (heat) is absorbed. • Ex. Cold packs • Endothermic rxns feel cold.

  23. Catalysts & Inhibitors • Catalyst—a substance that speeds up a chem rxn without being used in the rxn. • Written above the arrow in an equation. • Used in making plastics MnO2 • H2O + O2 H2O2 Heat as a catalyst • Inhibitors—prevent certain rxns from occurring. • Used in food preservatives to prevent spoiling.

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