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Chapter 5

Chapter 5. Chemical Reactions. The Nature of Chemical Reactions. Chemical reactions change substances into something new Growing, ripening, decaying, burning, digesting, baking, etc. Occur when substances go through a chemical changes. Ways to know a reaction happened. Change in color

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Chapter 5

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  1. Chapter 5 Chemical Reactions

  2. The Nature of Chemical Reactions • Chemical reactions change substances into something new • Growing, ripening, decaying, burning, digesting, baking, etc. • Occur when substances go through a chemical changes

  3. Ways to know a reaction happened • Change in color • Gas (bubbles) produced • Precipitate formed

  4. Terms & Symbols NaHCO3 + HC2H3O2 CO2 + H20 + NaC2H3O2 The state of the reactants and products is shown by symbols: s = solid l = liquid g = gas aq = aqueous solution

  5. Write the chemical equation • Solid aluminum and liquid bromine yield solid aluminum bromide. • Magnesium metal and water combine to form solid magnesium hydroxide and hydrogen gas

  6. Write the Word Equation • Zn(s) + HCl (aq)  ZnCl2(aq) + H2(g) • NH3(g) + O2(g)  NO2(g) + H2O (l)

  7. Energy & Reactions • Most chemical reactions need energy to get started • Activation energy • Breaking bonds requires energy • Forming bonds releases energy

  8. Exothermic reactions • Energy is

  9. Endothermic Reactions • Energy is

  10. HCl + Mg • What happened? • Reaction is: • HCl + NaHCO3 • What happened? • Reaction is:

  11. Balancing Equations • Law of Conservation of Mass: mass of reactants must = mass of products • If I start with 12 g of Carbon I have to end with 12 g of Carbon • Each side of the equations must contain the same number (mass) of atoms

  12. Balance using COEFFICIENTS • Never ever evereverever change the chemical formulas!!!

  13. Correct: • 2H2 + O2 2H2O • Incorrect: • H2 + O2 H2O2

  14. How to balance • Determine the number of atoms of each element on reactants and products side. • Balance elements one at a time with coefficients. • Make sure the number of atoms on each side of the equation are equal. • Put coefficients in lowest whole number ratio.

  15. Mg + O2 MgO

  16. Cu(SO4) + Al  Al2(SO4)3 + Cu

  17. Na2S + Ag(NO3)NaNO3 + Ag2S

  18. C2H6 + O2 CO2 + H2O

  19. Reaction Types • There are 5 main types of reaction: • Combination • Decomposition • Combustion • Single Replacement • Double Replacement

  20. Combination Reactions • 2 or more reactants join to form a product • H2(g) + O2(g) H2O(l) • Na(s) + Cl2(g) NaCl(s)

  21. Decomposition Reaction • Stuff is broken apart • H2O(l) H2(g) + O2(g)

  22. Combustion • Reactants always: Carbon cmpd. + O2 • Products always: CO2 + H2O • Burning is a combustion reaction! • CH4(g)+ O2(g) CO2(g) + H2O(l) • C6H12O6(s)+ O2(g)

  23. Single Replacement • Elements trade places • 3CuCl2(aq) + 2Al(s) 2AlCl3(aq)+ 3Cu (s) • Cl2(g)+ KBr(l)

  24. Activity Series Li K Ba Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Hg Ag Au • An element will replace another element only if it is BELOW it! • Na + AlCl3 • Cu + ZnCl2 F Cl BrI

  25. Double Replacement • cations switch places • Pb(NO3)2(aq) + 2KI(aq) PbI2(s)+ 2K(NO3)(aq) • NaCl(aq) + Ag(NO3)(aq) AgCl(s) + NaNO3 (aq)

  26. Reaction Rates & Equilibrium

  27. Ways to increase reaction rates • Increase temperature of reactants • Increase concentration of reactants • Add a catalyst – increases rate without taking part in the reaction

  28. A catalyst is not a reactant or a product, it is written above the arrow in a reaction • The decomposition of hydrogen peroxide is catalyzed by manganese dioxide • 2H2O2 --------> H2O + O2

  29. Activation energy (Ea) = the minimum amount of energy needed for a reaction to occur • A catalyst works by lowering activation energy Exothermic or endothermic?

  30. Equilibrium Systems • Some reactions are reversible • The products can react to form the reactants • Reversible reactions consist of two reactions happening at the same time! • H2 + F2 2HF What two reactions are happening?

  31. Chemical equilibrium – when the rate of the forward reaction = the rate of the reverse reaction • Amount of each reactant and product remains consistent over time • Reaction doesn’t stop!!!

  32. Does not mean equal amounts of reactants and products are present • No NET change is taking place

  33. Le Chatelier’s principle – if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress

  34. Add reactant & more product is formed • Remove product & more product is formed

  35. Adding energy • Endothermic = more products • Exothermic = more reactant

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