Chapter 8.1 : Describing Chemical Reactions


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Chapter 8.1 : Describing Chemical Reactions

  1. 1. Chemical equations and reactions<br />Chapter 8.1<br />
  2. 2. Objectives:<br />List three observations that suggest that a chemical reaction has taken place.<br />List three requirements for a correctly written chemical equation.<br />Write a word equation and a formula equation for a given chemical reaction.<br />Balance a formula equation by inspection.<br />
  3. 3. Describing Chemical Reactions<br />Chemical reaction<br />Defined as the process by which one or more substance are changed into one or more different substances.<br />Reactants: original substances<br />Products: resulting substances<br />According to law of conservation of mass:<br />total mass of reactants must equal total mass of products<br /><ul><li>Chemical equation
  4. 4. Defined as represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.
  5. 5. Example: this chemical equation shows the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide, and water</li></ul>(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)<br />
  6. 6. Indications of Chemical Reactions<br />Evolution of heat and light<br />Production of gas<br />Formation of a precipitate<br />Color change<br />Chemistry comes alive!<br />
  7. 7. Characteristics of Chemical Equations<br />Equation MUST represent known facts.<br /><ul><li>Must be actual chemical reaction</li></ul>MUST contain the correct formulas for the reactants and products.<br /><ul><li>Sodium Carbonate </li></ul>Na2CO3<br />The law of conservation of mass must be satisfied.<br /><ul><li>Same number of atoms on reactant side as on the product side.
  8. 8. Coefficients are added where necessary
  9. 9. Specifies relative number of moles of that substance</li></ul>Ex: Hydrogen gas reacts with oxygen gas to yield water<br /> H2 + O2 H2O<br />2<br />2<br />
  10. 10. 2<br /> H2 + O2 H2O<br />2<br />H – O <br />H – O <br />H <br />H <br />H - H<br />O = O<br />H - H<br />4 H’s<br />2 O’s<br />2 O’s<br />4 H’s<br /><ul><li> Satisfies the law of conservation of mass
  11. 11. Same atoms in reactants as the products</li></li></ul><li>Word and Formula Equations<br />Word equation - 1st step in writing chemical equation<br />Equation in which the reactants and products in a chemical reaction are represented by words<br />Only qualitative meaning (doesn’t give quantities)<br />Example: methane + oxygen carbon dioxide + water<br /> read as react to yield or yield (produce or form)<br /><ul><li> Equation is read as “methane and oxygen yield carbon dioxide and water”
  12. 12. formula equation - 2ndstep in writing chemical equation
  13. 13. Represents the reactants and products of a chemical reaction by their symbol
  14. 14. Example:
  15. 15. (g) represents that it is in gaseous state</li></ul>CH4 (g) + O2 (g) CO2 (g) + H2O (g)<br />
  16. 16. balance the equation– last step in writing a chemical equation<br />Satisfy law of conservation of mass by inserting coefficients<br /> # of atoms of reactant must = # of atoms of products<br />2<br />2<br />Reactant<br />Product<br />Step 1:<br />Type & number<br />C<br />C<br />1<br />1<br />Step 3:<br />When reactants = products its balanced<br />H<br />H<br />4<br />2<br />4<br />Step 2:<br />Add coefficients till reactants = products<br />O<br />O<br />2<br />3<br />4<br />4<br />CH4 (g) + O2 (g) CO2 (g) + H2O (g)<br />
  17. 17. Additional symbols used in chemical reactions<br />Examples:<br /> Yields<br /> Reversible reactions<br /> (s) or solid state<br /> (g) or gaseous state<br /> (l) liquid state<br /> (aq) aqueous (dissolved in water)<br /> reactants are heated<br />2 atm Pressure<br /> 0⁰ C Temperature<br /> MnO2 Catalyst<br />3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)<br />2HgO(s) 2Hg(l) + O2(g)<br />C2H2(s) + H2(g) pressure C2H6(g)<br />Pt<br />
  18. 18. Significance of a Chemical Equations<br />Coefficients of a chemical reaction indicate relative, not absolute, amount of reactants and products<br /><ul><li>H2(g) + Cl2(g) 2HCl(g)</li></ul> 1 molecule H2; 1 molecule Cl2; 2 molecules HCl<br /><ul><li>Shows smallest possible relative amounts</li></ul>Relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients<br />= 2.02 g H2<br />= 70.90 g Cl2<br />= 72.92 g HCl<br />1 mol H2 x 2.02 g H2<br />1 mol Cl2 x 70.90 g Cl2<br />2 mol HCl x 36.46 g HCl<br />1 mol H2<br />1 mol Cl2<br />1 mol HCl<br />
  19. 19. Reverse reaction for a chemical reaction has the same relative amounts of substances as the forward reaction<br />Balancing Chemical Equations<br />Identify the names of the reactants and the products, and write a word equation.<br />water hydrogen + oxygen<br />Write a formula equation by substituting correct formulas for the names of the reactants and the products.<br />H2O(g) H2(g) + O2(g)<br />
  20. 20. Balance the formula equation according to the law of conservation of mass.<br /><ul><li>Balance the different types of atoms one at a time.
  21. 21. First balance the atoms of elements that are combined and that appear only once on each side of the equation
  22. 22. Balance polyatomic ions that appear on both sides of the equation as single units.
  23. 23. Balance H atoms and O atoms after atoms of all other elements have been balanced</li></ul>H2O(g) H2(g) + O2(g)<br />2<br />2<br />Product<br />Reactant<br />H<br />H<br />2<br />2<br />4<br />4<br />O<br />O<br />1<br />2<br />2<br />Count atoms to be sure that the equation is balanced.<br />
  24. 24. Most common mistakes!!!!<br />Writing incorrect formulas for reactants or products<br />Trying to balance using subscripts<br /><ul><li>Can only use coefficients</li></ul>Practice 1<br />Write the word and formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in the water). Include symbols for physical states in the formula equation. Then balance the formula equation to give a balanced chemical equation.<br />Step 1: write word equation<br />Sodium oxide + water sodium hydroxide<br />
  25. 25. Step 2: convert to formula equation<br />Step 3: add all symbols<br />2<br />Na2O + H2O NaOH<br />(s) <br />(l) <br />(aq) <br />Step 4: balance<br />Reactant<br />Product<br />Na<br />2<br />1<br />Na<br />2<br />2<br />H<br />1<br />2<br />H<br />2<br />O<br />1<br />2<br />O<br />Practice 2<br />Translate the following chemical reaction<br />PbCl2(aq) + Na2CrO4(aq) PbCrO4(s) + 2 NaCl(aq)<br />Aqueous lead(II) chloride<br /> reacts with<br /> aqueous sodium chromate<br />to produce<br />a precipitate of lead(II) chromate<br />and aqueous sodium <br />chloride.<br />
  26. 26.
  27. 27. Practice 3<br />The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write the balanced chemical equation for the reaction.<br />zinc + hydrochloric acid zinc chloride + hydrogen<br />Zn(s) + <br />HCl(aq)<br />ZnCl2(aq)<br /> + H2(g)<br />2<br />Reactant<br />Product<br />Zn<br />1<br />1<br />Zn<br />1<br />H<br />2<br />2<br />H<br />2<br />1<br />Cl<br />2<br />Cl<br />
  28. 28. Practice 4<br />Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.<br />Aluminum carbide + <br />water<br />+ aluminum hydroxide<br /> methane<br />3<br />4<br />Al4C3(s) + <br /> H2O(l)<br />+ Al(OH)3(s)<br /> CH4(g)<br />12<br />Reactant<br />Product<br />4<br />4<br />1<br />Al<br />Al<br />3<br />3<br />1<br />C<br />C<br />24<br />24<br />7<br />15<br />2<br />H<br />H<br />12<br />1<br />3<br />12<br />O<br />O<br />
  29. 29. Practice 5<br />Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate. These products settle out, taking suspended solid impurities with them. Write a balanced chemical equation for the reaction.<br />Aluminum sulfate + <br />Calcium hydroxide<br /> + calcium sulfate<br /> aluminum hydroxide <br />Al2(SO4)3 (aq) + <br />2<br />3<br />3<br />Ca(OH)2(aq)<br />+ CaSO4(s)<br /> Al(OH)3(s)<br />Reactant<br />Product<br />2<br />1<br />2<br />Al<br />Al<br />3<br />1<br />3<br />SO4<br />SO4<br />3<br />3<br />1<br />1<br />Ca<br />Ca<br />6<br />2<br />3<br />6<br />OH<br />OH<br />