Chem1211 Stoichiometry of Chemical Reactions (Writing and Balancing Chemical Equations).

Chem1211 Stoichiometry of Chemical Reactions (Writing and Balancing Chemical Equations). 1. What does it mean to say an equation is balanced? Why is it important for an equation to be balanced? Solution An equation is balanced when the same number of each element is represented on the reactant and product sides. Equations must be balanced to accurately reflect the law of conservation of matter. 2. Consider molecular, complete ionic, and net ionic equations. (a) What is the difference between these types of equations? (b) In what circumstance would the complete and net ionic equations for a reaction be identical? Solution (a) Molecular equations are written without regard to the dissociation of any ionic reactants or products, with all formulas represented as neutral substances. Complete ionic equations more realistically represent all dissolved ions.Net ionic equations represent only those dissolved ions that are chemically or physically changed by the reaction, omitting any spectator ions. (b) If there are no spectator ions involved in a reaction, its complete and net ionic equations will be the same. 3. Balance the following equations: (a) PCl ( ) H O( ) POCl ( ) HCl( ) 5 2 3 s l l aq    (b) Cu( ) HNO ( ) Cu(NO ) ( ) H O( ) NO( ) 3 3 2 2 s aq aq l g     (c) H ( ) I ( ) HI( ) 2 2 g s s   (d) Fe( ) O ( ) Fe O ( ) 2 2 3 s g s   (e) Na( ) H O( ) NaOH( ) H ( ) 2 2 s l aq g    (f) 4 2 2 7 2 3 2 2 (NH ) Cr O ( ) Cr O ( ) N ( ) H O( ) s s g g    (g) P ( ) Cl ( ) PCl ( ) 4 2 3 s g l   (h) PtCl ( ) Pt( ) Cl ( ) 4 2 s s g   Solution (a) PCl ( ) H O( ) POCl ( ) 2HCl( ) 5 2 3 s l l aq    ; (b) 3 3 2 2 3Cu( ) 8HNO ( ) 3Cu(NO ) ( ) 4H O( ) 2NO( ) s aq aq l g     ; (c) H ( ) I ( ) 2HI( ) 2 2 g s s   ; (d) 2 2 3 4Fe( ) 3O ( ) 2Fe O ( ) s g s   ; (e) 2 2 2Na( ) 2H O( ) 2NaOH( ) H ( ) s l aq g    ; (f) (NH4 ) 2 Cr 2O7 (s) ¾¾® Cr 2O3 (s) + N2 (g) + 4H2O(g) ; (g) P ( ) 6Cl ( ) 4PCl ( ) 4 2 3 s g l   ; (h) PtCl ( ) Pt( ) 2Cl ( ) 4 2 s s g   4. Balance the following equations: (a) Ag( ) H S( ) O ( ) Ag S( ) H O( ) 2 2 2 2 s g g s l     (b) P ( ) O ( ) P O ( ) 4 2 4 10 s g s   (c) Pb( ) H O( ) O ( ) Pb(OH) ( ) 2 2 2 s l g s    OpenStax Chemistry 4.1: Writing and Balancing Chemical Equations Page 2 of 32 (d) Fe( ) H O( ) Fe O ( ) H ( ) 2 3 4 2 s l s g    (e) Sc O ( ) SO ( ) Sc (SO ) ( ) 2 3 3 2 4 3 s l s   (f) Ca (PO ) ( ) H PO ( ) Ca(H PO ) ( ) 3 4 2 3 4 2 4 2 aq aq aq   (g) Al( ) H SO ( ) Al (SO ) ( ) H ( ) 2 4 2 4 3 2 s aq aq g    (h) TiCl ( ) H O( ) TiO ( ) HCl( ) 4 2 2 s g s g    Solution (a) 2 2 2 2 4Ag( ) + 2H S( ) + O ( ) 2Ag S( ) + 2H O( ) s g g s l  ; (b) P ( ) 5O ( ) P O ( ) 4 2 4 10 s g s   ; (c) 2 2  2 2Pb( ) + 2H O( ) + O ( ) 2Pb OH ( ) s l g s  ; (d) 2 3 4 2 3Fe( ) + 4H O( ) Fe O ( ) + 4H ( ) s l s g  ; (e) 2 3 3 2 4  3 Sc O ( ) + 3SO ( ) Sc SO ( ) s l s  ; (f) 3 4 3 4 2 4     2 2 Ca PO ( ) + 4H PO ( ) 3Ca H PO ( ) aq aq aq  ; (g) 2 4 2 4 2  3 2Al( ) + 3H SO ( ) Al SO ( ) + 3H ( ) s aq aq g  ; (h) TiCl ( ) + 2H O( ) TiO ( ) + 4HCl( ) 4 2 2 s g s g  ] 5. Write a balanced molecular equation describing each of the following chemical reactions. (a) Solid calcium carbonate is heated and decomposes to solid calcium oxide and carbon dioxide gas. (b) Gaseous butane, C4H10, reacts with diatomic oxygen gas to yield gaseous carbon dioxide and water vapor. (c) Aqueous solutions of magnesium chloride and sodium hydroxide react to produce solid magnesium hydroxide and aqueous sodium chloride. (d) Water vapor reacts with sodium metal to produce solid sodium hydroxide and hydrogen gas. Solution (a) CaCO ( ) CaO( ) + CO ( ) 3 2 s s g  ; (b) 4 10 2 2 2 2C H ( ) + 13O ( ) 8CO ( ) + 10H O( ) g g g g  ; (c) 2   MgCl ( ) + 2NaOH( ) Mg OH ( ) + 2NaCl( ) 2 aq aq s aq  ; (d) 2 2 2H O( ) + 2Na( ) 2NaOH( ) + H ( ) g s s g  6. Write a balanced equation describing each of the following chemical reactions. (a) Solid potassium chlorate, KClO3, decomposes to form solid potassium chloride and diatomic oxygen gas. (b) Solid aluminum metal reacts with solid diatomic iodine to form solid Al2I6. (c) When solid sodium chloride is added to aqueous sulfuric acid, hydrogen chloride gas and aqueous sodium sulfate are produced. (d) Aqueous solutions of phosphoric acid and potassium hydroxide react to produce aqueous potassium dihydrogen phosphate and liquid water. Solution (a) 3 2 2KClO ( ) 2KCl( ) + 3O ( ) s s g  ; (b) 2 2 6 2Al( ) + 3I ( ) Al I ( ) s s s  ; (c) 2 4 2 4 2NaCl( ) + H SO ( ) 2HCl( ) + Na SO ( ) s aq g aq  ; (d) H PO ( ) + KOH( ) KH PO ( ) + H O( ) 3 4 2 4 2 aq aq aq l  7. Colorful fireworks often involve the decomposition of barium nitrate and potassium chlorate and the reaction of the metals magnesium, aluminum, and iron with oxygen. (a) Write the formulas of barium nitrate and potassium chlorate. (b) The decomposition of solid potassium chlorate leads to the formation of solid potassium OpenStax Chemistry 4.1: Writing and Balancing Chemical Equations Page 3 of 32 chloride and diatomic oxygen gas. Write an equation for the reaction. (c) The decomposition of solid barium nitrate leads to the formation of solid barium oxide, diatomic nitrogen gas, and diatomic oxygen gas. Write an equation for the reaction. (d) Write separate equations for the reactions of the solid metals magnesium, aluminum, and iron with diatomic oxygen gas to yield the corresponding metal oxides.(Assume the iron oxide contains Fe3+ ions.) Solution (a) Ba(NO3)2, KClO3; (b) 3 2 2KClO ( ) 2KCl( ) + 3O ( ) s s g  ; (c)  3 2 2 2 2Ba NO ( ) 2BaO( ) + 2N ( ) + 5O ( ) s s g g  ; (d) 2Mg(s) + O2 (g) ¾¾® 2MgO(s) 4Al(s) + 3O2 (g) ¾¾® 2Al 2O3 (s) 4Fe(s) + 3O2 (g) ¾¾® 2Fe2O3 (s) 8. Fill in the blank with a single chemical formula for a covalent compound that will balance the equation: Solution H2O 9. Aqueous hydrogen fluoride (hydrofluoric acid) is used to etch glass and to analyze minerals for their silicon content. Hydrogen fluoride will also react with sand (silicon dioxide). (a) Write an equation for the reaction of solid silicon dioxide with hydrofluoric acid to yield gaseous silicon tetra fluoride and liquid water. (b) The mineral fluorite (calcium fluoride) occurs extensively in Illinois. Solid calcium fluoride can also be prepared by the reaction of aqueous solutions of calcium chloride and sodium fluoride, yielding aqueous sodium chloride as the other product. Write complete and net ionic equations for this reaction. Solution (a) 2 4 2 4HF( ) + SiO ( ) SiF ( ) + 2 H O( ) aq s g l  ; (b) complete ionic equation: + – 2+ – + – 2 2Na ( ) + 2F ( ) + Ca ( ) + 2Cl ( ) CaF ( ) + 2Na aq aq aq aq s aq aq  ( ) + 2Cl ( ) , net ionic equation: – 2+ 2 2F ( ) + Ca ( ) CaF ( ) aq aq s  10. A novel process for obtaining magnesium from sea water involves several reactions. Write a balanced chemical equation for each step of the process. (a) The first step is the decomposition of solid calcium carbonate from seashells to form solid calcium oxide and gaseous carbon dioxide. (b) The second step is the formation of solid calcium hydroxide as the only product from the reaction of the solid calcium oxide with liquid water. (c) Solid calcium hydroxide is then added to the seawater, reacting with dissolved magnesium chloride to yield solid magnesium hydroxide and aqueous calcium chloride. (d) The solid magnesium hydroxide is added to a hydrochloric acid solution, producing dissolved magnesium chloride and liquid water. OpenStax Chemistry 4.1: Writing and Balancing Chemical Equations Page 4 of 32 (e) Finally, the magnesium chloride is melted and electrolyzed to yield liquid magnesium metal and diatomic chlorine gas. Solution (a) CaCO ( ) CaO( ) + CO ( ) 3 2 s s g  ; (b) 2  2 CaO( ) + H O( ) Ca OH ( ) s l aq  ; (c) 2 2     MgCl ( ) + Ca OH ( ) Mg OH ( ) + CaCl ( ) 2 2 aq aq s aq  ; (d) Mg OH ( ) + 2HCl( ) MgCl ( ) + 2H O( )  2 2 2 s aq aq l  ; (e) MgCl ( ) Mg( ) + Cl ( ) 2 2 l l g  11. From the balanced molecular equations, write the complete ionic and net ionic equations for the following: (a) K2 C2O4 (aq) + Ba(OH) 2 (aq) ¾¾® 2KOH(aq) + BaC2O4 (s) (b) Pb(NO ) ( ) H SO ( ) PbSO ( ) 2HNO ( ) 3 2 2 4 4 3 aq aq s aq    (c) CaCO ( ) H SO ( ) CaSO ( ) CO ( ) H O( ) 3 2 4 4 2 2 s aq s g l     Solution (a) + 2– 2+ – 2 4 + – 2 4 2+ 2– 2 4 2 4 2K ( ) + C O ( ) + Ba ( ) + 2OH ( ) 2K ( ) + 2OH ( ) + BaC O ( ) (complete) Ba ( ) + C O ( ) BaC O ( ) (net) aq aq aq aq aq aq s aq aq s   (b) 2+ – + 2– 3 4 + – 4 3 2+ 2– 4 4 Pb ( ) + 2NO ( ) + 2H ( ) + SO ( ) PbSO ( ) + 2H ( ) + 2NO ( ) (complete) Pb ( ) + SO ( ) PbSO ( ) (net) aq aq aq aq s aq aq aq aq s   (c) + 2– 3 4 4 2 2 + 2– 3 4 4 2 2 CaCO ( ) + 2H ( ) + SO ( ) CaSO ( ) + CO ( ) + H O( ) (complete) CaCO ( ) + 2H ( ) + SO ( ) CaSO ( ) + CO ( ) + H O( ) (net) s aq aq s g l s aq aq s g l   Chemistry 4: Stoichiometry of Chemical Reactions 4.2: Classifying Chemical Reactions 12. Use the following equations to answer the next five questions: i. H O( ) H O( ) 2 2 s l  ii. + – + – + – Na ( ) + Cl ( ) + Ag ( ) + NO ( ) AgCl( ) + Na ( ) 3 3 aq aq aq aq s aq aq  + NO ( ) iii. CH OH( ) + O ( ) CO ( ) + H O( ) 3 2 2 2 g g g g  iv. 2 2 2 2H O( ) 2H ( ) + O ( ) l g g  v. + – H ( ) + OH ( ) H O( ) 2 aq aq l  (a) Which equation describes a physical change? (b) Which equation identifies the reactants and products of a combustion reaction? (c) Which equation is not balanced? (d) Which is a net ionic equation? OpenStax Chemistry 4.1: Writing and Balancing Chemical Equations Page 5 of 32 Solution (a) i. The transition is from ice to liquid water. (b) iii. Combustion with oxygen generally produces both CO2 and H2O. (c) iii. The balanced equation is 3 2 2 2 2CH OH( ) + 3O ( ) 2CO ( ) + 4H O( ) g g g g  . (d) v. Only reacting ionic species are present. 13. Indicate what type, or types, of reaction each of the following represents: (a) Ca( ) + Br ( ) CaBr ( ) 2 2 s l s  (b) Ca OH ( ) + 2HBr( ) CaBr ( ) + 2H O( )  2 aq aq aq l  2 2 (c) C H ( ) + 9O ( ) 6CO ( ) + 6H O( ) 6 12 2 2 2 l g g g  Solution (a) oxidation-reduction (addition); (b) acid-base (neutralization); (c) oxidation-reduction (combustion) 14. Indicate what type, or types, of reaction each of the following represents: (a) H O( ) + C( ) CO( ) + H ( ) 2 2 g s g g  (b) 3 2 2KClO ( ) 2KCl( ) + 3O ( ) s s g  (c) Al OH ( ) + 3HCl( ) AlBr ( ) + 3H O( )  3 aq aq aq l  3 2 (d)  3 2 4 4 3 2 Pb NO ( ) + H SO ( ) PbSO ( ) + 2HNO ( ) aq aq s aq  Solution (a) oxidation-reduction (combustion); (b) oxidation-reduction; (c) acid-base (neutralization); (d) precipitation 15. Silver can be separated from gold because silver dissolves in nitric acid while gold does not. Is the dissolution of silver in nitric acid an acid-base reaction or an oxidation-reduction reaction? Explain your answer. Solution An oxidation-reduction reaction, because the oxidation state of the silver changes during the reaction. 16. Determine the oxidation states of the elements in the following compounds: (a) NaI (b) GdCl3 (c) LiNO3 (d) H2Se (e) Mg2Si (f) RbO2, rubidium superoxide (g) HF Solution (a) Na +1, I –1; (b) Gd +3, Cl –1; (c) Li +1, N +5, O –2; (d) H +1, Se –2; (e) Mg +2, Si –4; (f) Rb +1; 1 O – 2 ; (g) H +1, F –1