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Chapter 7

Chapter 7. Chemical Reactions. Homework. Assigned Problems (odd numbers only) “Questions and Problems” 7.1 to 7.31 (begins on page 200) “Additional Questions and Problems” 7.41 to 7.49 (page 221) “Challenge Questions” 7.51-7.57 (page 222). Evidence of a Chemical Reaction.

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Chapter 7

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  1. Chapter 7 Chemical Reactions

  2. Homework • Assigned Problems (odd numbers only) • “Questions and Problems” 7.1 to 7.31 (begins on page 200) • “Additional Questions and Problems” 7.41 to 7.49 (page 221) • “Challenge Questions” 7.51-7.57 (page 222)

  3. Evidence of a Chemical Reaction • Physical changes: • Involves no changes in chemical identity of a substance • No changes in physical properties (color, physical state, freezing point, boiling point) • Chemical changes: • A chemical reaction in which one or more substances changes to a different substance • Properties that matter exhibits as it undergoes changes in chemical composition

  4. Evidence of a Chemical Reaction • As a result of the chemical change, a chemical reaction has occurred which produces at least one new substance • Changes that identify chemical reactions (visual clues) • color change • precipitate formation • gas bubbles • flames • heat release

  5. Evidence of a Chemical Reaction A chemical change is a process that causes a change in the chemical composition of a substance • The creation of one or two new substances is characteristic of a chemical change A chemical change occurs when new substances are made • Conversion of material(s) into one or more new substances • These substances will have different properties from the original material

  6. Evidence of a Chemical Reaction Fe Fe2O3 Li LiOH, H2 HCO3-CO2 Na NaOH, H2

  7. The Chemical Equation • Chemical properties determine whether or not a substance can be changed to another substance • Chemical Reactions: • are processes that involve chemical changes in matter resulting in new substances • involve a rearrangement and exchange of atoms to produce new molecules • Elements are not changed during a reaction • Chemists have developed a shorthand way of stating chemical changes on paper: The chemical equation

  8. The Chemical Equation • A chemical equation is a written statement that uses symbols and formulas (or words) to describe the changes during a chemical reaction • It shows substances at the beginning of a reaction (reactants) • It shows substances formed in the reaction (products) • The reactants and products are separated by an arrow Reactants  Products

  9. The Chemical Equation For a chemical equation to be valid: • Only the reactants and products involved in the reaction are shown in the equation • The accurate formulas must be used for each of the substances • The law of conservation of mass must be obeyed: Equal numbers of the same kind of atom on the reactant and product side of the equation

  10. The Chemical Equation • The law of conservation of mass is complied with during a chemical reaction: • Matter is not created nor destroyed during a chemical reaction • At least one new substance is produced but the quantity of matter does not change • Every atom present as a reactant has to be present as a product • The sum of the masses of the products is always equal to the sum of the masses of the reactants

  11. How to Write Balanced Chemical Equations • A balanced chemical reaction has the same number of atoms of each element involved on each side of the equation • A balanced equation is consistent with the law of conservation of mass • Chemical reactions can be written as: • Word equations • Formula equations reactants products

  12. How to Write Balanced Chemical Equations • A balanced chemical reaction has the same number of atoms of each element on both sides of the arrow • Every atom must be accounted for (atoms are neither created nor destroyed) • Equations are balanced by placing a coefficient in front of one or more of the substances in the equation

  13. How to Write Balanced Chemical EquationsSymbols Used in Equations • To convey more information than just the type of chemical involved, symbols are used after a chemical formula to indicate its physical state • (g) = gas • (l) = liquid • (s) = solid • (aq) = aqueous, dissolved in water

  14. How to Write Balanced Chemical Equations • When magnesium metal burns in air it produces a white, powdery compound: magnesium oxide • Burning in air means reacting with O2 • Write the word equation • The reactants are to the left of the arrow • The products are to the right of the arrow • Two or more reactants or products are separated by a plus sign magnesium + oxygen magnesium oxide

  15. How to Write Balanced Chemical Equations • Indicate the physical state of each substance • Use the correct chemical symbol to indicate liquids and solids • Metals are solids, except for Hg which is liquid • Use molecular form for gases (H2, O2, N2, all halogens) • Identify polyatomic ions magnesium(s) + oxygen(g) magnesium oxide(s)

  16. How to Write Balanced Chemical Equations • Convert the word equation into a formula equation • Use the correct chemical symbol to indicate liquids and solids • There must be the same number of each kind of atom on the reactant and product side of the equation • Determine if the equation is balanced • If not equal, must BALANCE ___Mg (s) +___O2 (g) ___MgO(s)

  17. How to Write Balanced Chemical Equations • Unbalanced equations can be adjusted by the use of a coefficient placed to the left of a substance • Coefficients will adjust the number of reactant and/or products molecules or formula units • NEVER change the subscripts of a compound to balance an element • It changes the identity of the compound • Can change coefficients but never subscript numbers

  18. How to Write Balanced Chemical Equations Guidelines when writing balancing chemical equations • Write the unbalanced chemical equation • Pick the element that occurs in only one compound on both sides of the equation • The element that occurs as a free element on either side of the equation, balance it last

  19. 2 2 2 Balancing a Chemical EquationExample 1 2 2 Coefficient 1 Mg 1 Mg 2 O 1 O

  20. Balancing a Chemical Equation: Example 2 • When solid ammonium nitrite is heated it produces nitrogen gas and water vapor • Write the formula equation

  21. 2 4 Balancing a Chemical Equation Example 2 2 2 x N 2 x N 2 x O 1 x O 4 x H 2 x H

  22. Balancing a Chemical Equation: Example 3 • Nitrogen monoxide gas decomposes to produce dinitrogen monoxide gas and nitrogen dioxide gas • Write the formula equation

  23. 3 3 Balancing a Chemical Equation Example 3 3 1 x N 3 x N 1 x O 3 x O

  24. Balancing a Chemical Equation Example 4 • Liquid nitric acid decomposes to reddish-brown nitrogen dioxide gas, liquid water and oxygen gas. • Write the formula equation

  25. 4 4 4 2 2 4 4 7 6 12 12 4 2 Balancing a Chemical Equation Example 4 2 2 2 1 x N 1 x N 3 x O 5 x O 1 x H 2 x H

  26. Aqueous Solutions and Solubility:Compounds Dissolved in Water • An aqueous solution: • A homogeneous mixture of a substance with water • It contains one visible phase, cannot differentiate the various components • Same composition throughout with one substance dissolved into another • A uniform mixture requires complete interaction of the components

  27. Aqueous Solutions and Solubility:Compounds Dissolved in Water • Two parts: solvent and solute • Solute: substance being dissolved • Solvent: substance that dissolves the solvent • Most solutions are liquid but can be gaseous or solid

  28. Aqueous Solutions and Solubility:Compounds Dissolved in Water Water is an effective solvent for dissolving ionic compounds Water is neutral but the O atom is surrounded by many electrons and gives it a partial negative charge The other end which contains the H atom has a partial positive charge The positive ions are attracted to the O atom and negative ions to the H atom of water As an ionic compound dissolves, it becomes surrounded by water molecules

  29. Aqueous Solutions and Solubility:Compounds Dissolved in Water • When sodium chloride crystals are placed in water, they begin to dissolve • The attractive forces between the ions and water are stronger than forces between the ions in the crystal • Water molecules surround each ion, keeping them apart • The ions become uniformly dispersed throughout the solution

  30. K Cl Cl- K+ Aqueous Solutions and Solubility:Compounds Dissolved in Water • When ionic compounds dissolve in water the ions dissociate and separate into ions floating in water • Potassium chloride dissociates in water into potassium cations and chloride anions KCl(aq) = K+ (aq) + Cl- (aq)

  31. Cu SO4 Cu+2 SO42- Aqueous Solutions and Solubility:Compounds Dissolved in Water • Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions CuSO4(aq) = Cu2+(aq) + SO42-(aq)

  32. K+ SO4 K K SO42- K+ Aqueous Solutions and Solubility: Compounds Dissolved in Water • Potassium sulfate dissociates in water into potassium cations and sulfate anions K2SO4(aq) = 2 K+ (aq) + SO42-(aq)

  33. Electrolytes and Nonelectrolytes • Electrolytes • A substance that exists as ions in solution • Formed from an ionic compound that dissociates in water forming a solution with cations and anions • Strong electrolytes are ionic compounds that dissociate (nearly) completely in solution and exist as ions • Strong electrolyte solutions conduct electricity due to the presence of ions

  34. Electrolytes and Nonelectrolytes • Nonelectrolytes • Solute is a molecular substance (not ionic) • It does not form ions in solution (no dissociation) • Substance remains intact, dispersed throughout the solvent as individual molecules • Each molecule is separated by molecules of the solvent • These substances do not conduct electricity

  35. Electrolytes and Nonelectrolytes • Strong electrolyte: • Dissociates completely into ions • Conducts electricity • NaCl, HCl • Weak electrolyte: • Mainly whole molecules • Very few separate (into ions) • Conducts electricity less than strong electrolytes • Acetic acid, HF • Nonelectrolyte: • No dissociation into ions • Do not conduct electricity • Methanol, sucrose

  36. Electrolytes and Nonelectrolytes • Strong electrolytes are completely ionized when dissolved in water • Sodium chloride dissociates to form Na+ and Cl- • Good conductor of electricity

  37. Electrolytes and Nonelectrolytes • Weak electrolytes are only partially ionized when dissolved in water • Hydrofluoric acid only partially dissociates to form H+ and F- • Poor conductor of electricity

  38. Electrolytes and Nonelectrolytes • Nonelectrolyes are not ionized when dissolved in water • e.g. sugar and ethanol do not dissociate into ions in water • Do not conduct electricity

  39. Solubility:Soluble and Insoluble Salts • Not all ionic compounds are soluble in water • If water cannot overcome the ionic forces to separate the ions in solution: insoluble compound • The factors to determine solubility are complex which make predictions difficult • Through observation a series of statements or rules have developed to guide predictions called the solubility rules (See Table 7.2, page 210 and handout)

  40. Solubility Rules • Most compounds that contain NO3- and C2H3O2-ions are soluble in water • Most compounds that contain Li+, Na+, K+, or NH4+ ions are soluble in water • Most compounds that contain Cl-, Br-, I-ions are soluble, except AgX, PbX2, and Hg2X2 X = Cl-, Br-, I- • Most compounds that contain SO42- ions are soluble, except SrSO4, BaSO4, PbSO4, CaSO4

  41. Solubility Rules • Most compounds that contain OH- ions are slightly soluble (will precipitate). Exceptions: NaOH, KOH, are soluble and Ba(OH)2, Ca(OH)2 are moderately soluble • Most compounds that containS2-,CO32-, or PO43- ions are slightly soluble (will precipitate)

  42. Precipitation Reactions • Upon mixing two aqueous solutions, if two ions of an insoluble salt come into contact, a solid forms • This is a precipitation reaction • The solid that forms is called a precipitate • Can use the solubility rules to predict whether a solid will form when two ionic solutions are mixed

  43. Precipitation Reactions • Two aqueous clear solutions are mixed together: lead (II) nitrate and potassium iodide • The reaction between two solutes produces an insoluble product: lead (II) iodide

  44. Predicting Precipitation Reactions • There are no simple (basic) rules to predict which ionic compounds will be insoluble • Experimental observations have led to the guidelines for predicting solubility (see text: table 7.2, page 210 and handout) • These solubility rules will be used to predict the products formed in precipitation reactions

  45. Predicting Precipitation Reactions In solution, the cations of the two reactants exchange anions The chemical formulas are based on the charges on the ions Will the new interactions of cations and anions form an insoluble compound?

  46. Predicting Precipitation Reactions To predict the identities of the products: • Write the formulas of the two compounds being mixed as reactants in a chemical equation Separate all soluble ionic compounds into ions AB +CD A+ B-C+ D- Reactants

  47. Predicting Precipitation Reactions • Write the formulas of the potentially insoluble products that could form from the new combination of the ions • Exchange pairs • Charge balance each compound A+ B- C+ D- A+ D-C+ B- Products

  48. Predicting Precipitation Reactions • Use the solubility rules to determine whether any of the new products are insoluble • If all of the potentially insoluble products are soluble, there will be no precipitate (NO REACTION) • If any of the potential products are insoluble, use symbol (s) to indicate a solid forms • Balance the equations with coefficients

  49. Writing Equations for Precipitation Reactions • Aqueous solutions of calcium chloride and sodium carbonate are mixed • Write the formula of the two compounds • Write the formulas of the potentially insoluble products that could form from the reactants CO32- 2 Cl- 2 Na+ Ca2+ CaCO3 and NaCl

  50. Writing Equations for Precipitation Reactions Use the solubility rules to determine whether any of the new products are insoluble • NaCl is soluble (Rule 2, Rule 3) • CaCO3 is insoluble (Rule 6) Since one of the potential products is insoluble, write the formulas of the products of the reaction

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