Chapter 3 Chemical Reactions

1. Chapter 3Chemical Reactions

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4. Chemical Reactions Reactants: Zn + I2 Product: ZnI2

5. Chemical Reactions • Evidence of a chemical reaction: • Gas Evolution • Temperature Change • Color Change • Precipitation (insoluble species forms) • In general, a reaction involves a rearrangement or change in oxidation state of atoms from reactants to products.

6. Chemical Equations Chemical Equations show: • thereactantsandproductsin a reaction. • the relative amounts in a reaction. Example: 4 Al(s) + 3 O2(g)  2 Al2O3(s) • The numbers in the front are called stoichiometric coefficients • The letters (s), (g), (l) and (aq) are the physical states of compounds.

7. Reaction of Phosphorus with Cl2 Notice the stoichiometric coefficients and the physical states of the reactants and products.

8. Reaction of Iron with Cl2 Notice the stoichiometric coefficients and the physical states of the reactants and products.

9. Chemical Equations 4 Al(s) + 3 O2(g)  2 Al2O3(s) This equation states that: 4 Al atoms + 3 O2 molecules react to form 2 formula units of Al2O3 or... 4 moles of Al + 3 moles of O2 react to form 2 moles of Al2O3

10. Chemical Equations Law of the Conservation of Matter • Because the same number of atoms are present in a reaction at the beginning and at the end, the amount of matter in a system does not change. 2HgO(s) 2 Hg(l) + O2(g)

11. Lavoisier, 1788 Chemical Equations • Since matter is conserved in a chemical reaction, chemical equations must be balanced for mass! • In other words, there must be same number of atoms of the each kind on both sides of the equatoin.

12. Balancing Chemical Reactions • Steps in balancing a chemical reaction using coefficients: • Write the equation using the formulas of the reactants and products. Include the physical states (s, l, g, aq etc…) • Balance the compound with the most elements in the formula first using integers as coefficients. • Balance elements on their own last. • Check to see that the sum of each individual elements are equal on each side of the equation. • If the coefficients can be simplified by dividing though with a whole number, do so.

13. Balancing Chemical Equations: Example balance last C2H6 + O2 CO2 + H2O 2 C’s & 6 H’s 2 O’s 1 C & 2 O’s 2 H’s & 1 O

14. Balancing Chemical Equations: Example balance last C2H6 + O2 CO2 + H2O 2 C’s & 6 H’s 2 O’s 1 C & 2 O’s 2 H’s & 1 O balance H first ___C2H6 + O2 CO2 + ___ H2O 3 This side will always have an even # of O-atoms This side has an odd # of O-atoms

15. Balancing Chemical Equations: Example balance last C2H6 + O2 CO2 + H2O 2 C’s & 6 H’s 2 O’s 1 C & 2 O’s 2 H’s & 1 O balance H first ___C2H6 + O2 CO2 + ___ H2O 2 3

16. Balancing Chemical Equations: Example balance last C2H6 + O2 CO2 + H2O 2 C’s & 6 H’s 2 O’s 1 C & 2 O’s 2 H’s & 1 O balance H first ___C2H6 + O2 CO2 + ___ H2O 2 3 balance C next 4 2C2H6 + O2 ___ CO2 + 6H2O

17. Balancing Chemical Equations: Example balance last C2H6 + O2 CO2 + H2O 2 C’s & 6 H’s 2 O’s 1 C & 2 O’s 2 H’s & 1 O balance H first ___C2H6 + O2 CO2 + ___ H2O 2 3 balance C next 4 2C2H6 + O2 ___ CO2 + 6H2O balance O 7 2C2H6 + ____ O2 4CO2 + 6H2O

18. Balancing Chemical Equations: Example balance last C2H6 + O2 CO2 + H2O 2 C’s & 6 H’s 2 O’s 1 C & 2 O’s 2 H’s & 1 O balance H first ___C2H6 + O2 CO2 + ___ H2O 2 3 balance C next 4 2C2H6 + O2 ___ CO2 + 6H2O balance O 7 2C2H6 + ____ O2 4CO2 + 6H2O 4 C’s 12 H’s 14 O’s 4 C’s 12 H’s 14 O’s

19. Balancing Equations ___ Al(s) + ___ Br2(l)  ___ Al2Br6(s)

20. Balancing Equations: Practice ___C3H8(g) + ___ O2(g)  ___ CO2(g) + _____ H2O(g) ___B4H10(g) + ___ O2(g)  ___ B2O3(g) + ___ H2O(g)

21. Balancing Equations: Practice • Solid magnesium hydroxide reacts with hydrochloric acid to form aqueous magnesium chloride and water. • Write the balanced chemical equation for this reaction.

22. Balancing Equations: Practice • Solid magnesium hydroxide reacts with hydrochloric acid to form aqueous magnesium chloride and water. • Write the balanced chemical equation for this reaction. _ Mg(OH)2(s) + _ HCl(aq) 

23. Balancing Equations: Practice • Solid magnesium hydroxide reacts with hydrochloric acid to form aqueous magnesium chloride and water. • Write the balanced chemical equation for this reaction. _ Mg(OH)2(s) + _ HCl(aq)  _ MgCl2(aq) + _ H2O(l)

24. Balancing Equations: Practice • Solid magnesium hydroxide reacts with hydrochloric acid to form aqueous magnesium chloride and water. • Write the balanced chemical equation for this reaction. _ Mg(OH)2(s) + _ HCl(aq)  _ MgCl2(aq) + _ H2O(l) Balance with a coefficient of “2” in front of both HCl and water.

25. Balancing Equations: Practice • Solid magnesium hydroxide reacts with hydrochloric acid to form aqueous magnesium chloride and water. • Write the balanced chemical equation for this reaction. _ Mg(OH)2(s) + _ HCl(aq)  _ MgCl2(aq) + _ H2O(l) Balance with a coefficient of “2” in front of both HCl and water. Mg, Cl, O and H are now balanced.

26. Chemical Equations: Review • What Scientific Principles are used in the process of balancing chemical equations? • What symbols are used in chemical equations: gasses: _____ liquids: _____ solids: _____ aqueous species in solution: _____ • What is the difference between P4 and 4P in an eq.? • In balancing a chemical equation, why are the reactant and product subscripts not changed?

27. Chemical Equilibrium When writing chemical reactions one starts with: Reactants products 2NH3(g) N2(g) + 3H2(g) Some reactions can also run in reverse: 2NH3(g) N2(g) + 3H2(g) Under these conditions, the reaction can be written: Double arrows indicate “Equilibrium”.

28. Chemical Equilibrium Once equilibrium is achieved, reaction continues, but there is no net change in amounts of products or reactants.

29. Classifying Compounds • Salts (ionic compounds): Composed of a metal and non metal element(s). • Acids: Arrhenius definition Produce H+(aq) in water Examples: HCl, HNO3, HC2H3O2 • Bases: Arrhenius definition Produce OH(aq) in water Examples: NaOH, Ba(OH)2, NH3

30. Classifying Compounds • Molecular Compounds: • Covalently bonded atoms, not acids, bases or salts. • Compounds like alcohols (C2H5OH) or table sugar (C6H12O6) • These never break up into ions.

31. Classifying Compounds • Classify the following as ionic, molecular, acid or base.

32. Classifying Compounds • Classify the following as ionic, molecular, acid or base.

33. That which is dissolved (lesser amount) That which is dissolves (greater amount) Reactions in Aqueous Solutions Aqueous Solutions: Water as the solvent Solution = solute +solvent There are three types of aqueous solutions: Those with Strong Electrolytes Those with Weak Electrolytes & those with non-Electrolytes

34. K+(aq) + MnO4-(aq) Reactions in Aqueous Solutions Many reactions involve ionic compounds, especially reactions in water — aqueous solutions. KMnO4 in water

35. Ionic Compounds (CuCl2) in Water

36. Strong Electrolyte When ions are present in water, the solutions conduct electricity! Ions in solution are called ELECTROLYTES Examples of Strong Electrolytes: HCl (aq), CuCl2(aq) and NaCl (aq) are strong electrolytes. These dissociate completely (or nearly so) into ions. Strong Electrolytes conduct electricity well.

37. Strong Electrolytes HCl(aq), CuCl2(aq) and NaCl(aq) are strong electrolytes. These dissociate completely (or nearly so) into ions.

38. Weak Electrolytes Acetic acid ionizes only to a small extent, it is a weak electrolyte. Weak electrolytes exist in solution under equilibrium conditions. The small concentration of ions conducts electricity poorly. Weak electrolytes exit primarily in their molecular form in water.

39. Weak Electrolytes Weak electrolytic solutions are characterized by equilibrium conditions in solution: When acetic acid dissociates, it only partially ionizes. 95% 5% The majority species in solution is acetic acid in its molecular form. When writing a weak electrolyte in solution, one NEVERbreaks it up into the corresponding ions! ×

40. Weak Electrolytes Acetic acid ionizes only to a small extent, so it is a weak electrolyte. CH3CO2H(aq)  CH3CO2-(aq) + H+(aq)

41. Non-Electrolytes Some compounds dissolve in water but do not conduct electricity. They are non-electrolytes. Examples include: • sugar • ethanol • ethylene glycol Non-electrolytes do not dissociate into ions!

42. Species in Solution: Electrolytes Strong electrolytes: Characterized by ions only (cations & anions) in solution (water). Conduct electricity well Characterized by ions (cations & anions) & molecules in solution. Weak electrolytes: Conduct electricity poorly Non-electrolytes: Characterized by molecules in solution. Do not conduct electricity

43. Solutes in Aqueous Solutions

44. Solubility Rules • How do we know if a compound will be soluble in water? • For molecular compounds, the molecule must be polar. • We will discuss polarity later, for now I will tell you whether or not a molecular compound is polar… • For ionic compounds, the compound solubility is governed by a set of SOLUBILITY RULES! • You must learn the basic rules on your own!!!

45. Water Solubility of Ionic Compounds If one ion from the “Soluble Compound” list is present in a compound, then the compound is water soluble.

46. Types of Reactions in a Solution Precipitation Reactions: A reaction where an insoluble solid (precipitate) forms and drops out of the solution. Acid–base Neutralization: A reaction in which an acid reacts with a base to yield water plus a salt. Gas forming Reactions: A reaction where an insoluble gas is formed. Reduction and Oxidation Reactions (RedOx):A reaction where electrons are transferred from one reactant to another.

47. EXCHANGE:Precipitation Reactions EXCHANGE Acid-Base Reactions EXCHANGE Gas-Forming Reactions REACTIONS REDOX REACTIONS

48. Chemical Reactions in Water EXCHANGE REACTIONS The anions exchange places between cations. A precipitate forms if one of the products in insoluble. Pb(NO3) 2(aq) + 2 KI(aq)  PbI2(s) + 2 KNO3 (aq)

49. Precipitation Reactions The “driving force” is the formation of an insoluble solid called a precipitate. Pb(NO3)2(aq) + 2 KI(aq)  2 KNO3(aq) + PbI2(s) BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2 NaCl(aq) Precipitates are determined from the solubility rules.

50. Precipitation Reactions Which species is the precipitate? Pb(NO3)2(?) + 2KI(?)  2KNO3(?) + PbI2(?)