Introduction to Chemical Reactions

1. Introduction to Chemical Reactions Making new substances

2. How do you know when a chemical reaction takes place? Color Change Precipitate Formation

3. How do you know when a chemical reaction takes place? Gas Formation Odor

4. How do you know when a chemical reaction takes place? Temperature Change Change in Acidity

5. Representing Chemical Reactions • Chemists observe chemical reactions and have come up with a way to represent or model what is happening. • Making NaCl • Solid Sodium combines with Chlorine gas to make solid Sodium Chloride: 2Na (s) + Cl2 (g)  2NaCl

6. Chemical Equations are different from Numerical Equations • Numerical Equation: 3x + 2y = 47 • Chemical Equation 2Na + Cl2  2NaCl • ReactantA + Reactant B  Product • The reactants are used up in forming the product • The arrow  shows the direction of the reaction

7. Symbols used in Chemical Equations

8. Law of Conservation of Mass • In a chemical reaction, matter is neither created nor destroyed. • Atoms won’t change their identity (e.g. a Carbon atom can’t become an Iron atom) • This means that you have to have the same number of each type of atom on each side of the chemical equation. • Conservation of Mass Video

9. Balancing Equations After you write a chemical equation you have to balance it to make sure that the same number of atoms of each element are on each side. How would you balance this equation? Li + H2O  H2 + LiOH

10. Cl H H H Cl Cl Cl H reactants products reactants products H H Cl Cl Unbalanced and Balanced Equations H Cl Cl Cl H H H2 + Cl2 2 HCl (balanced) (unbalanced) H2 + Cl2 HCl 2 1 2 2 1 2 2 2

11. Meaning of Chemical Formula Chemical Symbol Meaning Composition H2O One molecule of water: Two H atoms and one O atom 2 H2O Two molecules of water: Four H atoms and two O atoms H2O2 One molecule of hydrogen peroxide: Two H atoms and two O atoms

12. Steps to Balancing a Chemical Equation

13. Another Example • CH4 (methane gas) + O2  CO2 + H2O 7 ≠ 6! Where did our atoms go?

14. Example Continued • Change the Coefficients to make the number of atoms of each element equal • Balance the Hydrogens: • CH4 + O2  CO2 + 2 H2O • Balance the Oxygens: • CH4 + 2 O2  CO2 + 2H2O

15. Example Continued • CH4 + 2 O2  CO2 + 2H2O • Are your coefficients in their simplest ratio? • Count your atoms again to check your work:

16. Try These! • C2H6 + O2 CO2 + H2O • Fe2O3 + H2SO4 Fe2(SO4)3 + H2O • Hint : balance the polyatomic ion first! • CaCl2 + AgNO3 AgCl + Ca(NO3)2

17. Review Matter is not destroyed or created Atoms are rearranged in chemical reactions Chemical equations represent chemical reactions You have to have the same number of each type of atom on the left and right hand side of a chemical equation

18. WARNING! Don’t mess with the insides of polyatomic ions – put a square around them, or label them as X – treat the WHOLE polyatomic ion as though it were an element! Don’t ever play around with subscripts (those little numbers that tell you how many atoms are in a molecule) e.g. C6H22O11

19. Types of Chemical Reactions Synthesis (Combination) reaction A + B  AB Decomposition reaction AB  A + B ASingle-replacement reaction A + BC  AC + B element compound compound element Ause activity series to predict AB + CD  AD + CB BDouble-replacement reaction compound compound compound compound Neutralization reaction HX + BOH  BX + HOH acid base salt water CH + O2 CO2 + H2O Combustion reaction (of a hydrocarbon)

21. Synthesis Reaction Direct combination reaction (Synthesis) 2 Na + Cl2 2 NaCl Na Na Cl Cl  Cl Cl Na Na General form: A + B  AB element or element or compound compound compound

22. Synthesis Reaction Direct combination reaction (Synthesis) 2 Na + Cl2 2 NaCl Na Cl Na+ Cl - Cl Cl - Na+ Na General form: A + B  AB element or element or compound compound compound

23. H H H H Decomposition Reaction Decomposition reaction 2 H2O + 2 H2 O2 H O O H + H O O H General form: AB A + B compound two or more elements or compounds

24. Single and Double Replacement Reactions Single-replacement reaction Mg + CuSO4 MgSO4 + Cu General form: A + BC  AC + B Double-replacement reaction CaCO3 + 2 HCl  CaCl2 + H2CO3 General form: AB + CD  AD + CB

25. Ca Activity Series Printable Version of Activity Series Element Reactivity Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2 Cu Hg Ag Pt Au Foiled again – Aluminum loses to Calcium Halogen Reactivity F2 Cl2 Br2 I2

26. Predict if these reactions will occur Al + MgCl2 3 2 2 3 Mg + AlCl3 Can magnesium replace aluminum? YES, magnesium is more reactive than aluminum. Activity Series No reaction Al + MgCl2 Can aluminum replace magnesium? NO, aluminum is less reactive than magnesium. Therefore, no reaction will occur. Activity Series Order of reactants DOES NOT determine how they react. No reaction MgCl2 + Al The question we must ask is can the single element replace its counterpart? metal replaces metal or nonmetal replaces nonmetal.

27. A + BC AC + B Single-Replacement Reactions Activity Series Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2 Cu Hg Ag Pt Au “Magic blue-earth” FeCl2 + Cu Fe + CuCl2 Yes Can Fe replace Cu? Zinc in nitric acid F2 Cl2 Br2 I2 2 Zn + HNO3 Zn(NO3)2 + H2 Yes Can Zn replace H? NO REACTION MgBr2 + Cl2 MgCl2 + Br2 No Can Br replace Cl? General Form

28. Solubility Rules

29. How would you prepare potassium nitrate (using a double replacement reaction)? potassium nitrate formation of water is a driving force. KOH + HNO3 KNO3+H2O H2O + + _________ Both potassium nitrate and calcium chloride are soluble (no driving force – no reaction!) 2 2 Ca(OH)2 KOH Ca(NO3)2 + KNO3 + Combine a potassium hydroxide solution with nitric acid to yield soluble potassium nitrate. KOH(aq) + HNO3(aq) KNO3(aq) + ? H2O The water could then be removed by distillation to recover solid potassium nitrate.

30. Na2CO3 FeCO3 FeCl2 NaCl + + (aq) (ppt) Predict if a reaction will occur when you combine aqueous solutions of iron (II) chloride with aqueous sodium carbonate solution. If the reaction does occur, write a balanced chemical equation showing it. (be sure to include phase notation) Balanced chemical equation iron (II) carbonate iron (II) chloride + sodium carbonate + sodium chloride CO32- CO32- Na1+ Fe2+ Fe2+ Cl1- Cl1- Na1+ CO3 FeCO3 Na2 Cl2 Fe NaCl (aq) (ppt) Using a SOLUBILITY TABLE: sodium chloride is soluble iron (II) carbonate is insoluble 2 (aq) (aq) Complete Ionic Equation Fe2+(aq) + 2Cl1-(aq) + 2Na1+(aq) + CO32-(aq) 2Na1+(aq) + 2Cl1-(aq) + FeCO3(s)

31. Molecular equations • Show all substances as elements or compounds using appropriate formulae • Identify as solid (s), gas (g), liquid (l) or dissolved (aq) • In these equations essential to show states Pb(NO3)2(aq) + 2KI(aq) = 2KNO3(aq) + PbI2(s)

32. Total ionic equations • For all dissolved substances show as constituent ions (except weak acids or other weak electrolytes) • All others show as molecular formula Pb(NO3)2(aq) + 2KI(aq) = 2KNO3(aq) + PbI2(s) Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq) = 2K+(aq) + 2NO3-(aq) + PbI2(s)

33. Net Ionic Equation: • Shows only the reacting species(species which are actively involved in the reaction…the one making the PPT) in the equation. • The net ionicequation is formed by omitting the spectator ions from the total ionic equation. For Example: We had: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) then: Ag+(aq)+ NO3-(aq)+ Na+(aq)+ Cl-(aq) → AgCl(s)+ Na+(aq)+ NO3-(aq) Canceling out spectator ions, we get: Ag+(aq)+ Cl-(aq) → AgCl(s)

34. Net ionic equations • Spectator ions are those ions that do not undergo a change; they do not participate in the chemical change and are the same on both sides of the equation • Remove all spectator ions from the equation Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq) = 2K+(aq) + 2NO3-(aq) + PbI2(s)

35. Let’s try an example putting all these ideas together • Example: • Write a formula equation, a total ionic equation, and a net ionic equation for the reaction which occurs when 0.2 M solutions of Al(NO3)3(aq) and MgS(aq) are mixed.

36. 1. Write the complete formula equation. Determine if there is a precipitate using“(s)” to identify the precipitate. The balanced formula equationis: 2 Al(NO3)3(aq) + 3 MgS(aq) → Al2S3(s) + 3 Mg(NO3)2(aq) 2. Write the complete ionic equation by breaking up the balanced formula equation into ions. Remember: • The precipitate remains in molecular form. • The number of ions is found by multiplying the subscript after the ion, by the coefficient in front of the molecule. • The final equation must still be balanced. We get: 2Al3+ (aq)+6NO3-(aq)+3Mg2+ (aq)+3S2-(aq)  Al2S3(s)+3Mg2+ (aq)+6NO3-(aq) 3. Write the net ionic equation by deleting the spectator ions and simplifying the coefficients if necessary. 2 Al3+ (aq) + 3 S2-(aq) → Al2S3(s)