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Chemical Reactions: Describing and Balancing Equations

Learn how to write word and skeleton equations, find formulas, and balance chemical equations in various types of reactions.

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Chemical Reactions: Describing and Balancing Equations

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  1. Chapter 7 CHEMICAL REACTIONS

  2. Reactions happen everyday in and around us!! Digesting food Photosynthesis Baking cookies

  3. All chemical reactions involve changing substances!! • Word Equations • Describing a chemical reaction using words Reactants  Products water  hydrogen + oxygen

  4. Practice!! • Write word equations for the following: • Pure copper can be produced by heating copper (II) sulfide with oxygen. Sulfur dioxide is also produced in this reaction. Copper (II) sulfide + Oxygen  Copper + Sulfur dioxide

  5. Describing Chemical Change • Formula or Skeleton Equation • Translate the words to formulas • Skeleton equations do not indicate the relative amounts of the reactants and products water  hydrogen + oxygen H2O  H2 + O2

  6. To write an equation, you must write the correct formula first!!Finding Formulas Review • Elements • Is it an atom or a diatomic molecule (HON FClBrI)? • Atom  single atom • Silver • Ag • Diatomic  subscript 2 • Chlorine • Cl2

  7. Finding Formulas Review • Compounds • Is it molecular (nonmetals) or ionic (metal or ammonium)? • Molecule  Translate prefix • Carbon dioxide • CO2 • Ionic  find charges, criss-cross reduce • Barium nitrate • Ba2+ and NO3- • Ba(NO3)2

  8. Finding Formulas Review • Common names • Water • H2O • Ammonia • NH3 • Salt • NaCl • Sugar • C12H22O11

  9. heat Symbols Used in Chemical Equations + (s) (l) (g) (aq) Used to separate two reactants or two products “Yields” separates reactants from products reversible reaction solid liquid gas dissolved in water heat must be added to reaction catalyst added Pt

  10. Practice!! • Write a skeleton equation for the following: • When calcium carbonate is heated, calcium oxide and carbon dioxide are produced. CaCO3 (s) CaO (s) + CO2 (g) When solid mercury (II) sulfide is heated with oxygen, liquid mercury metal and gaseous sulfur dioxide are produced HgS (s) + O2 (g) Hg (l) + SO2 (g)

  11. Balancing Equation Rules • Make sure you have the right formulas. • Balance the atoms by changing the coefficients. • Start with atoms that appear only once on each side. Usually you will do hydrogen and oxygen last. • If a polyatomic ion is on both sides of the equation, treat it as one unit and balance it as a single unit. • Balancing tricks • If you are having trouble getting it to balance, double the first compound and try again. • Double check to make sure all atoms and ions are balanced and the coefficients are in the lowest possible ration.

  12. Make sure you have the right formulas. ___H2 + ___O2___H2O 2 1 2 ___H2 + ___O ___H2O 1 1 1

  13. Balance the atoms by changing the coefficients. ___H2 + ___O2___H2O 2 2 Can’t erase the 2 Can’t add a 2

  14. Start with atoms that appear only once on each side. Usually you will do hydrogen and oxygen last. ___H2SO4 + ___NaOH ___Na2SO4 + ___H2O 1 2 1 2 • Do Na and S first. 2. Do H next. 3. Do O last.

  15. If a polyatomic ion is on both sides of the equation, treat it as one unit and balance it as a single unit. 2 ___Mg + ___AgNO3___Mg(NO3)2 + ___Ag 2 The anion, NO3 , is on both sides 2 3 3 2 ___H3PO4 + ___(NH4)2CO3___H2CO3 + ___(NH4)3PO4 NH4 , CO3 , and PO4 appear on both sides

  16. If you are having trouble getting it to balance, double the first compound and try again. ___C2H6 + ___O2___CO2 + ___H2O 2 3 How can you make this = 7 ? 4 + 3 = 7 ___C2H6 + ___O2___CO2 + ___H2O 2 7 4 6 8 + 6 = 14

  17. 6. More Practice! ___Pb(OH)4 + ___H2SO4___Pb(SO4)2 + ___H2O

  18. 7. Double check to make sure all atoms and ions are balanced and the coefficients are in the lowest possible ratio. ___H2 + ___O2___H2O 4 2 4 ___H2 + ___O2___H2O 2 1 2

  19. Five Reaction Types There are five different types of reactions that we will study! • Synthesis • Decomposition • Combustion • Single Replacement • Double Replacement

  20. Five Reaction Types • Synthesis (combination) Reactions - 2 or more substances combining to make a single product. A + B  AB 2K + Cl2 2KCl CaO + H2O  Ca(OH)2 Mg + O2

  21. + Synthesis

  22. Five Reaction Types • Decomposition Reaction - single reactant breaking down into simpler substances. AB  A + B 2H2O  2H2 + O2 CaCO3 CaO + CO2 Ag2O 

  23. Decomposition

  24. Five Reaction Types • Combustion Reactions - an element or compound reacting with oxygen(O2), often producing heat or light. A + O2(g) AO or CO2 + H2O • Element Combustion - also a combination reaction. 2Mg(s) + O2(g) 2MgO(s) • Hydrocarbon Combustion - oxygen reacting with a substance containing hydrogen and carbon. In complete hydrocarbon combustion, the products will always be CO2 and H2O. 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O(g) C3H8(g) + O2(g)

  25. Combustion

  26. Five Reaction Types • Single Replacement Reaction - atoms of an element replacing the atoms of a second element in a compound. Only occurs if the elemental atoms are more reactive (chemically active) than the atoms they are attempting to replace in the compound. A + BX  B + AX X + AY  Y + AX

  27. Single Replacement Copper is added to silver nitrate

  28. Five Reaction Types Single Replacement cont. • Li is higher (more reactive) than Na atoms so the following reaction would occur. Li + NaOH  Na + LiOH • Zn is lower (less reactive) than Al and is therefore not active enough to replace the Al in the compound and a reaction would NOT occur. Zn + AlCl3 NR (No Reaction) Activity Series Li K Ca Na Mg Al Zn

  29. Five Reaction Types Single Replacement cont. Mg + LiNO3 Na + Zn(NO3)2 Br2 + NaI  Activity Series Li K Ca Na Mg Al Zn

  30. Double Replacement

  31. Five Reaction Types • Double replacement - exchange of two positive ions between two compounds. AX + BY  AY + BX A and B are trading places Na2S(aq) + Cd(NO3)2(aq) CdS(s) + 2NaNO3(aq) Ca(OH)2(aq) + HCl(aq)

  32. Practice! • Li3N + NH4NO3 • HBr + Al(OH)3 

  33. Predicting ProductsTo predict the products of a chemical reaction, you must recognize the possible type of reaction that the reactants can undergo! • Hydrocarbon Combustion - Are O2 and a hydrocarbon the reactants?  CO2 and H2O • Decomposition - Only 1 reactant  break apart into elements • Synthesis - Reactants are both elements?  combine to make one product • Single replacement - Reactants an element (not O2) and a compound  check the activity series, replace like ions or NR • Double Replacement - Reactants are both compounds?  trade the like ions

  34. Steps to finding net ionic equations • Predict Products • Balance • Find states (not soluble= (s), soluble = (aq)) • Write ionic equations (separate aqueous into ions) • Write net ionic equation by canceling spectator ions (can be NR)

  35. Net Ionic Equations • AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) • We can write this equation more realistically if we recognize that most ionic compounds dissociate or separate into cations and anions when they dissolve in water. • Complete Ionic Equation- An equation that shows dissolved ionic compounds as their free ions. • Ag+(aq) + NO3-(aq)+Na++Cl-  AgCl(s)+Na+(aq)+ NO3- (aq) • This equation can be simplified and made more useful by eliminating the ions that do not participate in the reaction. You do this by canceling the ions that appear on both sides of the equation.

  36. Net Ionic Equations • Spectator Ions- Ions that are not directly involved in a reaction. • What are the spectator ions in the previous problem? • Rewrite the equation leaving out the spectator ions. • Net Ionic Equation- indicates only the particles that actually take part in the reaction. Ag+ (aq) + Cl- (aq)  AgCl (s)

  37. Net Ionic Equations • In writing balanced net ionic equations, you must also balance the electric charge. • What is the net ionic equation of the following? (First write the complete ionic equation) Pb (s) + AgNO3 (aq)  Ag (s) + Pb(NO3)2 Pb (s) + Ag+ (aq)  Ag(s) + Pb2+ (aq) Although this equation is balanced with respect to atoms, the electrical charges do not balance. Pb (s) + 2Ag+ (aq)  Ag(s) + Pb2+ (aq)

  38. Solubility Rules and Activity Series • SOLUBILITY RULES •  1.      Alkali metal compounds, acetates, nitrates, and ammonium compounds are all soluble.  •  2.      Hydroxides of alkali metals and NH4+1, Ca+2, Sr+2, and Ba+2 are soluble.  All others are insoluble. •  3.      All halides (chlorides etc.) are soluble except for those containing Ag+1, Pb+2, and Hg2+2. •  4.      Most sulfates are soluble, except for BaSO4, SrSO4, Ag2SO4, PbSO4, and CaSO4. •  5.      Most phosphates, carbonates, chromates and sulfides are insoluble (except those of the alkali metals and ammonium).  •  6.      In addition, all acids are soluble!

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