Chapter 7 Chemical Reactions

1. Chapter 7 Chemical Reactions Integrated Chemistry and Physics

2. 7.1 Chemical Equations • The substances involved in a chemical reaction before the change occurs are called the reactants • The new substances formed after the change are called the products • We say that reactants yield products

3. 7.1 Using Equations to Represent Reactions • Reactants  Products • C(s) + O2(g)  CO2(g) • 1 carbon reacts • with 1 oxygen molecule • yields 1 molecule of carbon dioxide • The letters behind the chemical indicates whether the substance is solid (s) or (cr), liquid (l), aqueous solution (aq), or gas (g).

4. 7.1 Writing Chemical Equations • A chemical equation is a shorthand for the expression of the reaction. • For example: Hydrogen gas reacts with Oxygen gas to produce Dihydrogen Oxide : can be written in shorthand as: H2 + O2  H2O

5. 7.1 Conservation Obeyed • In any chemical reaction, the conservation of mass is observed • Principle of the conservation of mass states: matter can not be created or destroyed in a chemical reaction • The mass of the reactants and the mass products must be the same

6. 7.1 Coefficients • A number in front of compounds or elements in a chemical equation represent the number of particles in the reaction • The coefficients are used to balance the equation to obey the law of conservation of mass H2 + O2  H2O 2 2

7. Balancing Equations • When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. • Changing the subscripts changes the compound.

8. Steps to Balancing Equations There are four basic steps to balancing a chemical equation. • Write the correct formula • Find the number of atoms for each element on each side. • Determine where to place coefficients. • Check your answer to see if: • The numbers of atoms on both sides of the equation are now balanced. • The coefficients are in the lowest possible whole number ratios. (reduced)

9. 7.1 Balancing Equations • Keep track of the elements in a table, • adjusting the amount of atoms as you put in coefficients • You must go back and forth putting in coefficients until all of the atoms’ numbers are equal

10. 7.1 Balancing Equations • Fe(s) + O2(g)  Fe2O3(s) • Focus on one element at a time. • Fe(s) + O2(g)  Fe2O3(s) • There are 2 O’s on the left and 3 on the right. • You cannot change subscripts. • Fe(s) + 3O2(g)  2Fe2O3(s) • Fe is now wrong. • 4Fe(s) + 3O2(g)  2Fe2O3(s) Fe + O2  Fe2O3 Fe 1 O 2 X 4 Fe 2 O 3 X 4 X 6 X 6

11. H 2 N 2 H 3 N 1 7.1 Balancing Equations • Try to balance H2 + N2  NH3 3 2 X 6 X 6 X 2

12. Na 2 S 9(1+8) O 3 Na 2 S 2 O 3 7.1 Balancing Equations • Try to balance Na2SO3 + S8  Na2S2O3 8 8 X 16 XXXX16 X 24 X 16 X 16 X 24

13. Balancing Equations • Try to balance • CH4 + O2  CO2 + H2O • CH4 + 2O2  CO2 + 2H2O

14. 7.1 Counting with Moles • 12 eggs is a eggs? • 144 pencils is a of pencils? • 500 sheets of paper is a ? • 6.02 x 1023 of anything is a ? Mole

15. 7.1 UNITS Not this type of Mole

16. 7.1 Or this type of Mole

17. 7.1 A Mole is a unit. • 6.02 x 1023 is a mole -602,000,000,000,000,000,000,000 • If you counted paper at the rate of one sheet per second it would take you 19,089,294,774,226,281 years to count a mole of paper. • It is a big number because atoms are small.

18. 7.1 The Mole • A mole of sugar weighs about one half pound (between 180 and 342 g’s) but contains how many molecules of sugar? • 602,000,000,000,000,000,000,000 • 6.02 x 1023 molecules.

19. 7.1 Avogadro's Number How do we get Avogadro’s Number? Relative Mass in grams=6.02 X 1023 Actual Mass in AMU’s 1 mole = ?? grams = 6.02 X 1023 molecules

20. 7.1 Molar Masses • A chemical reaction is like a recipe. • A specific number of atoms or molecules react to form a specific product. • Consider: 1C(s) + 1O2(g)  1CO2(g) • This recipe calls for 1 carbon atom and 1 oxygen molecule to make 1 molecule of CO2 , carbon dioxide.

21. 7.1 Molar Masses • Because atoms and molecules are too small to see you can’t count them out. • To make CO2 we need to weigh atoms or molecules. • To get the right proportions, we need to know the relative masses. • We do! • The atomic masses = the relative masses.

22. 7.1 Molar Masses • On the periodic table the atomic mass is usually placed under the symbol for the atom. • One mole of Carbon is equal to 12.0 grams of carbon.

23. 7.1 Molar Masses • C(s) + O2(g)  CO2(g) • The atomic mass of C is 12.0. • The atomic mass of one oxygen atom is 16.0. • The oxygen molecule contains two oxygen atoms and has a relative mass of (2 x 16) or 32. • Therefore 12.0 g of carbon will react with 32.0 g of O2. • To form 44 grams of CO2.

24. 7.1 Chemical Calculations • Take a look at the formation of water.

25. C 5 H 12 O 3 C 1 H 2 O 3 7.1 Balancing Equations • Try to balance # 8 C5H12O + O2  CO2 + H2O 2 10 12 15 X 10 XX 24 X 4 X 24 XX 32 X 10 X 21 X 32

26. End Section 1

27. 7.2 Types of Reactions

28. 7.2 Types of Reactions • There are six types of chemical reactions we will talk about: • Synthesis reactions • Decomposition reactions • Single displacement reactions • Double displacement reactions • Combustion reactions • Oxidation-Reduction reactions • You need to be able to identify the type of reaction

29. 7.2 1. Synthesis reactions • Synthesis reactions occur when two substances react and form a single compound. • reactant + reactant  1 product • Basically: A + B  AB • Example: 2H2 + O2  2H2O • Example: C+ O2  CO2

30. 7.2 Synthesis Reactions • Here is another example of a synthesis reaction

31. 7.2 2. Decomposition Reactions • Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds • 1 Reactant  Product + Product • In general: AB  A + B • Example: 2 H2O  2H2 + O2

32. 7.2 Decomposition Reactions • Another view of a decomposition reaction:

33. 7.2 3. Single Replacement • Single Replacement Reactions occur when one element replaces another in a compound. • A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). • element + compound product + product A + BC  AC + B (if A is a metal)OR A + BC  BA + C (if A is a nonmetal) (remember the cation always goes first!)

34. 7.2 Single Replacement Reactions

35. 7.2 Single Replacement Reactions • Sodium chloride solid reacts with fluorine gas NaCl(s) + F2(g)  NaF(s) + Cl2(g) Note that fluorine replaces chlorine in the compound • Aluminum metal reacts with aqueous copper (II) nitrate Al(s)+ Cu(NO3)2(aq) Cu(s) + Al(NO3)3(aq) 2 2 2 2 3 3

36. 7.2 4. Double Replacement Reactions • Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound • Compound + compound  product + product • AB + CD  AD + CB

37. 7.2 Double Replacement Reactions • Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together • Example: AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq) • Another example: K2SO4(aq) + Ba(NO3)2(aq)  KNO3(aq) + BaSO4(s) 2

38. 7.2 5. Combustion Reactions • Combustion reactions occur when a hydrocarbon reacts with oxygen gas. • This is also called burning!!! In order to burn something you need the 3 things in the “fire triangle”:1) A Fuel (hydrocarbon)2) Oxygen to burn it with3) Something to ignite the reaction (spark)

39. 7.2 Combustion Reactions • In general: CxHy + O2  CO2 + H2O • Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide) • Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)

40. 7.2 Combustion • Example • C5H12 + O2 CO2 + H2O • Write the products and balance the following combustion reaction: • C10H22 + O2  CO2 + H2O 5 8 6 31 20 22 2

41. 7.2 Oxidation-Reduction • Many of these 5 types of reactions are also oxidation-reduction reactions • Oxidation: Loss of electrons • Reduction: Gain of electrons

42. 7.2 You can’t have one… without the other! • Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons. • Reduction has to occur at the cost of oxidation LEO the lion says GER! ose lectrons xidation ain lectrons eduction GER!

43. 7.2 Another way to remember • OIL RIG s s eduction xidation ose ain

44. 7.2 Mixed Practice • State the type and balance the following reactions: • BaCl2 + H2SO4 • C6H12 + O2  • Zn + CuSO4  • Cs + Br2  • FeCO3  BaSO4 + HCl 2 6 9 6 CO2 + H2O ZnSO4 + Cu 2 2 CsBr FeO + CO2

45. From 7.3 Signs of Chemical Change Energy change • Exothermic • Endothermic

46. 7.3 Why do reactions take place? • Reactions take place is because of energy • Energy is somehow involved in all reactions • Some reactions will gain energy from the surroundings • Some reactions will give off energy

47. Chemical Bonds and Energy • Chemical reactions involve the breaking and making of chemical bonds. • It takes energy to break bonds. • Energy is released when we make bonds.

48. ∆H: How much energy is transferred? • Energy is needed to break chemical bonds • Energy is given out when bonds are made • ∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

49. energy products energy taken in ∆H is positive reactants course of reaction 7.3 Energy Level DiagramsEndothermic Reactions It gets cold!!!

50. energy given out ∆H is negative energy reactants products course of reaction 7.3 Exothermic Energy Level Diagrams It gets HOT!!!