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Chemical Reactions

Chemical Reactions. Chapter 8. Chemical reactions…. involve a rearrangement of the ways atoms are grouped together. Chemical Reaction –. Ex: Remember, in a chemical change, a new substance is formed!. 2H 2 + O 2 → 2H 2 O. Signs a Chemical Reaction has occured. Color Change Solid forms

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Chemical Reactions

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  1. Chemical Reactions Chapter 8

  2. Chemical reactions… • involve a rearrangement of the ways atoms are grouped together.

  3. Chemical Reaction – Ex: Remember, in a chemical change, a new substance is formed! 2H2 + O2→ 2H2O

  4. Signs a Chemical Reaction has occured • Color Change • Solid forms • Bubbles form • Heat and/or flame produced or heat is absorbed

  5. Parts of a chemical reaction: 2H2 + O2→ 2H2O Reactants Products Yield Sign • Reactants– substances that are reacting; the starting substances • Products – new substances that are made; substances formed during reaction

  6. Physical states of compounds are often given in a chemical equation.

  7. reactants products Conservation of Mass • - During a chemical reaction, mass is not created or destroyed. • – Mass is conserved! • -All atoms present in the reactants must be present in the products. • -Same number of each type atom on both sides of arrow

  8. Balancing Chemical Equations - The number of atoms of each element must be the same on both sides of the yield sign. 2H2 + O2→ 2H2O 4 Hydrogen 2 Oxygen 4 Hydrogen 2 Oxygen The equation is balanced!

  9. When balancing an equation, you can only change the coefficient before the compound. Never change the subscript! 2Ca(NO3)2 coefficient subscript

  10. Multiply the coefficient by any subscripts to determine the number of atoms of each element. 2 calcium atoms 4 nitrogen atoms 12 oxygen atoms For Calcium: For Nitrogen: For Oxygen: 2Ca(NO3)2 2 2 x___ 6 distribute - just like math class! coefficient subscript 2 x 1 = 2 2 x 2 = 4 2 x 6 = 12

  11. Balancing equations: • Step 1: Determine the formulas for reactants and products • Step 2: Write reactants on the left and products on the right Example: Aluminum + oxygen  aluminum oxide Al + O2 Al2O3

  12. Step 3: Count the number of each element in the reactants and products. Al + O2 Al2O3 1 Al, 2 O 2 Al, 3 O • Step 4: Balance the elements one at a time by using coefficients (you should not write 1s). Do not change the formulas as you balance! 4Al + 3O2 2Al2O3 4 Al, 6 O 4 Al, 6 O

  13. Finally, • Step 5: Check to make sure each atom is balanced, and make sure all the balanced coefficients are in the lowest possible ratio.

  14. CH4 + 2O2→ CO2 + 2H2O Reactants 1 carbon atom 4 hydrogen atoms 2 oxygen atoms Products 1 carbon atom 2 hydrogen atoms 3 oxygen atoms 4 hydrogen atoms 4 oxygen atoms 4 oxygen atoms What went in came out… the reaction is balanced!

  15. CH4 + 2O2→ CO2 + 2H2O What went in came out… the reaction is balanced!

  16. 2Al + 6HCl → 2AlCl3 + 3H2 Reactants 1 aluminum atom 1 hydrogen atom 1 chlorine atom Products 1 aluminum atom 2 hydrogen atoms 3 chlorine atoms 2 aluminum atoms 2 aluminum atoms 6 hydrogen atoms 6 hydrogen atoms 6 chlorine atom 6 chlorine atoms Congratulations… it’s balanced!

  17. Practice 1.The reaction below shows the process of oxidizing iron. Balance the equation by calculating the coefficients. • 2, 3, 1 • 1, 1, 1 • 4, 3, 2 • 4, 2, 2 4 3 2

  18. Practice 2. Which chemical equation supports the law of conservation of mass? • 2H2O → H2 + O2 • Zn + HCl → ZnCl2 + H2 • Al4C3 + H2O → CH4 + Al(OH)3 • CH4 + 2O2 → CO2 + 2H2O

  19. 2 2 • Practice 3. What are the coefficients that will balance the chemical reaction above? • 2, 1, 1 • 3, 4, 2 • 2, 2, 1 • 4, 3, 2

  20. 2K + 2H2O → 2KOH + H2 • Practice 4. What is the coefficient for H2O when the equation above is balanced? • 1 • 2 • 3 • 4

  21. Ways to Classify Reactions • There are 5 general types of chemical reactions: • Combination • Decomposition • Single-replacement • Double replacement • Combustion

  22. Types of Reactions • Combination or synthesis reactions- occur when two elements combine to form a single compound as a product. X + Y  XY • A synthesis reaction may also involve the combination of an element plus a compound or the combination of two compounds, always forming a single compound as a product.

  23. Examples: Ca + Cl2 CaCl2 PCl3 + Cl2 PCl5 Na2O + H2O  2NaOH Rules to remember: Any metallic oxide plus water yields a metallic hydroxide, as in K2O + H2O  2KOH Any nonmetallic oxide plus water yields an acid N2O5 + H2O  2HNO3

  24. Decomposition reaction- occurs where one compound breaks down into two or more elements or compounds. Heat, electricity, or a catalyst is usually necessary. XY  X + Y Examples: 2NaCl  2Na + Cl2 H2SO3 H2O + SO2

  25. Rules to remember: • When heated, metal carbonates break down to yield metal oxides and carbon dioxide • Example: MgCO3 MgO + CO2 • Hydrogen peroxide breaks down into water and oxygen gas • H2O2 H2O + O2 • Carbonic acid decomposes into water and carbon dioxide • H2CO3 H2O + CO2

  26. Single replacement reaction(or single displacement)… • occurs when one element displaces another element in a compound. • A new element and a new compound are formed. A + BC  B + AC

  27. Details about single replacement reactions… • Active metals displace less active metals or hydrogen from their compounds in aqueous solution. Consult an activity series of metals. • A + BY  AY + B

  28. In general, Group I and Group II metals and aluminum are most active, transition metals are more active than hydrogen, and the jewelry metals (Cu, Ag, Au) and Hg are the least active. You do not need to memorize the activity series. Examples: Mg + NiCl2 Zn + CuSO4 2HNO3 + Pb 

  29. More single replacement… • Some nonmetals can also displace other nonmetals: active halogens replace less active halogens from their compounds in aqueous solution. • Fluorine at the top of group 7A is the most active, with activity decreasing as you move down the group. • MX + Y  MY + X

  30. b) Some nonmetals can also react in single replacement reactions: active halogens replace less active halogens from their compounds in aqueous solutions. Fluorine at the top of the group is the most active, with the activity decreasing as you move down the group. Examples: 2NaCl + F2 NaF + Cl2 Cl2 + 2KI  2KCl + I2

  31. Double replacement reactions • Two compounds (in aqueous solution) react to form two new compounds. These are also called exchange reactions. • AB + CD  AD + CB

  32. Double replacement reactions require one of three kinds of chemical change: • Formation of a precipitate (solid) • Formation of a gas • Formation of water

  33. Types of double replacement… Formation of a precipitate (a solid product) BaNO3(aq) + K2CrO4(aq) 2KNO3(aq) + BaCrO4(s)

  34. Types of double replacement… • Formation of a gas • Since H2CO3 and H2SO3 are unstable, if produced in a reaction, they will decompose. • H2CO3decomposes into H2O and CO2. • H2SO3 decomposes into H2O and SO2.

  35. Types of double replacement… • Formation of water • Includes acid-base reactions… acid + base  water and salt HCl + NaOH  NaCl + H2O

  36. Combustion Reactions • Involve combining with oxygen • These reactions produce energy so rapidly that a flame results • CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

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