Chemical Reactions

1. Chemical Reactions What makes what? When do we know when we are seeing one?…

2. The Nature of Chemical Reactions • Chemical reactions change substances • Familiar examples: grow, ripen, decay, burn, respiration, digestion, photosynthesis… • Production of gas, color change, new products, and change in temperature • Chemical reactions re-arrange atoms • Matter is not destroyed, just rearranged • Reactants make the products. Look for the → • Isooctane (C8H18) + Oxygen (O2) → CO2 + H2O • The arrow points to the new products from the reactants

3. Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: release input change in color change in odor production of new gases or vapor input or release of energy difficult to reverse

4. How Reactions Occur • Collisions • For a chemical reaction to occur, the atoms/molecules must actually come in contact with another substance Atoms collide into one another causing a chemical change and formation of new products This will be addressed later in this unit

5. Energy and Reactions • Energy can be found in various forms: • heat, electricity, light, chemical (nuclear) & mechanical • Energy must be added to break the old bonds so that new bonds may be created • When the new bonds form, energy is released • Other reactions may produce electricity or other energy forms • Energy is conserved in chemical reactions • The total energy before the reaction must be the same energy of the products and their surroundings

6. Giving and Taking Energy • Reactions may be exothermic or endothermic • Exothermic means releasing chemical energy • Things get/feel warmer • Endothermic means absorbing • Things get/feel cooler • Energy may be added from other sources • heat for cooking, sunlight for photosynthesis,… • Endergonic = endothermic • Exergonic = exothermic

7. Chemical Reactions • Depict the kind of reactantsand productsand their relative amounts in a reaction 4 Al(s) + 3 O2(g) → 2 Al2O3(s) The numbers in the front are called stoichiometric coefficients

8. Additional Symbols Used in Chemical Equations “Yields”; indicates result of reaction Used to indicate a reversible reaction A reactant or product in the solid state; also used to indicate a precipitate formation Alternative to (s), but used only to indicate a precipitate A reactant or product in the liquid state A reactant or product in an aqueous solution (dissolved in water) A reactant or product in the gaseous state (s) (l) (aq) (g)

9. Additional Symbols Used in Chemical Equations Alternative to (g), but used only to indicate a gaseous product Reactants are heated Pressure at which reaction is carried out, in this case 2 atm Pressure at which reaction is carried out exceeds normal atmospheric pressure Temperature at which reaction is carried out, in this case 0 °C Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction D 2 atm pressure 0 oC MnO2

10. Chemical Equations 4 Al(s) + 3 O2(g) → 2 Al2O3(s) This equation means: 4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3 or 4 Al moles + 3 O2moles yield 2 moles of Al2O3 4 g Al + 3 g O2 yield 2 g Al2O3

11. Word Equations A WORD EQUATION describes chemical change using the names of the reactants and products Methane + oxygen → carbon dioxide + water Reactants Products CH4(g) + 2 O2(g)→ CO2(g) + 2 H2O(g) Write the word equation for the reaction of methane gas with oxygen gas yielding carbon dioxide and water.

12. The Five Basic Types of Reactions • Synthesis • Decomposition • Combustion • Single-Displacement or “single-replacement” • Double-Displacement or “double-replacement” • Acid-Base Reaction: • Often labeled as a type of reaction as it is a specific type of double-displacement

13. Synthesis Combine simpler substances to make more complex ones A + B→ AB 2Na + Cl2→ 2NaCl 6CO2 + 6H2O → C6H12O6 + 6O2

14. Decomposition Break complex substances into simpler ones AB→ A+ B 2H2O → 2H2 + O2 • You are looking for: • one reactant • a catalyst (heat or something on the yield sign) • small gas molecules in the products (H2, O2, etc.)

15. Combustion Uses oxygen (O2) as a reactant, normally with a hydrocarbon 2CH4 + 4O2→ 2CO2 + 4H2O excellent oxygen supply means cleaner “burning” 2CH4 + 3O2→ 2CO + 4H2O limited oxygen supply means a messier “burn” 2CH4 + 2O2→ 2C + 4H2O poor oxygen supply means a dirty “burn”

16. Single-Displacement Atoms of one element take the place of another element in a compound XA + B→ BA + X 3CuCl2 +2Al → 2AlCl3 + 3Cu Generally, a more reactive element will take the place of a less reactive one

17. Activity Series M O S T A C T I V E L E A S T Useful to determining if a chemical reaction will take place Halogen Activity Series Fluorine F Chlorine Cl Bromine Br Iodine I *This is based on electron affinity

18. Single-Replacement Reactions Activity Series Fe + CuCl2 FeCl2 + Cu Can Fe replace Cu? Yes MgCl2 + Br2  MgBr2 + Cl2 Can Br replace Cl? No Zn + HNO3  Zn(NO3)2 + H2 Can Zn replace H? Yes Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2 Cu Hg Ag Pt Au M O S T A C T I V E L E A S T F2 Cl2 Br2 I2

19. Double-Displacement The exchange of ions between compounds • Atoms of one element in a compound switch places with an element in another compound AX + BY→ AY + BX Pb(NO3) + K2CrO4→ PbCrO4 + 2KNO3 Possible results: gas, precipitate, covalent molecule

20. Acid-Base Reaction This is also called a Neutralization Reaction AX + BY→ AY + BX A = Hydrogen H Y = Hydroxide OH AY + BXone is H2O and the other is an ionic compound HCl + NaOH→ NaCl + H2O

21. An Additional Sub-Type Reaction:Oxidation/Reduction Reactions • The discovery of the electrons and their role in chemical reactions enlightened this reaction type • Electrons are transferred in redox (reduction/oxidation) reactions • Rust: 4Fe + 3O2→ 2Fe2O3 • Substances that accept electrons are “reduced” • Substances that give up electrons are “oxidized” • “Oxidation” may also be the gain of an oxygen or the loss of a hydrogen (As in respiration and combustion) • “Reduction” may also be the loss of an oxygen or the gain of a hydrogen (As in respiration and combustion) Looks like combustion, but there are no hydrocarbons here.

22. Na2S + Zn(NO3)2→ZnS + 2NaNO3 • Na has been oxidized • Na begins bonded to S, no O’s • As a product, Na is bonded to O (along with other elements) • Zn has been reduced • Zn begins by being bonded to O (along with other elements) • As a product, Zn has lost the O

23. Why do redox reactions occur? • “Free radicals” • These are fragments of a molecule with at least one electron available for bonding • Similar to valence e-, but connected to molecules instead of atoms • Making of polymers, combustion of rocket fuel, the burning of coal or oil all involve the formation of radicals

24. Balancing Chemical Reactions • Follow the Law of Conservation of Mass (already covered) • Balancing is done with coefficients, not subscripts. • Changing subscripts will change the substances • CH4 + 2O2 →CO2 + 2H2O • So? What does it mean? • Methane requires 2 oxygen molecules to combust and produce carbon dioxide and water, any less would not yield the same products, or it simply would not react. • AND, each burned methane yields 1 molecule of carbon dioxide and 2 molecules of water, if supplied with enough oxygen.

25. Coefficients • Molecules is self explanatory, but can you “see” 1 or 2 molecules of anything? • Mole ratio (a.k.a. molar ratio) is just the coefficients • For 2Mg +O2→2MgO; the ratio is 2:1:2 • The moles for the above equation are as follows: • Remember to use the atomic masses as grams. • Mg = 24.3 g/mol, so 2Mg = 48.6 g • O = 16.0 g/mol, so O2 = 32.0g • (subscripts work the same way for determining masses.) • 2MgO = 80.6g, or MgO = 40.3g/mol

26. How Reactions Occur • The Collision Theory • For a chemical reaction to occur, the atoms/molecules of each substance must actually come in contact with one other • Thus, they collide causing a chemical change and formation of new products

27. Orientation • Compounds are three-dimensional objects made of tiny microscopic atoms • When they collide, they have to hit with the correct orientation to transfer electrons, atoms or even synthesis a larger product • A good analogy would be two basketball players attempting to pass the ball between them. If one was facing the wrong way, he/she would never be able to receive the pass (wrong orientation)

28. Particle Collisions

29. Activation Energy (Ea) Reactants Time Activation Energy and Reactions • Biology Recall: • Some chemical reactions require energy to start the reaction. This is known as activation energy(Ea) • This is the energy required for a collision to be hard enough for the compounds to react • Like tackling someone in football but not hard enough to bring him down Energy Products

30. Activation Energy (Ea) • Activation energy has a direct influence on reaction rates • High Ea means collisions may not have enough energy (momentum) to react • Low Ea means more collisions will cause reactions to occur between the reactants

31. Factors Affecting Reactions • The Nature of Reactants • Some chemicals are more reactive by nature due to their chemical properties • Pressure • Increasing gas pressure causes the chemicals to come closer together because the system is now smaller and thus collide more frequently • (Decrease Volume) • Concentration • Increased concentrations mean more of the substance in a given space, thus more likely to have collisions with other material • Surface Area • Increasing surface area will allow for more contact surface for collisions • Pulverizing or grinding, like in a mortar and pestle, is a good example • Temperature • KMT - Increasing heat increases kinetic energy of the particles causing more collisions and more likelihood of chemical reaction • Catalysts (Enzymes are biological catalysts) • Catalysts lower the Ea for reactions, thus increasing the rate • Inhibitors are those that slow down reactions

32. Standard Reactions & Reversible Reactions • Some reactions go to completion and are done • Others are reversible (⇄) • The reaction goes one way until it is unbalanced, then it goes the other way • Equilibrium = Dynamic balanced state • Le Châtelier’s Principle: • Ex: N2 + 3H2⇄ 2NH3 (gas) + heat • ↑ heat and rxn moves to the left • ↑gas pressure and rxn moves to the right (to the side with less separate molecules) • Equilibrium is always sought by nature

33. Picture Le Châtelier’s Principle: • Raise the gas pressure and the reaction gets too heavy here because of the different kinds of molecules colliding. • Raise the heat, the reaction gets too heavy here. • N2 + 3H2⇄ 2NH3 (gas) + heat So the reaction goes to make more on the other side. So the reaction goes to make more on the other side.