Chemical Reactions (Chapter 13)

Transcription

1 Chemical Reactions (Chapter 13) coefficients reactants products By4 Student Learning Objectives Utilize chemical equations to determine the amounts of reactants, products, and energy change. Characterize acids and bases. Describe reactions involving oxygen. How is a chemical equation balanced? A chemical reaction is the process of forming new substances. Atoms are combined to form molecules. Molecules separate into atoms. There must be the same numbers of each type of atom on each side of the equation. Example: Al + Br 2 Al 2 Br 6 1

2 1) Pb + PbO 2 + H 2 SO 4 PbSO 4 + H 2 O 2) C 3 H 8 + O 2 CO 2 + H 2 O 3) Al + HCl AlCl 3 + H 2 4) BaO + H 2 O Ba(OH) 2 There are always x atoms or molecules in 1 mole of a substance. The atomic mass number is equal to the grams/mole of that substance. Avogadro s Number x particles mole Examples: Hydrogen & Water 1) Calculate the amount of grams in one mole. Determine the number of particles in one gram. a) Gold b) NaCl c) Cu(NO 3 ) 2 2) How many moles are in 2 grams of carbon dioxide? How many molecules are in 2 grams of carbon dioxide? 3) How many grams of each reactant are needed and how many grams of product are produced in the reaction? Al + Br 2 Al 2 Br 3? 2

3 What are exothermic and endothermic reactions? The amount of chemical potential energy may change during a chemical reaction. Exothermic Reaction Energy released Reactants higher PE Endothermic Reaction Energy absorbed Products higher PE Activation Energy H 2 + Cl 2 2HCl Bond Strength Bond Energy is the energy required to break 1 mole of bonds. Bond Energy to Break = Bond Energy Released The net bond energy indicates whether the process was exothermic or endothermic. 3

4 Examples 1) N 2 + H 2 N 2 H 4 2) H 4 C 2 + O 2 CO 2 + H 2 O What can cause reaction rates to vary? 1. Temperature 2. Concentration 3. Surface area 4. Catalysts A catalyst accelerates the reaction. Provides different pathway Lowers activation energy Reappears as a product 1) What do you have that uses a catalyst? 2) Why are there warnings on some medications not to break, crush, or chew the pill? 3) Why are photographic prints processed in the dark? 4

5 What is an acid? What is a base? Acids contain H + and H ions, and are substances that donate hydrogen ions. Bases contain hydroxide (OH ) ions, or are substances that accept a hydrogen ions. Proton Donor Baking soda (NaHCO 3 ) Proton Acceptor Example: HCl + NaHCO 3 H 2 CO 3 + NaCl Which substances are behaving as an acid, and which substances are behaving as a base? CH 3 CO 2 H + H 2 O CH 3 CO 2- + H 3 O + Strong acids & bases completely ionize in water. HNO 3 - nitric acid HCl - hydrochloric acid H 2 SO 4 -sulfuric acid LiOH - lithium hydroxide NaOH - sodium hydroxide (lye) KOH - potassium hydroxide (lye) 5

6 The neutralization reaction of an acid and a base produces water and a salt. Acid Base Water Salt HCl + NaOH H 2 O + NaCl HBr + KOH H 2 O + KBr ph A substance that has an equal number of acid and base ions is neutral. (ph balanced) ph Solution 1 to 6 Acidic 7 Neutral 8 to 14 Basic Substanc e ph Battery Acid 0.5 Stomach Acid Cola 2.5 Orange 3.5 Juice Coffee 5.0 Milk 6.5 Human Blood Hand Soap ) If the water you drink has a ph of 6.8, is it acidic, basic, or neutral? 2) Blood has buffers to help control the natural ph your body needs. The amount of CO 2 in the blood stream can change ph. What do we do naturally that can change the ph of blood? 6

7 What is acid rain? Acid Rain is moisture that has a ph of less than 5. Rain, snow, sleet, or fog Rainwater normally has a ph of 5.6 to 6.2 Acids may combine with dust (dry deposition) Water naturally reacts with CO 2 in the atmosphere, forming a weak acid. water + carbon dioxide carbonic acid (H 2 CO 3 ) Human-produced exhaust emissions of sulfur and nitrogen oxides form strong acids. sulfates + water sulfuric acid (H 2 SO 4 ) nitrates + water nitric acid (HNO 3 ) Question: Where would you expect a higher level of acidity in the atmosphere, WY or CA? Why? What is the result of an oxidation-reduction reaction? Redox reactions involve a transfer of electrons, and often the addition or subtraction of oxygen. Oxidation Reactant loses electrons Oxygen Added Reduction Reactant gains electrons Oxygen Removed The two processes, oxidation and reduction, always occur simultaneously. 7

8 Redox Examples Corrosion is a redox reaction between a metal and oxygen (O 2 ). Combustion is a redox reaction between a non-metal and oxygen (O 2 ). Redox Examples Corrosion is a redox reaction between a metal and oxygen (O 2 ). Combustion is a redox reaction between a non-metal and oxygen (O 2 ). 1) Which element is oxidized and which is reduced? a) HgS + O 2 Hg + SO 2 b) SnO 2 + 2C Sn + 2CO 2 2) Is the iron oxidized or reduced in the formation of rust? 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) 3) Is a combustion reaction, like the combustion of methane, exothermic or endothermic? CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) + energy 8

9 How do batteries and fuel cells work? Batteries have 2 diodes; one that is oxidized and one that is reduced. Acidic solution reacts with metals Reactions free electrons Anode Cathode Oxidation Reduction Metal loses e Metal gains e Negative ( ) Positive (+) Battery : Where do you store your batteries? Why? A fuel cell uses a membrane to separate the reactants. Hydrogen gas is oxidized. Oxygen gas is reduced. Product is pure water vapor. : Where do you store your batteries? Why? A fuel cell uses a membrane to separate the reactants. Hydrogen gas is oxidized. Oxygen gas is reduced. Product is pure water vapor. 9

10 What is electrolysis? Electrolysis is the use of electric current to cause chemical change. Reverses chemical reactions Splits compounds Example: Al 2 O 3 + Energy Al 2 + O 2 Question: How do you recharge your cell phone battery? What is electrolysis? Electrolysis is the use of electric current to cause chemical change. Reverses chemical reactions Splits compounds Example: Al 2 O 3 + Energy Al 2 + O 2 Question: How do you recharge your cell phone battery? 10