Chapter 6 CONSERVATION OF ENERGY AND MATTER

Transcription

1 Chapter 6 CONSERVATION OF ENERGY AND MATTER

2 Identifying Chemical Change (6.1) Chemical reactions: Process in which the physical and chemical properties of the original substance change as new substances are formed. Reactants: starting materials Products: ending materials

3 II. Chemical Equations Chemical equations: ways to express chemical reactions (reactants on left of arrow, products on right of arrow) Zn + Cu(NO 3 ) 2 Zn(NO 3 ) 2 + Cu Reactants Products

4 2 Na + Cl 2 2 NaCl Properties of the product are very different than the properties of the reactants.

5 Clues of Chemical Change Change of color Light is given off Change in temperature Production of a gas (Bubbles) Production of a solid

6 What clue is there that this is a chemical reaction? A burning campfire can warm you up on a cold day. Dissolving an antacid tablet in water produces a fizzy drink Adding acid to milk produces curds of cottage cheese.

7 What clue is there that this is a chemical reaction? Maple leaves changing color in the fall.

8 Balancing Chemical Equations (6.2) Law of Conservation of Matter: number of atoms of each element must be the same before and after a chemical reaction Ca + O 2 CaO Never change the subscripts Add coefficients 2Ca + O 2 2CaO atoms are not the same

9 Steps for balancing equations 1. Write a formula equation with correct symbols and formulas. 2. Count the number of atoms of each element on each side of the arrow. 3. Balance atoms by using coefficients. 4. Check your work by counting atoms of each element again.

10 Diatomic molecules

11 Example Balance CH 4 + O 2 CO 2 + H 2 O CH 4 + 2O 2 CO 2 + 2H 2 O What is wrong with this one? 2CH 4 + 4O 2 2CO 2 + 4H 2 O Always use lowest whole-number ratio.

12 D. Complete Chemical Equations Include the phase or state of matter CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (l) g gas l liquid s solid aq aqueous Aqueous means it has been dissolved in water to make a solution

13 I. Stoichiometry Calculations that relate quantities of substances. The study of the quantitative, or measurable relationships, that exist in chemical formulas and chemical reactions. Everything manufactured in this country requires stoichiometry.

14 A. Balanced Chemical Equations N 2 H 4 + 2H 2 O 2 N 2 + 4H 2 O 1 molecule 2 molecules 1 molecule 4 molecules Can multiply by any number and keep it balanced. Can multiply by Avogadro s Number (6.02x10 23 ) 1 mole 2 moles 1 mole 4 moles

15 B. Mole-Mole problems NH 4 NO 3 N 2 O + 2H 2 O 1 mole 1 mole 2 moles 2.25 moles?? Molar ratios: NH 4 NO 3 to N 2 O 1:1 NH 4 NO 3 to H 2 O 1:2 N 2 O to H 2 O 1:2

16 Dimensional Analysis Create a conversion factor from molar ratios. The unit you want on top, and the unit you re trying to cancel on bottom.

17 Solve 2.25 moles NH 4 NO 3 x 1molN 2O 1molNH NO mol N 2 O 2.25 moles NH 4 NO 3 x 2molH 2O 1molNH NO mol H 2 O

18 Law of Conservation of Matter 2H 2 + O 2 2H 2 O 2 mole 1 mole 2 mole 2g 2molx 1 mol 32g 1molx 1 mol 4g + 32 g = 36 g 18g 2molx 1 mol

19 Solving Stoichiometry Problems A. Mass-Mass Problems Chemical equations don t show mass relationships, only mole to mole ratios. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O How many grams of H 2 O would be produced by 1.5 g of C 6 H 12 O 6?

20 Stoichiometry Problems Quantity of Given Quantity of unknown Convert to Moles Moles of Given Find Molar ratio Moles of unknown Convert to desired units

21 Solve 1.5gC H O x 1molC H O x 180g 6molH 1molC 6 H 2 O 12 O 6 x 18g 1molH 2 O 0.90gH 2 O

22 Chemical Reactions that Involve Heat Thermochemistry: study of changes in heat in chemical reactions. Endothermic: absorbs heat; temp. goes down Exothermic: releases heat; temp. goes up

23 sur sur Endothermic reaction sur sur sur sur Exothermic reaction sur sur

24 Exothermic reaction Camping stove C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) kJ The energy released in creating new bonds is greater than the energy needed to break the old bonds. Joule (J) is used to measure heat and energy.

25 Endothermic reaction C (s) + H 2 O (g) + 113kJ CO (g) + H 2(g) H 2 O (l) + 286kJ ½O 2(g) + H 2(g) More energy is needed to break the old bonds than is released in forming the new bonds. Energy is stored in new bonds.

26 Heat and Enthalpy Changes Enthalpy (H): almost the same thing as energy; when pressure is held constant, it is the heat absorbed or released in a chemical reaction. Total Energy = P.E. + K.E. of particles

27 Enthalpy Change ΔH = H products - H reactants

28 Standard enthalpy change: ΔHº - enthalpy change when products and reactants are in their standard state. Sign of ΔH Process Heat Positive endothermic absorbed Negative exothermic released

29 Enthalpy problems C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) ΔHº = -2043kJ C (s) + H 2 O (g) CO (g) + H 2(g) ΔHº = +113kJ 1 mol C & 1 mol H 2 O 113kJ of heat 2 mol C & 2 mol H 2 O? Works like stoichiometry.

30 Enthalpy problems C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) ΔHº = -2043kJ C (s) + H 2 O (g) CO (g) + H 2(g) ΔHº = +113kJ 1 mol C & 1 mol H 2 O 113kJ of heat 2 mol C & 2 mol H 2 O 226 kj Works like stoichiometry.

31 Calorimetry Endothermic: heat absorbed; temp. goes down Exothermic: heat released; temp. goes up Study of heat flow and heat measurement Calorimetry experiments determine the heats (ΔH) of reactions by making accurate measurements of temperature changes produced in a calorimeter.

32 Calorimetry Heat capacity: amount of heat needed to raise the temperature of an object by 1ºC; depends on mass and composition. Specific heat: amount of heat needed to raise the temperature of 1 gram of an object by 1ºC. Specific heat of water = J/g-ºC

33 Calorimetry Calorie: amount of heat needed to raise the temperature of 1 gram of water by 1ºC. 1 calorie = J 1 kcal = 1 Cal. = 1000 cal Calorimeter: insulator can be simple like a foam cup

34 Figure 10.6: A coffee-cup calorimeter.

35 Calculations q = (mass)(specific heat)(temp.change) q heat absorbed or released in a calorimeter. m mass C specific heat ΔT change in temp. T final - T initial

36 What is Heat? B. Kinetic Theory- heat results from the motion and vibration of the particles of matter. Heat: the transfer of kinetic energy from a hotter object to a colder object.

37 6.6 Production of Electrical Energy Electrochemistry: chemical reactions that produce electricity Electrochemical cell or Galvanic cell Oxidation: loss of electrons (more positive); occurs at anode Reduction: gain of electrons (more negative); occurs at the cathode

38 Figure 18.5: Schematic of a battery. 38 Copyright by McDougal Littell. All rights

39 Half-reactions Oxidation Half Reactions Cu Cu e - Ag Ag + + e - 2Ag + (aq) + Cu (s) Ag (s) + Cu 2+ (aq) Salt bridge allows ions to flow between half reactions.

40 Figure 18.6: Schematic of one cell of the lead battery. 40 Copyright by McDougal Littell. All rights

41 41 Figure 18.7: A common dry cell battery. Copyright by McDougal Littell. All rights reserved.

42 Figure 18.9: Cathodic protection of an underground pipe. 42 Copyright by McDougal Littell. All rights