Presentation on theme: "Chapter 5 “Thermochemistry”"— Presentation transcript:
1 Chapter 5 “Thermochemistry” Mr Daniel : OHS AP ChemistryCredit to:Charles Page High SchoolStephen L. Cotton
2 Section 11.1 The Flow of Energy – Heat and Work OBJECTIVES:Explain how energy, heat, and work are related.Classify processes as either exothermic or endothermic.Identify the units used to measure heat transferDistinguish between heat capacity and specific heat.
3 Energy Transformations Thermochemistry – study of heat changes that occur during chemical and physical changes.Energy - capacity for doing work or supplying heatweightless, odorless, tastelessEnergy stored within chemical substances is called chemical potential energyGasoline contains a significant amount of chemical potential energy
4 Energy Transformations Heat (“q”) is energy that transfers from one object to another, because of a temperature difference between them.only changes of heat can be detected!flows from warmer cooler objectAll chemical reactions and changes in physicalstate involve either:release of heat, orabsorption of heat
5 Exothermic and Endothermic Processes When studying heat changes there are two important categories:the system - the part of the universe on which you focus your attention (a test tube, a room, a molecule, the Earth etc.)the surroundings - includes everything else in the universeTogether, the system and it’s surroundings constitute the universe
6 Exothermic and Endothermic Processes Thermochemistry is concerned with the flow of heat from the system to it’s surroundings, and vice-versa.The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed.All the energy is accountedfor as work, stored energy,or heat.
7 Exothermic and Endothermic Processes Endothermic: Heat flowing into a system from it’s surroundingsdefined as positive change.q has a positive valueThe system gains heat as the surroundings cool down
8 Exothermic and Endothermic Processes Exothermic: Heat flowing out of a system intoit’s surroundingsdefined as negative changeq has a negative valueThe system loses heatas the surroundings heat up
9 Exothermic and Endothermic Every reaction has an energy change associated with it. For example, the…** Gummy Bear Sacrifice!! ** (next week!!!)Exothermic reactions release energy, usually in the form of heat.Endothermic reactions absorb energyChemical Energy is storedin the form of bondsbetween atoms.
10 Units for Measuring Heat Flow A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC.a Calorie, (written with a capital C), is calories and refers to the energy in food(100 Calorie candy bar is 100,000 calories)1 Calorie = 1 kilocalorie = 1000 calories
11 Units for Measuring Heat Flow The Joule is the SI unit of heat and energy and is related to the calorie:4.184 J = 1 calNamed after James Prescott Joule
12 Heat Capacity and Specific Heat Heat Capacity (Cp) - the amount of heat needed to increase the temperature of an entire object exactly 1 oCCop = Joules/oCHeat capacity is dependent on both the object’s mass and its chemical composition
13 Heat Capacity and Specific Heat Specific Heat Capacity (c) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oCOften called simply “Specific Heat”c = Joules/ gram oC
15 Heat Capacity and Specific Heat Water has a HUGE specific heat value, when is compared to other chemicals:C (H2O) = 4.18 J/(g oC), orC (H2O) = 1.00 cal/(g oC)It heats slowly andIt cools slowlyConsequently, water maintains a relatively constant temperature!
16 Heat Capacity and Specific Heat Heat calculations for various materials are done with the formula: q = mass (g) x c x T q = change in heatc = specific heatT = change in temperatureUnits are either J/(g oC) or cal/(g oC)
18 Section 11.2 Measuring and Expressing Enthalpy Changes OBJECTIVES:Describe how calorimeters are used to measure heat flow.Construct thermochemical equations.Solve for enthalpy changes in chemical reactions by using heats of reaction.
19 CalorimetryCalorimetry - the precise measurement of the heat into or out of a system for chemical and physical processes.The device used to measure the absorption or release of heat in chemical or physical processes is called a Calorimeter
20 CalorimetryFoam cups are excellent heat insulators, and are commonly used as simple calorimetersFor systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system
21 A foam cup calorimeter – here, two cups are nestled together for better insulation
22 Calorimetry Changes in heat = Changes in enthalpy = H q = H These terms are often used interchangeablyThus, q = H = m x c x TH is negative for an exothermic processH is positive for an endothermic process
23 CalorimetryCalorimetry experiments can be performed at a constant volume using a device called a “bomb calorimeter” - a closed systemUsed by nutritionists to measure energy content of food
25 C + O2 → CO2 + 395 kJ ∆ H= -395 kJ C + O2 Energy 395kJ CO2 ® Reactants ProductsC + O2395kJCO2∆ H= kJ
26 ExothermicIn an exothermic rxn the products are lower in energy than the reactants and energy is released.ΔHrxn = -395 kJThe negative sign indicates that energy is released.
27 CaCO3 → CaO + CO2 CaCO3 + 176 kJ → CaO + CO2 CaO + CO2 Energy CaCO3 ® ReactantsProductsCaO + CO2176 kJCaCO3∆H = kJ
28 EndothermicThe products are higher in energy than the reactants and energy is absorbed.ΔHrxn = +176 kJThe positive sign indicates that energy is absorbed
29 Heat of ReactionA heat of reaction is the heat change for the equation.The physical state of reactants and products must also be given.Standard conditions for the reaction is kPa (1 atm.) and 25 oC (different from STP)
30 Thermochemical Equations An equation that includes energy is called a thermochemical equationCH4 + 2O2 ® CO2 + 2H2O kJ1 mole of CH4 releases kJ of energy.When you make kJ you also make 2 moles of water
31 CH4 + 2 O2 ® CO2 + 2 H2O kJIf grams of CH4 are burned completely, how much heat will be produced?1 mol CH4802.2 kJ10. 3 g CH416.05 g CH41 mol CH4= 514 kJThus, ΔH = kJ for this reaction
33 EnthalpyEnthalpy: The heat content a substance has at a given temperature and pressureCan’t be measured directly because there is no set starting pointThe reactants start with a heat contentThe products end up with a heat contentSo we can measure how much enthalpy changesDH (delta H) is the measure of change in heat
34 Enthalpy Enthalpy: Amount of heat in a system (H) . If heat is released, DH is negative (exothermic)If heat is absorbed, DH is positive (endothermic)
35 EnergyChange is downΔH is <0ExothermicReactantsProducts
36 EnergyChange is upΔH is > 0EndothermicReactantsProducts
37 Heat of ReactionHeat of Reaction: The heat that is released or absorbed in a chemical reactionEquivalent to DHC + O2(g) ® CO2(g) kJC + O2(g) ® CO2(g)DH = kJIn a thermochemical equation, it is important to indicate the physical stateH2(g) + 1/2O2 (g)® H2O(g) DH = kJH2(g) + 1/2O2 (g)® H2O(l) DH = kJ
38 Heat of Combustion C + O2(g) ® CO2(g) + 393.5 kJ C + O2(g) ® CO2(g) The heat from the reaction that completely burns 1 mole of a substance:C + O2(g) ® CO2(g) kJC + O2(g) ® CO2(g)DH = kJ
39 Section 17.3 Heat in Changes of State OBJECTIVES:Classify the enthalpy change that occurs when a substance melts, freezes, boils, condenses, or dissolves.
40 Section 17.3 Heat in Changes of State OBJECTIVES:Solve for the enthalpy change that occurs when a substance melts, freezes, boils condenses or dissolves.
41 Heat in Changes of State Heat of Fusion (Hfus) : the heat absorbed by a substance in melting from a solid to a liquidFor H2O: Hfus = 334 J/ gMolar Hfus = 6.01 kJ/ molq = mass x Hfus (there is no temperature change)
42 Molar Heat of Solidification (Hsolid) = heat lost when one mole of liquid solidifies (or freezes) to a solidFor H2O: Hfus = -334 J/ gMolar Hfus = kJ/ molq = mass x Hsolid (no temperature change)
43 Heat in Changes of State Heat absorbed by a melting solid is equal to heat lost when a liquid solidifiesThus, Hfus = -HsolidNote Table 17.3, page 522
45 Heats of Vaporization and Condensation When liquids absorb heat at their boiling points, they become vapors.Molar Heat of Vaporization (Hvap) = the amount of heat necessary to vaporize one mole of a given liquid.q = mass x Hvap (no temperature change)For H2O: Hfus = 2260 J/ gMolar Hfus = 40.7 kJ/ mol
46 Heats of Vaporization and Condensation Condensation is the opposite of vaporization.Molar Heat of Condensation (Hcond) = amount of heat released when one mole of vapor condenses to a liquidHvap = - Hcond
47 Heats of Vaporization and Condensation Note Figure 17.10, page 523The large values for water Hvap and Hcond are the reason hot vapors such as steam is very dangerousYou can receive a scalding burn from steam when the heat of condensation is released!H20(g) H20(l) Hcond = kJ/mol
49 Heat of Solution (Ch 11)Heat changes can also occur when a solute dissolves in a solvent.Molar Heat of Solution (Hsoln) = heat change caused by dissolution of one mole of substanceSodium hydroxide provides a good example of an exothermic molar heat of solution:
50 Heat of Solution NaOH(s) Na1+(aq) + OH1-(aq) Hsoln = - 445.1 kJ/mol The heat is released as the ions separate (by dissolving) and interact with water, releasing kJ of heat as Hsoln -thus becoming so hot it steams!H2O(l)