2 Definitions Energy – capacity for doing work or supplying heat. Thermochemistry – study of energy changes that occur during phase changes and chem. rxns.Chem. Potential Energy – energy stored in chemical bonds.
3 Example Lots of energy stored in bonds! Energy difference. 5473 kJ/mol Little energy stored in bonds.
4 HeatRepresented by q.Energy that transfers from one object to another because of a Temp. difference between them.Heat flows from warm cool until the two objects are at the same Temp.
5 Exothermic vs. Endothermic In exothermic processes, the system loses heat as its surroundings warm up.q has a negative value b/c the system is losing heat.In endothermic processes, the system gains heat as its surroundings cool down.q has a positive value b/c the system is gaining heat.
6 Potential Energy Diagram of Ice Melting at 0ºC. Time Potential Energy IceWaterIs the melting of ice an endothermic or an exothermic process? How can you tell?
7 Measuring Heat Flow SI Unit of heat flow: Joule (J) Common unit used in chemistry: calorie (cal)Amt. of heat needed to raise 1 gram of water by 1ºC.1 cal = JFood Calorie (capital “C”) = 1000 cal, or 1 kilocalorie = 4184 J
8 What do Calories mean in food? 10 grams of sugar has 41 Calories.When 10 grams of sugar are burned, 41 kcal (170 kJ) of energy are released.Your body “burns” food for energy.In order to use the energy available in 10 grams of sugar, you must do 41 kcal worth of work.
9 Heat CapacityAmount of heat needed to raise an object’s temperature by 1°C.Depends on the chemical composition and the mass of the object.EXAMPLE: 1 gram of water requires 1 cal to raise its temperature by 1°C.100. g of water require 100. cal to raise the temp. by 1°C.
10 Same temperature change Heat Capacity1 g H2O10 g H2OSame temperature change
11 Specific Heat (c)Amt. of heat needed to raise 1 gram of a substance’s temperature by 1ºC.Expressed in J/g ºC, or cal/g ºCThe higher a substance’s specific heat, the more energy it takes to heat it.Substance’s with low specific heats heat up and cool down quickly (most metals, e.g.)
12 Some Specific Heats Substance Specific Heat J/gºC cal/g ºC 4.18 1.00 Water4.181.00Grain alcohol2.40.58Ice2.10.50Steam1.70.40Chloroform0.960.23Aluminum0.900.21Iron0.460.11Silver0.240.057Mercury0.140.033
13 Specific Heat (c) c = heat / (mass x change in Temp.) c = q / (m x ΔT) q = m x c x ΔT
14 Example ProblemThe temperature of a 95.4-g piece of Cu increases from 25.0ºC to 48.0ºC when the Cu absorbs 849 J of heat. What is the specific heat of Cu?SOLUTION: q = m x c x ΔT849 J = (95.4 g) c (48.0ºC – 25.0ºC)849 J = (95.4 g) c (23.0ºC)849 J = (2190 gºC) cc = J/gºCBased on what you know about metals, does this answer make sense?
15 Example ProblemWhen 435 J of heat is added to 3.4 g of olive oil at 21ºC, the temperature increases to 85ºC. What is the specific heat of olive oil?SOLUTION: q = m x c x ΔT435 J = (3.4 g) c (85ºC – 21ºC)435 J = (3.4 g) c (64ºC)435 J = (220 gºC) cc = 2.0 J/gºC
16 Example ProblemHow much heat is required to raise the temperature of g of mercury by 52ºC? The specific heat of mercury is 0.14 J/gºC.SOLUTION: q = m x c x ΔTq = (250.0 g)(0.14 J/gºC)(52ºC)q = 1800 J = 1.8 kJ
17 Enthalpy ChangesEnthalpy (H) – the heat content of a system at constant pressure.Enthalpy change (ΔH) – the heat that enters or leaves a system at constant pressure.q = ΔHNeg. ΔH = exothermic processPos. ΔH = endothermic process
18 Thermochemical Equations Enthalpy change can be written as a reactant or a product.Reactant endothermicProduct exothermicExample: The reaction of calcium oxide with water is exothermic.It produces 65.2 kJ of heat per mole of CaO reacted.CaO(s) + H2O(l) Ca(OH)2(s) kJ
21 Thermochemical Equations and Stoichiometry You can use thermochemical equations in stoichiometry.How much heat energy is produced when 55.0 grams of ethanol is burned completely?C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g) kJGiven: 55.0 g C2H5OHWant: kJConversion factors:1 mol C2H5OH produces kJ when burned1 mol C2H5OH = g/mol55.0 g C2H5OH= 1550 kJ
22 Thermochemical Equations and Stoichiometry 0.500 grams of methane gas are burned completely beneath a container that holds 100. grams of water, originally at 20.0º. If all of the heat from the combustion reaction goes into the water, what will the water’s final temperature be?CH4(g) + 2O2(g) CO2(g) + 2H2O(g) kJFirst find out how much total heat is released.Given: g CH4(g)Want: kJConversion factors:1 mol CH4 = g CH41 mol CH4 produces 803 kJ when completely burned0.500 g CH4= 25.0 kJ
23 Thermochemical Equations and Stoichiometry The combustion of 5.00 grams of methane releases 250. kJ of heat.Now we’ll calculate how hot the water in the container will get if it absorbs all of the heat.First convert 25.0 kJ to Jq = m x c x T2.50x104 J = (100. g) (4.18 J/gºC) T2.50x104 J = (418 J/ºC) TT = 59.8ºCThe water will get 59.8ºC warmer.The final temperature will be 20.0ºC ºC = 79.8ºC.25.0 kJ= 2.50x104 J