2 Energy The ability to do work or cause a change Work- using force to move somethingSymbol is wThermochemistry- studies energy changes in reactionsq is heatHeat flows from high temperature to low temperature
3 The Universe Can be divided into 2 pieces System- the part you are investigatingSurroundings- the rest of the universeLaw of conservation of energy-Energy can’t be created or destroyedThe energy of the universe is constantEnergy change of System + Energy change of surroundings = 0
4 Exothermic Surroundings System Energy System releases energy Heat flows outSurroundings get hotterq is negativeSystemSurroundingsEnergy
5 Endothermic Surroundings System Energy System absorbs energy Heat flows inSurroundings get coolerq is positiveSystemSurroundingsEnergy
6 Units of Energy Energy is measured in Joules or calories calorie is amount of heat to change 1 g of water by 1 CFood Calories are kilocalories1Calorie = 1000 calories1 cal = J
7 Heat capacity How much heat it takes to heat an object by 1C Affected by two thingsWhat the substance isMass of the objectSpecific heat is the amount needed to heat 1 g by 1COnly depends on the substance
8 Heat capacityThe higher the specific heat the more energy it takes to change its temperature.Pizza burning the roof of your mouthThe same amount of heat is released when an object cools down
9 Heat capacity Equation q = m T C Heat = mass x temp x specific change heat
10 How much heat is needed to change the temperature of 12 g of silver with a specific heat of cal/gC from 25C to 83 C?
11 If you put 6500 J of heat into a 15 g piece of Al at 25 C , what will the final temperature be? ( C = 0.90 J/gC )
12 Calorimetry Measuring heat. Use a calorimeter. Two kinds Constant pressure calorimeter (called a coffee cup calorimeter)An insulated cup, full of water.q = m T CFor water C is 1 cal/gºCDissolve chemicals, measure temp before and after
13 Calorimetry Enthalpy (H) – heat content at constant pressure Coffee cup calorimeter measure how much heat content changesHH = qWe will use heat and change in enthalpy interchangeablyIf temperature goes up exothermic
14 ExampleA chemical reaction is carried out in a coffee cup calorimeter. There are 75.8 g of water in the cup, and the temperature rises from 16.8 ºC to 34.3 ºC. How much heat was released?
15 CalorimetrySecond type is called a bomb calorimeter. (constant volume)Material is put in a container with pure oxygen.The container is put into a container of water.Wires are used to start the combustion.
16 Bomb Calorimeter thermometer stirrer full of water ignition wire Steel bombsample
18 CalorimetryRun first with a known amount of heat to find the heat capacity of the calorimeter (cal/ ºC)Put in your unknown and run a second timeMultiply temperature change by the heat capacity to find heat of unknown
19 Thermochemistry Every reaction has an energy change associated with it Energy is stored in bonds between atomsMaking bonds gives energyBreaking bonds takes energy
20 In terms of bonds C O O C O Breaking this bond will require energy C O Making these bonds gives you energyIn this case making the bonds gives you more energy than breaking them
21 Exothermic The products are lower in energy than the reactants Releases energyOften release heat
22 C + O2 ® CO2+ 395 kJEnergyReactantsProductsC + O2-395kJCO2
23 When will a reaction be exothermic A) When breaking the bonds of the reactants takes more energy than making the bonds of the products.B) When breaking the bonds of the reactants takes less energy than making the bonds of the productsC) When you put in energy to break the bondsD) When you get energy by breaking bonds
24 Endothermic The products are higher in energy than the reactants Absorbs energyAbsorb heat
25 CaCO3 + 176 kJ ® CaO + CO2 CaCO3 ® CaO + CO2 Energy Reactants Products
26 Chemistry Happens in MOLES An equation that includes energy is called a thermochemical equationCH4 + 2 O2 ® CO2 + 2 H2O kJEnergy is a product in this example1 mole of CH4 makes kJ of energy.When you make kJ you make 2 moles of water
27 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=515 kJ
28 CH4 + 2 O2 ® CO2 + 2 H2O kJHow many liters of O2 at STP would be required to produce 23 kJ of heat?
29 CH4 + 2 O2 ® CO2 + 2 H2O kJHow many grams of water would be produced with 506 kJ of heat?
30 Heat of ReactionThe heat that is released or absorbed in a chemical reactionEquivalent to DHC + O2(g) ® CO2(g) kJC + O2(g) ® CO2(g) DH = kJIn thermochemical equation it is important to say what stateH2(g) + ½ O2 (g)® H2O(g) DH = kJH2(g) + ½ O2 (g)® H2O(l) DH = kJ
31 EnergyChange is downDH is <0ReactantsProducts+ heat
32 EnergyChange is upDH is > 0Reactants + heatReactantsProducts
33 Choose all that apply... C(s) + 2 S(g) ® CS2(l) H = 89.3 kJ Which of the following are true?A) This reaction is exothermicB) It could also be writtenC(s) + 2 S(g) kJ ® CS2(l)C) The products have higher energy than the reactantsD) It would make the water in the calorimeter colder
34 Heat of CombustionThe heat from the reaction that completely burns 1 mole of a substance at 25C and 1 atmC2H4 + 3 O2 ® 2 CO2 + 2 H2OC2H6 + O2 ® CO2 + H2O2 C2H O2 ® 4 CO2 + 6 H2OC2H6 + (7/2) O2 ® 2 CO2 + 3 H2OAlways exothermic
35 Heat and phase change Melting and vaporizing are endothermic Breaking things apartFreezing and condensing are exothermicForming connections
36 Heat of Fusion Heat of fusion-Hfus- heat to melt one gram q = Hfus x mFor water 80 cal/g or 334 J/gSame as heat of solidificationBook uses molar heat of fusion- heat to melt one mole of solidq = Hfus x n
37 Heat of Vaporization Hvap- heat to change one gram of liquid to gas q = Hvap x mFor water 540 cal/g or 2260 J/gSame as heat of condensation
38 Calculating Heat If there is a temperature change q = m T C If there is a phase changeq = Hfus x m or q = Hsolid x mq = Hvap x m or q = Hcond x mIf there is both, do them separately and add.
39 ExampleAmmonia has a heat of fusion of 332 cal/g. How much heat to melt 15 g of ammonia?
40 ExampleMethanol has a heat of vaporization of J/g. How much heat will be absorbed by 23 g of ethanol vaporizing?
41 ExampleButane, C4H10 , absorbs energy as it vaporizes. If 25.3 g of butane absorb 1630 cal by vaporizing, what is the heat of vaporization of butane?
42 ExampleHow much heat does it take to turn 25 g of water at 22C into steam at 100 C ?
43 Heat of SolutionHsoln- heat change when one mole of solute is dissolved.q = Hsoln x nSometimes endothermicAmmonium nitrate for cold packsSometimes exothermicAcids and bases
44 Standard Heat of Formation The DH for a reaction that produces 1 mol of a compound from its elements at standard conditionsStandard conditions 25°C and 1 atm.Symbol isThe standard heat of formation of an element is 0This includes the diatomics
45 What good are they? There are tables (pg. 530) of heats of formations For most compounds it is negativeBecause you are making bondsMaking bonds is exothermicThe heat of a reaction can be calculated by subtracting the heats of formation of the reactants from the products
48 Why Does It Work? If H2(g) + 1/2 O2(g)® H2O(l) DH=-285.5 kJ then H2O(l) ® H2(g) + 1/2 O2(g) DH = kJIf you turn an equation around, you change the sign2 H2O(l) ® 2 H2(g) + O2(g) DH = kJIf you multiply the equation by a number, you multiply the heat by that number.Twice the moles, twice the heat
49 Why does it work?You make the products, so you need their heats of formationYou “unmake” the reactants so you have to subtract their heats.