Presentation on theme: "Chapter 17 “Energy and Chemical change”"— Presentation transcript:
1 Chapter 17 “Energy and Chemical change” Pre-AP ChemistryBMHSAnahit Pivazyan
2 Section 17.1 The Flow of Energy – Heat and Work OBJECTIVES:Explain how energy, heat, and work are related.
3 Section 17.1 The Flow of Energy – Heat and Work OBJECTIVES:Classify processes as either exothermic or endothermic.
4 Section 17.1 The Flow of Energy – Heat and Work OBJECTIVES:Identify the units used to measure heat transfer.
5 Section 17.1 The Flow of Energy – Heat and Work OBJECTIVES:Distinguish between heat capacity and specific heat capacity (also called simply specific heat).
6 Energy Transformations “Thermochemistry” - concerned with heat changes that occur during chemical reactionsEnergy - capacity for doing work or supplying heatweightless, odorless, tastelessif within the chemical substances- called chemical potential energy
7 Energy Transformations Gasoline contains a significant amount of chemical potential energyHeat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them.only changes can be detected!flows from warmer cooler object
8 Exothermic and Endothermic Processes Essentially all chemical reactions and changes in physical state involve either:release of heat, orabsorption of heat
9 Exothermic and Endothermic Processes In studying heat changes, think of defining these two parts:the system - the part of the universe on which you focus your attentionthe surroundings - includes everything else in the universe
10 Exothermic and Endothermic Processes Together, the system and it’s surroundings constitute the universeThermochemistry is concerned with the flow of heat from the system to it’s surroundings, and vice-versa.
11 Exothermic and Endothermic Processes The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed.All the energy is accounted for as work, stored energy, or heat.
12 Exothermic and Endothermic Processes Heat flowing into a system from it’s surroundings:defined as positiveq has a positive valuecalled endothermicsystem gains heat (gets warmer) as the surroundings cool down
13 Exothermic and Endothermic Processes Heat flowing out of a system into it’s surroundings:defined as negativeq has a negative valuecalled exothermicsystem loses heat (gets cooler) as the surroundings heat up
14 Exothermic and Endothermic Fig. 17.2, page on the left, the system (the people) gain heat from it’s surroundings (the fire)this is endothermic (q is positive)On the right, the system (the body) cools as perspiration evaporates, and heat flows to the surroundingsthis is exothermic (q is negative)
15 Exothermic and Endothermic Every reaction has an energy change associated with itExothermic reactions release energy, usually in the form of heat.Endothermic reactions absorb energyEnergy is stored in bonds between atoms
16 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.Used except when referring to fooda Calorie, (written with a capital C), always refers to the energy in food1 Calorie = 1 kilocalorie = 1000 cal.
17 Units for Measuring Heat Flow The calorie is also related to the Joule, the SI unit of heat and energynamed after James Prescott Joule4.184 J = 1 calHeat Capacity - the amount of heat needed to increase the temperature of an object exactly 1 oCDepends on both the object’s mass and its chemical composition
18 Heat Capacity and Specific Heat Specific Heat Capacity (abbreviated “C”) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oCoften called simply “Specific Heat”Note Table 17.1, page 508 (next slide)Water has a HUGE value, when it is compared to other chemicals
19 Table of Specific Heats Note the tremendous difference in Specific Heat.Water’s value is VERY HIGH.
20 Heat Capacity and Specific Heat For water, C = 4.18 J/(g oC) in Joules, and C = 1.00 cal/(g oC) in calories.Thus, for water:it takes a long time to heat up, andit takes a long time to cool off!Water is used as a coolant!
21 Heat Capacity and Specific Heat To calculate, use the formula: q = mass (in grams) x T x Cheat is abbreviated as “q”T = change in temperatureC = Specific HeatUnits are either: J/(g oC) or cal/(g oC)
25 Section 17.2 Measuring and Expressing Enthalpy Changes OBJECTIVES:Solve for enthalpy changes in chemical reactions by using heats of reaction.
26 CalorimetryCalorimetry - the measurement of the heat into or out of a system for chemical and physical processes.Based on the fact that the heat released = the heat absorbedThe device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”
27 CalorimetryFoam cups are excellent heat insulators, and are commonly used as simple calorimeters under constant pressure.See picture on the next slideFor systems at constant pressure, the “heat content” is the same as a property called Enthalpy (H) of the system(They are good because they are well-insulated.)
28 A foam cup calorimeter – here, two cups are nestled together for better insulation
29 Calorimetry Changes in enthalpy = H q = H These terms will be used interchangeably in this textbookThus, q = H = m x C x TH is negative for an exothermic reactionH is positive for an endothermic reaction
30 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
33 C + O2 → CO2 + 395 kJ Energy Reactants Products ® C + O2 395kJ given offCO2
34 ExothermicThe products are lower in energy than the reactantsThus, energy is released.ΔH = -395 kJThe negative sign does not mean negative energy, but instead that energy is lost.
35 CaCO3 → CaO + CO2 CaCO3 + 176 kJ → CaO + CO2 Energy Reactants Products CaO + CO2176 kJ absorbedCaCO3
36 EndothermicThe products are higher in energy than the reactantsThus, energy is absorbed.ΔH = +176 kJThe positive sign means energy is absorbed
37 Chemistry Happens in MOLES 1 mole of CH4 releases 802.2 kJ of energy. 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
38 Thermochemical Equations The heat of reaction is the heat change for the equation, exactly as writtenThe physical state of reactants and products must also be given.Standard conditions (SC) for the reaction is kPa (1 atm.) and 25 oC (different from STP)
39 CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(l) + 802.2 kJ 1If grams of CH4 are burned completely, how much heat will be produced?Convert moles to desired unitConvert to molesStart with known value1 mol CH4802.2 kJ10. 3 g CH416.05 g CH41 mol CH4= 514 kJRatio from balanced equationΔH = -514 kJ, which means the heat is released for the reaction of 10.3 grams CH4
41 EnthalpyThe 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 changes
42 Enthalpy Symbol is H Change in enthalpy is DH (delta H) If heat is released, the heat content of the products is lowerDH is negative (exothermic)If heat is absorbed, the heat content of the products is higherDH is positive (endothermic)
43 = Exothermic (heat is given off) EnergyChange is downΔH is <0= Exothermic (heat is given off)ReactantsProducts
44 = Endothermic (heat is absorbed) EnergyChange is upΔH is > 0= Endothermic (heat is absorbed)ReactantsProducts
45 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 indicate the physical stateH2(g) + 1/2O2 (g)® H2O(g) DH = kJH2(g) + 1/2O2 (g)® H2O(l) DH = kJ
46 Heat of CombustionThe heat from the reaction that completely burns 1 mole of a substance:C + O2(g) ® CO2(g) kJC + O2(g) ® CO2(g) DH = kJ