Chapter 17.1 - ThermochemistryHeat and Chemical Change,Masuk High SchoolJ. K. Stoelzel,2,Section 17.1a The Flow of Energy - Heat,OBJECTIVES:Explain the relationship between energy and heat.,3,Section 17.1aThe Flow of Energy - Heat,OBJECTIVES:Distinguish between heat capacity and specific heat.,4,Energy and Heat,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,5,Energy and Heat,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,Exothermic and Endothermic Processes,Essentially all chemical reactions, and changes in physical state, involve either:release of heat, orabsorption of heat,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,Exothermic and Endothermic Processes,Together, the system and its surroundings constitute the universeThermochemistry is concerned with the flow of heat from the system to its surroundings, and vice-versa.,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.,Exothermic and Endothermic Processes,Fig. 17.3, p.517 - heat flowing into a system from its surroundings:defined as positiveq has a positive valuecalled endothermicsystem gains heat as the surroundings cool down,Endothermic Reaction,reaction that absorbs energyreactants have lower PE than products,2Al2O3 + energy 4Al + 3O2,Exothermic and Endothermic Processes,Fig. 17.2, p.516 - heat flowing out of a system into its surroundings:defined as negativeq has a negative valuecalled exothermicsystem loses heat as the surroundings heat up,B. Exothermic Reaction,reaction thatreleases energyproducts have lower PE than reactants,2H2(l) + O2(l) 2H2O(g) + energy,14,Exothemic 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,15,Heat Capacity and Specific Heat,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.,16,Heat Capacity and Specific Heat,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 oC,17,Heat Capacity and Specific Heat,Specific Heat Capacity - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC (abbreviated “C”)often called simply “Specific Heat”Water has a HUGE value, compared to other chemicals,18,Heat Capacity and Specific Heat,For water, C = 4.18 J/(g oC), and also C = 1.00 cal/(g oC) 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!,19,Heat Capacity and Specific Heat,To calculate, use the formula:q = mass (g) x T x Cheat abbreviated as “q”T = change in temperatureC = Specific HeatUnits are either J/(g oC) or cal/(g oC)Sample problem 17-1, page 513,20,Section 17.1b Measuring and Expressing Heat Changes,OBJECTIVES:Construct equations that show the heat changes for chemical and physical processes.,21,Section 17.1b Measuring and Expressing Heat Changes,OBJECTIVES:Calculate heat changes in chemical and physical processes.,Calorimetry,Calorimetry - the accurate and precise measurement of heat change 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,Calorimetry,Foam cups are excellent heat insulators, and are commonly used as simple calorimetersFig. 17.4, page 519For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system,Calorimeter,Calorimetry,Changes in enthalpy = Hq = 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,Calorimetry,Calorimetry experiments can be performed at a constant volume using a device called a “bomb calorimeter” - a closed systemSample 17-2, page 521,27,C + O2 CO2,395kJ,+ 395 kJ,28,In terms of bonds,C,O,O,Breaking this bond will require energy.,