Measuring Energy Changes Introducing Heat Capacity and Specific Heat
Before Today s Discussion Begins Remember the differences between temperature, thermal energy, and heat Temperature is the average kinetic energy of all particles in a material It is an intensive property the amount of material DOES NOT affect the magnitude of this property Thermal energy is the TOTAL energy of motion of molecules, atoms, or ions Thermal energy is directly proportional to the temperature within a given system + molecules = + particle movement = + thermal energy= + temperature Everything in the universe will contain some thermal energy if at a temperature above 0 Kelvin (the absolute scale) Heat is the thermal energy TRANSFER that results from a difference in temperature
Introducing Heat Capacity So, how do we relate a temperature to the amount of thermal energy? With heat capacity! Defined as the heat required to raise the temperature of an object by 1 C Heat Capacity = Heat supplied Temperature = J
Measuring Energy Changes Look back to the Mind Catalyst What are the two factors that determined the amount of energy (heat) contained by a substance? Amount of substance being heated (number of grams) Temperature change (number of degrees) However, identical masses of different substances may contain different amounts of thermal energy even if at the same temperature! So, identity of the substance is also a factor! Different substances respond differently to being heated Thus, different heat capacities Do metals have relatively high or low heat capacities? When in your life have you noticed this? What about water?
Relating Heat Capacities to Amount of a Substance More heat is required to raise the temperature of a large sample of a substance by 1 C than is needed for a smaller sample Specific heat capacity takes into account amount of the substance present Defined as the amount of energy (in calories or joules) required to change the temperature of ONE GRAM of substance by 1 C Specific Heat Capacity C = Heat capacity Mass = J g = J g Specific heat capacities vary depending on state of matter! The larger the specific heat of a substance, the less its temperature will change when it absorbs a particular amount of heat energy Thus, more heat must be added to increase the temperature of a substance a given number of degrees
Real-World Application of Specific Heat Capacity Oceans of Climate Change
Calculating the Amount of Heat Absorbed or Released From a Substance We can use specific heat capacity as a conversion factor to calculate how much heat is absorbed or lost from a substance as long as its mass and change in temperature are known! To do so, use the equation below: Heat = Mass Specific Heat Temperature Change q = mc T q is energy (heat) m is mass of sample in grams C is specific heat capacity ΔT is change in temperature in C
Practice! How much heat is absorbed when 500 g of water with a specific heat of 4.184 J g goes from 25 C to 35.0 C? How much heat is absorbed when 500 g of copper with a specific heat of 0.385 J g goes from 25 C to 35 C? A 50.0 g block of ice with specific heat of 2.087 J absorbed 333 J g of heat energy. How much does the temperature of ice rise?
Practice! The temperature of a silver coin (C = 0.24 J g ) falls by 353 C as it releases 5,550 Joules of heat. What is the mass of the coin?
Measuring H Using Calorimetry
Determining ΔH using Calorimetry Remember, we can easily measure heat changes ( q) of any process in the laboratory Under the conditions of constant pressure - as it is in the laboratory - we equate q with H The process of measuring the change in heat of a chemical or physical change is done with a technique called calorimetry More specifically, calorimetry is the process of measuring heat based on observing the temperature change when a body absorbs or releases energy as heat energy Calorimetry is based on First Law of Thermodynamics The total heat of the system and the surroundings remains constant
Performing Calorimetry To be able to observe energy changes, we must be able to isolate our system from the rest of the universe We use an insulated device called a calorimeter to measure this energy (heat) change A typical device is a coffee cup calorimeter Reaction is open to the atmosphere Therefore, constant pressure and q = H
How Does Calorimetry Work? In calorimetry: The heat released by the system is equal to the heat absorbed by its surroundings The heat absorbed by the system is equal to the heat released by its surroundings q system + q surroundings = 0 Assume the calorimeter does not absorb or leak any heat
How Does Calorimetry Work? In other words: The heat generated by the reaction (system) is absorbed by the water (surroundings) We know the mass of the water, mwater We know the change in temperature of the water, Twater We also know that water has a specific heat of c water = 4.184 J/ C g. So, we can calculate the enthalpy of reaction by using the relationship: qsys = H = qsurr qrxn = H rxn = qwater qrxn = H rxn = -(mwater Cwater Twater)
Quantifying Energy Exchanges using Constant-Pressure Calorimetry Remember, enthalpies of reactions are often expressed in terms of energy per moles of reacting substances or moles of produced substances Divide calculated energy by amount of substance May need to use stoichiometry to find amount of substance
Calorimetry Example http://mutuslab.cs.uwindsor.ca/schurko/animations/heatca pacitymetals/heat_metal.htm
Determination of H Using a Bomb Calorimeter A bomb calorimeter is a device used to measure heat of combustion at constant volume Therefore, q H The steel jacket isolates the system so that the heat produced by the combustion is taken up by calorimeter q rxn = q calorimeter Bomb Calorimeter Animation