Transcript Slide 1
Warm Up Can you explain why the colder block melts the ice faster? Colder block is actually the same temperature as warmer block … both are at room temperature. Block made of metal simply conducts heat faster. Heat flows from your hand faster into it and out of block into ice faster as well. The Flow of Energy – Heat Chapter 8.1 Introduction to Thermochemistry • Thermochemistry (or Thermodynamics) is the study of heat, energy, and temperature changes that occur during chemical reactions (One of the main courses of study in Physical Chemistry) • Law of Conservation of Energy (1st Law of Thermodynamics) – The law of conservation of energy states that energy may neither be created nor destroyed. Therefore the sum of all the energies in the system is a constant. (Whether in a reaction flask or the universe) James Joule (1818-1889) Joule studied the nature of heat, and discovered its relationship to mechanical work. This led to the Law of conservation of energy, and this led to the development of the First law of thermodynamics. The SI derived unit of energy, the joule, is named for James Joule. He worked with Lord Kelvin to develop the absolute scale of temperature. Joule also made observations of magnetostriction, and he found the relationship between the current through a resistor and the heat dissipated, which is now called Joule's first law. Temperature vs. Heat • Temperature is the measure of the average kinetic energy of the particles in a substance – Measure using a thermometer – Rises when heat is added – Falls when heat is removed • Heat (q) is the energy that is transferred between two objects – Cannot be measured directly – Can only detect the changes caused by heat Thermochemical Reactions • System – reactants and products • Surroundings – the reaction container Exothermic Rxn Endothermic Rxn Heat/Energy flows out of the system Heat/Energy flows into the system Heat/Energy is a product Heat/Energy is a reactant q is negative q is positive Exothermic vs. Endothermic Exothermic Heat Endothermic Heat State Properties The state of a system is described by giving its … -composition -temperature -pressure -energy -entropy -volume These above are some examples of state properties State properties depend only on the state of a system, not the way the system reached that state. Measuring Heat • The Joule (J) is the SI unit of heat and energy • The calorie (cal) is also used to measure heat capacity and is defined by the amount of heat needed to raise the temperature of 1g of pure water by 1C • 1 J = 0.2390 cal 1 cal = 4.184 J 1 kcal = 4.184 kJ • NOTE: The energy contained in food is measured in Calories (Cal) = 1000 calories = 1 kcal Hmmmm….. Which of these pictures contains more heat? Heat Capacity vs Specific Heat • Heat capacity (C) is the amount of heat needed to raise the temperature of a substance by 1C q = C x Dt (Dt = tfinal – tinitial) (units J/C or J/K) • Specific heat (c) is the amount of heat needed to raise the temperature of one gram of a substance by 1C q = c x m x Dt (Dt = tfinal – tinitial) (units J/gC or J/gK) Examples of specific heat capacities Susbtance Specific heat (cal/gc) Water 1.00 Ice 0.50 Steam 0.40 Paper 0.33 Aluminum 0.21 Sand 0.19 Iron 0.11 Copper 0.09 Gold 0.03 Homework Read 8.1 and 8.2 Problems 1, 3, and 5 Due tomorrow