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Chemistry – Oct 22 , 2018. P3 Challenge – Distinguish between a triple point and a critical point. Turn in States Phet Activity if not yet Turn in PT Units HMK if not yet. Chemistry – Oct 22 , 2018. Objective – Energy and Heat Capacity Solutions and Solubility Assignments
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Chemistry – Oct 22, 2018 • P3 Challenge – Distinguish between a triple point and a critical point. Turn in States Phet Activity if not yet Turn in PT Units HMK if not yet
Chemistry – Oct 22, 2018 • Objective – • Energy and Heat Capacity • Solutions and Solubility • Assignments • Heat Worksheet • Agenda – • Energy • Heat • Heat Capacity • Heating Curve of Water (time permitting)
What is Energy? • Energy = Ability to do work • Law of Conservation of Energy: Energy is neither created nor destroyed. • It can only be transformed from one form to another or transferred from one system to another. • Total energy of a system = Kinetic Energy + Potential Energy • Kinetic energy – Energy of motion • Potential energy – Stored energy
Kinetic Energy Types • Thermal energy – Energy of random motion of atoms • Electromagnetic energy – Energy of light • Sound energy – Energy of motion of atoms due to pressure differences • Electrical energy – Energy of the flow of electrons
Potential Energy Types • Gravitational potential energy – Energy of position within Earth’s gravity • Electrical potential energy – Energy of separation of electric charge (voltage) • Chemical potential energy – Energy contained within chemical bonds • Nuclear potential energy – Energy contained within the nucleus
Heat • Heat is the transfer of thermal energy either to or from a system. • Heat is given the symbol, Q. • Positive heat transfers heat to a system. (Endothermic process) • Temperature of substance increases • Negative heat releases heat to the surroundings. (Exothermic process) • Temperature of substance decreases • Each substance absorbs/releases at a characteristic rate called it’s heat capacity. A substance with a high heat capacity requires a large amount of energy to change its temperature.
Initial state Final state Endothermic process • An endothermic process is one that absorbs heat. • You must heat an endothermic process to make it occur. • Energy is a reactant. • Q, heat is positive • The system gets hotter. • Surroundings gets colder. • T = Tfinal– Tinitialis positive Tf > Ti E Energy
Energy Initial state E Final state Exothermic process • An exothermic process is one that releases heat. • An explosion is an exothermic process (easy way to remember) • Energy is a product. • Q, heat is negative • The system gets colder. • Surroundings gets hotter. • T = Tfinal– Tinitialis negative Tf< Ti
Specific heat capacity, c • If the heat capacity is per gram of material, it is the specific heat capacity, c. • Specific heat capacity is the amount of energy needed to raise the temperature of 1 gram of substance by 1 degree Celsius. Typical units are cal/g◦C or J/gK. • ◦C and K are interchangeable for these units. • Problems may use either Joules or calories. • Given a choice, use Joules form. • Note water’s extremely high value • 1 Food Calorie = 1 kcal (note the capital C)
Measuring heat, q • For any change in temperature from Tinitial to Tfinal, ΔT = Tfinal – Tinitial • For a temperature increase, ΔT is positive. • For a temperature decrease, ΔT is negative. • Heat exchanged during the temperature change is given by: • Where q is the heat, m is the mass of the substance, c is the specific heat capacity of the substance and ΔT is the temperature change.
Heat calculations • Ex: How much heat is needed to raise 2.50 g of water from 25 ◦C to body temperature of 37 ◦C? • What mass of aluminum can be heated from 5°C to 45°C by using 275 J of heat? (Specific heat capacity of Al = 0.900 J/g°C)? • What is the final temperature when 36.4 J of of heat is lost from a 23.8 g piece of lead initially at 85°C? (Specific heat capacity of Pb= 0.128 J/g°C)? q = m = c = ΔT =
Heating Curve of Water • Heat and temperature are not the same. • Phase changes occur at a constant temperature, but still require heat to happen. • Not all phase changes require the same amount of heat • It takes more energy to boil than to melt. • It takes more heat to warm water than ice or steam the same amount. • The slopes of the ice and steam lines are greater than the line for water.
Exit Slip - Homework • Exit Slip: How much heat is needed to warm a 450 g copper pan from 25°C to 150°C ? (Use joules) • What’s Due? (Pending assignments to complete.) • Heat Worksheet • What’s Next? (How to prepare for the next day) • Start reviewing for Test 2 Oct 31