6.1-6.3 Heat, The Nature of Energy and The First Law of Thermodynamics
Thermochemistry The study of the relationship between chemistry and energy. If you were to place an ice pack on a leg injury, why does your leg become cold? (in which direction does heat flow?) Heat always flows from warm object to cold object, so loss of heat from your leg is the reason your leg becomes cold. Energy – The capacity to do work. Work – The result of a force acting over a distance. Heat – the flow of energy caused by temperature differences. Heat and work are both ways to transfer energy.
Thermochemistry (Continued) In order to track energy changes, a system, what is being investigated, should be identified. In an energy exchange, energy is transferred between the system and the surroundings. Example: If I place sodium bicarbonate in water, and then I touch the beaker that the contents are in, and it feels cold to the touch, what type of thermochemical reaction took place in the beaker? This would be an endothermic reaction. The sodium bicarbonate (the system) absorbs heat from the water. The solution then becomes cold. If the beaker is touched, heat flows from your warm hand, into the cold water. Everything other than the baking soda are the surroundings.
Thermochemistry (Continued) If I pick a ball off the ground and hold it in the air, you increase the balls __________energy. Potential energy – the energy associated with the position or composition of an object. Kinetic energy – the energy associated with the motion of an object. Thermal energy – the energy associated with the temperature of an object. Chemical energy – the energy associated with the relative position of electrons and nuclei in atoms and molecules, is also a form of potential energy. Law of conservation of energy – energy is neither created nor destroyed. Energy is transferred from one object to another and it can assume different forms.
Units of Energy Joule (J) – The SI unit of energy. Equal to kg m2 / s2 We often use kJ rather than J. calorie (cal) – 1 cal = 4.184 joules A dietary calorie is denoted with a capital C (Cal) 1 Cal = 1 kcal = 1000 cal
The First Law of Thermodynamics The law of conservation of energy – The total energy in the universe is constant! When a bond is broken, energy is _________. When a bond is formed, energy is _________. Absorbed in order to break a bond. When a bond is formed, energy is released. The amount of energy released will be the same amount of energy that is absorbed by the surroundings. Internal energy (E) – the sum of the kinetic and potential energies of all the particles that compose the system. A state function – its value depends only on the state of the system, not on how the system arrived at that state. Δ𝐸=𝐸final products −Einitial(reactants)
Summary of Energy Flow If the reactants have a higher internal energy than the products, ΔEsys is negative and energy flows out of the system into the surroundings. (Exothermic reaction) Bonds formed. If the energy of the reactants have a lower internal energy than the products, ΔEsys is positive and energy flows into the system from the surroundings. (Endothermic reaction) Bonds broken. ΔEsys = - ΔEsurr ΔE = w + q (where w is work and q is heat)
Let’s Try a Practice Problem Identify each energy exchange as heat or work according and determine whether the sign of heat or work (relative to the system) is positive or negative. An ice cube melts and cools the surrounding beverage. q is + A metal cylinder is rolled up a ramp. w is + Steam condenses on skin, causing a burn. q is -
Let’s Try Another! A cylinder and piston assembly (defined as the system) is warmed by an external flame. The contents of the cylinder expand, doing work on the surroundings by pushing the piston outward against the external pressure. If the system absorbs 559 J of heat and does 488 J of work during the expansion, what is the value of ΔE? ΔE = q + w = 559 J + (-488J) = 71 J
6.1-6.3 pgs. 287-288 #’s 37, 38, 40, 42, 44 Read 6.4-6.6 pgs. 256-268