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Heat of Formation Enthalpy of Formation Enthalpy Summary Enthalpy of formation Heat of Formation 2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(s) •Using enthalpies of Standard Enthalpy formation, calculate the of Formation Hf (kJ/mol) standard change in NH3(g) -46 enthalpy for the NO2 (g) 34 H2O(l) -286 thermite reaction: Al2O3(s) -1676 •This reaction occurs Fe2O3(s) -826 when a mixture of CO2 -394 CH OH -239 powdered aluminum and 3 (l) C8H18(l) -269 iron(III) oxide is ignited with a magnesium fuse. Enthalpy Methanol (CH3OH) is often Standard Enthalpy Summary used as a fuel in high- of Formation Hf (kJ/mol) performance engines in race NH3(g) -46 •When a reaction is reversed, the cars. Using the data in Table, NO2 (g) 34 H O -286 magnitude of H remains the same, compare the standard enthalpy 2 (l) Al2O3(s) -1676 but the sign changes. of combustion per gram of Fe2O3(s) -826 •When the balanced equation for a methanol with that per gram of CO2 -394 CH OH -239 reaction is multiplied by an integer, gasoline. Gasoline is actually a 3 (l) C8H18(l) -269 the value of H for that reaction mixture of compounds, but must be multiplied by the same assume for this problem that integer. gasoline is pure liquid octane (C8H18). Summary II •The change in enthalpy for a given reaction can be calculated from the enthalpies of Laws of Thermodynamics formation of the reactants and products: • o o o Hrxn = SmHf (products) - SnHf (reactants) • Elements in their standard states are not o include in the Hrxn calculations. That is, o Hf for an element in its standard state is 0. 1 Intro Entropy and heat Most natural events involve a decrease in total energy and an increase in disorder. • Entropy is a measure of randomness or disorder of a system. • The energy that was “lost” was converted to heat. • Entropy gets the symbol S • An increase in energy/motion decreases the • Heat energy is the kinetic energy of an overall order (organization) of the system. object. • Energy is never created nor destroyed it • The more motion in an object the more simply changes form. random it becomes. Thermodynamics Heat • Thermo – heat; dynamic- change or motion. • ~is the total thermal energy of an object. • Thermodynamics- conversion of heat energy • Like all energy, heat is measured in joules (J) (into other forms or objects). • The terms hot, warm, cool and cold are st • 1 Law of Thermodynamics- the amount of always relative. energy in the universe is constant. Law of conservation of energy. • Heat is only noticed when there is a transfer • 2nd Law of Thermodynamics- Spontaneous from one object to another. processes, ones that happen on their own, • Heat always flows from hot to cold. involve an increase in entropy. Entropy in the • This due to the 2nd law of thermodynamics. universe is always increasing. Kinetic Energy Temperature • Temperature is a measure of the intensity • All atoms/molecules in any object are of the heat energy present. moving. • This is measured by the average kinetic • The faster they are moving the more kinetic energy of all atoms/molecules present. energy an object has. • In other words, it’s the average amount of • The heat energy or thermal energy is the heat energy that will transfer. total (sum of) kinetic energy of all particles • Temperature is measured in Fahrenheit, in an object. Celsius or Kelvin. 2 Converting between temperature Units of Temperature units • Here are some common temperatures in the • Kelvin = 273 + Celsius different scales • (9/5 Celsius) + 32 = Fahrenheit COMMON TEMPERATURES F C K • Convert 65° F to C and K freezing point of water 32 0 273 • 18° C, 291 K room temperature (comfortable) 68 20 293 • Convert 301 K to C and F human body temperature 98.6 37 310 Boiling point of water 212 100 373 • 28° C, 82° F Two objects • When two objects of different temperatures are Penny and water next to each other heat will transfer from the • Here is a hot penny about to higher temperature to the lower temperature be dropped in a large cup of object. water. • By the 2nd law of thermodynamics • The penny will have a higher • This is not necessarily the object with more temperature, intensity of heat energy. energy, but the water will have to have more thermal • Consider a hot penny dropped into a large cup energy due to the amount of of room temperature water. water present. The Zeroth Law of Penny and water Thermodynamics • When dropped in, the penny sizzles, and the heat flows • If two different systems or objects are at from the penny into the water. thermal equilibrium with the same object, • This is obvious, since you can then they must be at thermal equilibrium with each other. now touch the penny. • Even though the water had • Another way to state this is they must all be at the same temperature and therefore no more total energy than the transfer of heat will take place. penny. • This increases entropy. 3 Matter without heat energy Zeroth Law of thermodynamics • Solids have the lowest amount of kinetic • So back to the penny in the cup of water. energy, however their particles still vibrate. • If you cool it until molecules no longer • They are now at thermal equilibrium. vibrate... • If a spoon is introduced to the cup of water o o it will also reach a thermal equilibrium with • it is theorized this occurs at -273.15 C or - 459 the water. F or 0 K • By this law, if the spoon and penny are • This is called absolute zero. It is when all removed, they are at thermal equilibrium, or motion stops. the same temperature. • Scientists have made it to 0.000 000 02 K Third Law of Thermodynamics Problems with forcing extremes • Whenever you are heating something, heat is • As the temperature of a body approaches escaping somewhere. absolute zero, all processes cease and the • Why can’t you melt steel on your stove? entropy approaches a minimum value. • Natural gas flames are in the range of 1800-2000o C • This minimum value is almost zero, but not quite. • Steel melts around 1400oC, depending on the alloy. • The law continues that… • As it gets hotter more heat escapes to the surrounding area. • It is impossible for any procedure, no matter how idealized, to reduce any system to • You eventually reach a point where the amount of absolute zero in a finite number of steps. heat escaping the surrounding area equals the amount going into the substance. • Laws explain what, not why. • So it isn’t getting heated anymore. Reverse the problem Cont. • Whenever you are cooling something, (removing heat, there is NO cold energy) • Now of course you can correct this problem heat is always entering from somewhere. by getting a hotter flame, or by better • This is why there is such difficulty getting insulating the area around the steel. to absolute zero. • However, you would run into this same • How do you stop any heat from being able problem again at a higher temperature. to enter a substance? • No one knows. • Further corrections would be needed. • Which leads us right back to the Third Law of Thermodynamics. “It can’t be done” as a summary of all attempts so far. 4.
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