Dec 4 9:41 AM. Dec 4 9:41 AM. Dec 4 9:42 AM. Dec 4 9:42 AM. Dec 4 9:44 AM. Dec 4 9:44 AM. Mostly coal, petroleum and natural gas

Transcription

1 Quality vs Quantity of Energy Your textbook discusses the idea that although energy is conserved we face an energy crisis because the quality of energy we have available to do work like heat our homes and run industry is decreasing. This happens because when we use energy to do work we lower its usefulness or its quality. Another way of looking at it is concentrated energy spread energy use to do work One form of concentrated energy that we use today is in the form of fossil fuels. What are fossil fuels? Remains of decayed plants over millions of years Hydrocarbons with stored energy from photosynthesis What are the types of fossil fuels? Mostly coal, petroleum and natural gas What is petroleum? What are the components of petroleum and how are they separated? Dec 4 9:41 AM Dec 4 9:41 AM The Greenhouse Effect and Global Warming What is the Greenhouse Effect? Visible light waves emitted by the Sun pass through the Earth s atmosphere. These waves strike the Earth and get reradiated (bounce off) at longer wavelengths. The visible waves get changed into infrared or heat waves. These heat waves cannot escape back into space because they get absorbed (or trapped) by certain gases in our atmosphere. The trapped heat waves cause the atmosphere of our planet to increase in temperature. This is a good thing because it keeps the Earth from freezing. This phenomenon is called the Greenhouse effect because we observe the same kind of effect in a greenhouse. In a greenhouse, the visible light waves from the Sun pass through the glass, strike the ground, and get reradiated out as heat or infrared waves. These longer waves cannot pass through the glass, and thus get trapped in the greenhouse. In this way the greenhouse stays warm. What are the greenhouse gases? These are gases whose molecular structure absorbs heat or infrared waves. The gases are: carbon dioxide, CO2 methane, CH4 water vapor, H2O Dec 4 9:42 AM Dec 4 9:42 AM What is global warming? Global warming is a theory that our planet is heating up because we are adding more and more greenhouse gases into our atmosphere. The main gas is carbon dioxide, which is a product of the combustion of fossil fuels like coal and gasoline. The idea is that the more carbon dioxide we put in our atmosphere, the more heat waves get trapped and the warmer the planet gets. What are possible consequences of global warming? Models predict the following: Changing climates around the world Melting of polar ice caps resulting in rising sea levels displacing millions of people Increase incidence of diseases such as malaria Thermodynamics Thermodynamics is the study of energy and its interconversions. It also studies whether reactions will occur that is, the spontaneity of reactions. To a chemist a spontaneous reaction means one that can occur. To determine the spontaneity of a reaction, three factors must be considered: 1. the enthalpy change ( H) 2. the entropy change ( S) 3. the temperature (in Kelvin) Dec 4 9:44 AM Dec 4 9:44 AM 1

2 It is impossible to measure the enthalpy of a substance. It is possible to measure the change in enthalpy a substance undergoes during a physical or chemical change: H = H products H reactants Problem Write the equation showing the standard heat of formation of solid ammonium chloride. In an endothermic reaction heat is absorbed and the H is positive. In an exothermic reaction heat is released and H is negative. This change in enthalpy is known as the heat of the reaction. The heat of reaction is a generic term. Chemists often refer to particular kinds of reactions, such as the heat of combustion or the heat of fusion. One important kind of reaction involves the formation of a compound from its elements under standard conditions. This is termed the standard heat of formation of a compound, as is expressed in kj/mol of compound formed. It has the symbol Hf. Hf = heat involved in the formation of a compound from its elements in their standard states The standard conditions referred to here are not the same as STP conditions we discussed when studying the gas laws. Here the standard conditions refer to: 25 o C or 298K 760 mm Hg = 1 atm = kpa Based on Table I, which compound is most stable? Which compound is least stable? Illustrate with PE diagram of each noting the ΔH s. The standard heats of formation of various compounds can be easily looked up in the Handbook of Chemistry and Physics but we will avail ourselves to reference table I (discussed earlier in chapter 9), which gives the values for generic heats of reactions. Keep in mind that only those reactions involving elements forming compounds are heats of formation type reactions. Note from the above table that those compounds having large negative heats of formation are stable, while those having positive heats of formation are unstable. Why is this? Recall that in exothermic reactions the products have a lower PE compared to the reactants and are energetically, or we should say thermodynamically, more stable. The more exothermic the reaction, the bigger the H and the more stable the compound. Conversely compounds formed via endothermic reactions have a higher PE compared to the elements from which they formed, and are thermodynamically less stable Ways to Calculate H 1. via constant pressure calorimeter where Q = H = mc T 2. via Hess Law of Constant Heat Summation when a reaction can be expressed as the algebraic sum of a sequence of two or more other reactions, then the heat of the reaction is the sum of the heats of these other reactions. For example Note the following: H is an extensive property it depends on the number of moles; hence, if we double the moles of product we must double the H; if we halve the moles we halve the H If we reverse a reaction we must reverse the sign of H Sn(s) + Cl2(g) SnCl2(s) SnCl2(s) + Cl2(g) SnCl4(s) H = kj H = kj Net: Dec 4 9:46 AM Dec 4 9:46 AM 2

3 Use the following equations to calculate the standard heat of formation of sulfur dioxide gas: S(s) + O2(g) SO2(g) 1. S(s) + 3/2 O2(g) SO3(g) ΔH = 395 kj 2. 2 SO2(g) + O2(g) 2 SO3(g) ΔH = 198 kj 3. via heats of formations: H = Σn Hfproducts Σn Hfreactants note the Hf of an element is taken to equal zero; keep in mind H is an extensive property (depends on n) ΔH f C 2H 6(g) = 84 kj/mol ΔH f CO 2(g) = 394 kj/mol ΔH f H 2O(l) = 286 kj/mol calculate the H o for the reaction: C 2H 6(g) + 5/2O 2(g) 2CO 2(g) + 3H 2O(l) Dec 4 9:47 AM Dec 4 9:47 AM Demonstration In this demonstration a M&M will be burned in a pure source of oxygen gas. The oxygen gas is obtained by decomposing a metal chlorate. a) Write an equation for the combustion of glucose: b) Calculate the standard ΔH for this combustion, given the following andard sδh f values: ΔH f glucose = 1275 kj/mol ΔH f H 2O(g) = 242 kj/mol ΔH f CO 2(g) = 394 kj/mol Entropy Entropy is a measure of the disorder or randomness of a system. Entropy has the symbol S and a change in entropy is given by the symbol S. c) If an M&M weighs.80 g and is 90.0% glucose, calculate the heat energy involved in the combustion of an M&M: d) If the heat energy in (c) is added to g of water at a temperature of 25.0oC, what will be the final temperature of the water? S = S products S reactants A relative measure of the entropy of a system can be seen by looking at the change in state (phase) the system undergoes: solid liquid gas increasing entropy Chemists have observed that systems tend to go towards states of maximum entropy. Tell that to your parents the next time they yell at you for your messy room! The second law of thermodynamics can be stated as: The entropy of the universe is always increasing Your textbook describes the cause of this entropy due to energy spread (i.e. exothermic reactions) and matter spread (i.e. i.e. gas occupies volume it is placed in). Dec 4 9:48 AM Dec 4 9:48 AM Spontaneity of a Reaction and Gibbs Free Energy Chemists have observed the following: Nature favors exothermic reactions those having a negative H; that is, those reactions in which energy is released (energy spread) tend to occur or are spontaneous. Nature favors increasing disorder those reactions having a positive S; that is, those reactions in which the products have a greater randomness compared to the reactants (matter spread) tend to occur or are spontaneous. The mathematician Willard Gibbs proposed a thermodynamic concept to simultaneously incorporate the concepts of entropy and enthalpy. He called this concept free energy. It is now called Gibbs free energy in his honor and is given the symbol G. The free energy change, or G (or Gibb s free energy) is thought of as the net driving force of a reaction. It is expressed mathematically as: G = H T S when G = reaction is spontaneous G = + reaction is not spontaneous G = 0 reaction is at equilibrium Note that to a chemist the term spontaneous has the specific meaning that the reaction can occur. It does not mean it will occur instantaneously. 3

4 Case 1. S(s) + O2(g) SO2(g) + heat Case 2. N2(g) + 3H2(g) + heat 2NH3(g) Case 3. NH4Cl(s) + heat NH4Cl(aq) Case 4. Mg(s) + Cl2(g) MgCl2(s) + heat Use thermodynamic principles to explain the spontaneity of the following reaction: The H for a reaction = +293kJ and the S = J/K. Calculate G at 25 o C. Will the reaction be 2KClO 3(s) + heat 2KCl(s) + 3O 2(g) spontaneous at 100 o C? Dec 4 9:51 AM 4

5 Demonstration How can you test for the presence of ammonia gas? Consider the reaction between two solid ionic compounds, ammonium thiocyanate and barium hydroxide octahydrate: 2NH 4SCN(s) + Ba(OH) 2. 8H 2O(s) Ba(SCN) 2(aq) + 2NH 3(g) + 10 H 2O(l) The two solids are mixed in a flask which is placed on a wet piece of wood. The water surrounding the flask froze yet the water inside the flask did not freeze. Can you offer an explanation? What is the sign for the enthalpy change, H? Explain. What is the sign for the entropy change, S? Explain. Is the reaction spontaneous at room temperature? Which factor, H or S, drives the reaction? Explain. Dec 4 9:51 AM Dec 4 9:52 AM 5