Unit 4: Thermochemistry

Transcription

1 Unit 4: Thermochemistry The making and breaking of bonds only happen as a result of energy being exchanged. Some reactions give off energy and some take in energy. This unit is all about the energy of reactions. In it we will study thermodynamics, which is the way that energy moves from one entity to another. Quality Core standards V. INTEGRATING THE MACROSCOPIC, MICROSCOPIC, AND SYMBOLIC WORLDS B. Kinetics, Equilibrium, and Thermodynamics 3. Chemical Processes and Heat; Calorimetry a) Explain the law of conservation of energy in chemical reactions b) Describe the concept of heat, and explain the difference between heat energy and temperature c) Explain physical and chemical changes as endothermic or exothermic energy changes d) Solve heat capacity and heat transfer problems involving specific heat, heat of fusion, and heat of vaporization e) Calculate the heat of reaction for a given chemical reaction when given calorimetric data 4. Enthalpy and Entropy f) Define enthalpy and explain how changes in enthalpy determine whether a reaction is endothermic or exothermic g) Compute ΔH rxn from ΔH f º values and explain why the ΔH f º values for elements are zero h) Explain and apply, mathematically, the relationship between ΔH rxn º (forward) and ΔH rxn º (reverse) i) Define entropy and explain the role of entropy in chemical and physical changes, and explain the changes that favor increases in entropy Unit 4 Learning Targets 1. I can determine the amount of heat gained or lost for a specific substance 2. I can use the heat relationship between two objects to determine missing information in a given situation where there is heat transfer occurring. 3. I can interpret an enthalpy diagram and decide if a reaction is endothermic or exothermic. 4. I can apply stoichiometric calculations (mass-mole-molar ratio-heat) to equations involving heat.

2 5. I can apply Hess's Law in order to determine the heat of a reaction 6. I can determine if a reaction is spontaneous using Gibb s Free Energy Key Formulas q rxn =q sur q=mcδt q= energy c=specific heat m=mass T=temperature ΔG=ΔH-TΔS G=gibbs free energy H=enthalpy S=enrtropy H rxn = H prod Key Vocabulary H react Calorimetry: The study of heat transfer between systems. A calorie is a unit of heat measure and is defined to be the amount of heat energy required to raise 1 gram of water 1 degree Celsius Enthalpy: A state of heat energy. We calculate the change in enthalpy by measuring the heat gained or lost in the surrounding environment. We use the change in enthalpy to determine if a reaction is endothermic or exothermic Entropy: A measure of disorder in a system. The more chaotic a system, the greater the entropy it has. Entropy of the universe is constantly increasing. Most systems will naturally progress from order to disorder, much like my classroom after I clean it. Free Energy: Energy available to do work for us. The amount of free energy in the universe is constantly decreasing. Eventually, there will be no more free energy and, therefore, no more work can be done. Exothermic: Energy is released into the surrounding environment Endothermic: Energy is removed from the surrounding environment Specific Heat: The amount of heat energy required to raise one gram of a substance one degree Celcius. The specific heat of substances is unique to that substance. Specific heat is typically calculated experimentally, but many experiments have been done such that many specific heats have been defined and recorder in table form. Heat Capacity: The amount of heat required to raise the mass one degree Celsius. This is not specialized to just one gram of the substance, but refers to the entire mass of a sample Spontaneous: Energy of reactants moves to a lower state releasing energy for doing work Heat of Fusion: The amount of heat required to melt a substance. We will typically work with Kilojoules per mole Heat of Vaporization: The amount of heat required to cause a liquid substance to turn to a gas.

3 Laws of Thermodynamics Zeroth Law: If two systems are in thermal equilibrium with each other and one of the two is also at thermal equilibrium with another, then all three are in thermal equilibrium with each other. Think about it like the transitive property in math class If A=B and B=C, then A=C 1 st Law: Energy is always conserved. You can t lose it. You can t create it. Energy just moves from one system to another. You can t get something for nothing. Energy doesn t get wasted. It sometimes doesn t get used for something useful, but it is never lost. 2 nd Law: The entropy of a system will naturally increase as it becomes more chaotic and less orderly. It would require outside energy to bring a system back into order or to hold it in a state of order. The total entropy of the universe is constantly increasing 3 rd Law: The entropy of as perfect crystal at absolute zero is zero. No system can have entropy of zero and no system can reach absolute zero. Absolute zero and zero entropy would mean that you have mass present with no energy. No energy would mean that it isn t even vibrating, electrons aren t orbiting, yikes! E=mc 2 can t equal zero if there s mass present. Everything we think we know would have to change. Calorimetry Heat is not created or destroyed but it can move from one system to another. If one system s temperature increases then another must have given up enough energy to cause the temperature increase. Heat lost equals heat gain, plain and simple. We can calculate the amount of energy gained or lost using q = mc T Where q is the amount of heat (measured in Joules or calories), m is mass, c is the specific heat of the material with which we are working, and T is the change in temperature for the material with which we are working. q rxn = q sur The heat gained or lost (that s why the values are in absolute value symbols) from a reaction either comes from or goes into the surrounding environment.

4 Enthalpy Diagrams The first diagram shows that the state of the reactants is higher than the state of the products. That being the case, energy must be given off as the reaction proceeds in order to lower the state. When energy is given off, the change in enthalpy is negative. The second diagram shows the state of the reactants is lower than the state of the products. The change in enthalpy is positive and this reaction requires energy form an outside source in order to proceed as written. Hess s Law Hess s Law is very similar to the elimination process for solving systems of equations in your math class. The total enthalpy of a reaction is independent of the reaction pathway. This means that if a reaction is carried out in a series of steps, the enthalpy change ( H) for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps. This idea is also known as Hess s law. Here are some rules for using Hess s law in solving problems: 1. Make sure to rearrange the given equations so that reactants and products are on the appropriate sides of the arrows. 2. If you reverse equations, you must also reverse the sign of H. 3. If you multiply equations to obtain a correct coefficient, you must also multiply the H by this coefficient. Finally, in doing Hess s law problems, it s often helpful to begin by working backward from the answer that you want. In other words write the final equation first. Try it out. Example Given the following equations H 3 BO 3(aq) HBO 2(aq) + H 2 O (l) H rxn = kj H 2 B 4 O 7(aq) + H 2O (l) 4HBO 2(aq) H rxn = kj H 2 B 4 O 7(aq) 2B 2 O 3(s) + H 2 O (l) H rxn = 17.5 kj find the H for this overall reaction: Explanation Multiply the first equation by 4: 2H 3 BO 3(aq) B 2 O 3(s) + 3H 2 O (l)

5 4H 3 BO 3(aq) 4HBO 2(aq) + 4H 2 O (l) H rxn = 4(-0.02 kj) = Reverse the second equation: Leave the last equation as is: 4HBO 2(aq) H 2 B 4 O 7(aq) + H 2 O (l) H rxn = kj H 2 B 4 O 7(aq) 2B 2 O 3(s) + H 2 O (l) H rxn = 17.5 kj Cross out common terms and you are left with: 4H 3 BO 3(aq) 2B 2 O 3(s) + 6H 2 O (l) H rxn = 28.8 kj Divide the above equation and the enthalpy by 2 and you see that the answer is 14.4 kj (the reaction is endothermic). Gibbs Free Energy G= H-T S Free energy is the difference between the change in enthalpy and the product of temperature and change in entropy. If the change in free energy is negative then the reaction proceeds spontaneously. Free energy in the universe wants to decrease and entropy wants to increase. This equation shows that some reactions will always be spontaneous, some will be always nonspontaneous, and others are dependent upon the temperature of the reaction. H S G Spontaneity nonspontaneous or - High T nonspontaneous@lowt or - low T nonspontaneous@hight spontaneous at all T