Thermochemistry AP Chemistry Lecture Outline

Transcription

1 Thermochemistry AP Chemistry Lecture Outline Name: thermodynamics: the study of energy and its transformations -- thermochemistry: the subdiscipline involving chemical reactions and energy changes Energy kinetic energy: energy of motion; KE = ½ mv 2 -- all particles have KE -- Thermal energy is due to the KE of particles We measure the average KE of particles as... potential energy: stored energy Chemical potential energy is due to electrostatic forces between charged particles. -- related to the specific arrangement of atoms in the substance Units of energy are joules (J), kilojoules (kj), calories (cal), or nutritional calories (Cal or kcal) -- conversions: system: the part of the universe we are studying surroundings: everything else -- Typically in chemistry, a closed system is one that can exchange energy but not matter with its surroundings. -- Usually, energy is transferred to... (1)...or... (2) force: a push or pull exerted on an object Work is done when a force moves through a distance. Equation: Heat is often referred to as an amount of energy transferred from a hotter object to a colder one. Energy is also defined as the capacity to do work or transfer heat. Find the kinetic energy of 1.0 mol of argon atoms moving at 650 m/s. Find the kinetic energy of a single argon atom moving at 650 m/s. 1

2 first law of thermodynamics = the law of conservation of energy -- energy is transferred between its various forms, but the total amount remains the same (pretty much) internal energy (E) of a system: the sum of all the KE and PE of the components of a system -- The change in the internal energy of a system is found by: E = E final E initial For chemistry, this equation becomes: E is + if E final E initial (i.e., system... ) E is if E final E initial (i.e., system... ) Clearly, E can t very well be found using the above equations. In terms of measurable quantities: E = q + w q = heat; w = work; ** KEY: Sign conventions are based on the system s point of view. Endothermic processes result in heat... e.g., Exothermic processes result in heat... e.g., state function: a property that is independent of the pathway -- From physics, displacement is a state function; distance is not. -- e.g., a bowling ball that starts from rest at the top of a hill and rolls down What quantity is the state function? What quantities are not? 2

3 Heat (q) and work (w) are NOT state functions, but their sum (i.e., E) is. charged battery dead battery To go further, we must introduce the concept of enthalpy (H). -- Enthalpy is defined as... H = E + PV where E = system s internal energy P = pressure of the system V = volume of the system -- H is a state function, and so H is, too. -- There is much that could be said about enthalpy, but what you need to know is: If a process occurs at constant pressure, the change in enthalpy of the system equals the heat lost or gained by the system. i.e., H = H final H initial = q P P indicates constant pressure conditions. When H is +, the system... When H is, the system... Enthalpy is an extensive property, meaning that the amount of material matters. enthalpy of reaction: For exothermic reactions, the heat content of the reactants is larger than that of the products. 2 H 2 O 2 (l) 2 H 2 O(l) + O 2 (g) H = 196 kj Find the enthalpy change when 5.00 g of H 2 O 2 decompose at constant pressure. 3

4 H for a reaction and its reverse are the opposites of each other. 2 H 2 (g) + O 2 (g) 2 H 2 O(g) ( H = kj) 2 H 2 O(g) 2 H 2 (g) + O 2 (g) ( H = kj) Enthalpy change depends on the states of reactants and products. 2 H 2 (g) + O 2 (g) 2 H 2 O(g) ( H = kj) 2 H 2 (g) + O 2 (g) 2 H 2 O(l) ( H = kj) Calorimetry: the measurement of heat flow -- device used is called a... heat capacity of an object: amount of heat needed to raise object s temp. 1 K = 1 o C molar heat capacity: amt. of heat needed to raise temp. of 1 mol of a substance 1 K specific heat (capacity): amt. of heat needed to raise temp. of 1 g of a substance 1 K i.e., molar heat capacity = molar mass X specific heat We can find the heat a substance loses or gains using: where q = heat m = mass of substance c P = substance s specific heat T = temperature change Large beds of rocks are used in some solar-heated homes to store heat. Calculate the quantity of heat absorbed by 58.0 kg of rocks if their temperature increases from 20.0 o C to 36.0 o C and their specific heat is 0.85 J/g-K. 4

5 With a coffee-cup calorimeter, a reaction is carried out under constant pressure conditions. -- Why is the pressure constant? -- If we assume that no heat is exchanged between the system and the surroundings, then the solution must absorb any heat given off by the reaction. i.e., q soln = q rxn -- For dilute aqueous solutions, it is a safe assumption that c P = When 50.0 ml of M AgNO 3 and 50.0 ml of HCl are mixed in a constantpressure (i.e., a coffee-cup) calorimeter, the mixture s temperature increases from o C to o C. Calculate the enthalpy change for this reaction. List any assumptions you make. Combustion reactions are studied using constant-volume calorimetry. This technique requires a bomb calorimeter. -- the heat capacity of the bomb calorimeter (C cal ) must be known -- again, we assume that no energy escapes into the surroundings, so that the heat absorbed by the bomb calorimeter equals the heat given off by the reaction -- the heat of reaction is then given by the simple equation: q rxn = C cal T A g sample of lactic acid, HC 3 H 5 O 3, is burned in a bomb calorimeter having a heat capacity of kj/ o C. The temperature of the material in the calorimeter increases from o C to o C. Calculate the heat of combustion of lactic acid per gram and per mole. 5

6 NOTE: In a bomb calorimeter, the heat transferred is actually equal to the change in internal energy E, not the change in enthalpy H. Recall that since H = E + PV, then H = E + (PV). However, in most cases, the difference between H and E in bomb calorimetry is very small, on the order of 0.1%, so we don t worry about it. Hess s Law The H rxn s have been calculated and tabulated for many basic reactions. Hess s law allows us to put these simple reactions together like puzzle pieces such that they add up to a more complicated reaction that we are interested in. By adding or subtracting the H rxn s as appropriate, we can determine the H rxn of the more complicated reaction. Given the following, calculate the enthalpy (i.e., heat) of reaction for the conversion of graphite to diamond. C (graphite) + O 2 (g) CO 2 (g) H = kj C (diamond) + O 2 (g) CO 2 (g) H = kj Calculate H for the reaction NO(g) + O(g) NO 2 (g) given the following: NO(g) + O 3 (g) NO 2 (g) + O 2 (g) H = kj O 3 (g) 3 / 2 O 2 (g) H = kj O 2 (g) 2 O(g) H = kj 6

7 enthalpy of formation ( H f ): the enthalpy change associated with the formation of a compound from its constituent elements -- also called heat of formation When finding the standard enthalpy of formation ( H o f ), all substances must be in their standard states. The standard state of a substance has arbitrarily been chosen to be the state of the substance at 25 o C (298 K). If more than one form of the element exists at 298 K, then the standard state is the most stable form, e.g., O 2 rather than O By definition, H o f for the most stable form of any element in its standard state is zero. e.g., -- H f values are always for 1 mol of substance, so the units are usually kj/mol. -- Many H f values have been tabulated. standard enthalpy of a reaction ( H o rxn): -- Using Hess s law, we can easily calculate H o rxn from the H f o of all R and P. -- equation: H o rxn = n H f o (products) m H f o (reactants) where n and m are the coefficients in the balanced equation Using standard enthalpies of formation, calculate the energy change for the combustion of 246 g of ethanol, CH 3 CH 2 OH. Calculate the H f o of CuO, given that: CuO(s) + H 2 (g) Cu(s) + H 2 O(l) H o rxn = kj 7

8 Energy, Food, and Fuel fuel value: the energy released when 1 g of a material is combusted -- measured by calorimetry Food The body runs on glucose, C 6 H 12 O When it is in the blood stream, glucose is called blood sugar. -- Our bodies produce glucose out of the foods that we consume. carbs: fats: proteins: 4 kcal/g; quickly broken down into glucose; not much can be stored as carbs 9 kcal/g; broken down slowly; insoluble in water; easily stored for future use 4 kcal/g; contain nitrogen which ends up as urea, (NH 2 ) 2 CO after digestion Fuels fossil fuels: coal, petroleum, natural gas -- products of what used to be living things -- nonrenewable coal gasification: coal is treated with superheated steam to make the gases CH 4, H 2, and CO -- most impurities (e.g., sulfur compounds) are easily removed in this process -- the fuel gases can be easily transported by pipeline and then burned for fuel Nuclear energy, from the splitting or fusing of atoms, is also nonrenewable. -- a lot of bang for your buck, but there is the problem of hazardous waste disposal Renewable energy sources include: solar wind geothermal hydroelectric biomass Solar heating could be used to generate CO and H 2 gases, which could be burned... Solar (or photovoltaic) cells directly convert solar energy to electricity. Problems with solar energy: 8