Thermochemistry PDF

**Heat Calculations** Terminologies: - Energy is the capacity to do work. - Work is the directed energy change resulting from a process. - Forms of energy: - Kinetic energy is the energy produced by a moving object. - Potential energy is energy available by virtue of an objec...

**Heat Calculations** Terminologies: - Energy is the capacity to do work. - Work is the directed energy change resulting from a process. - Forms of energy: - Kinetic energy is the energy produced by a moving object. - Potential energy is energy available by virtue of an object's position. - Chemical energy is energy stored within the structural units of chemical substances. - Thermal energy is the energy associated with the random motion of atoms and molecules. - Radiant energy is solar energy, that is energy that comes from the sun. - Heat is the transfer of thermal energy between two bodies that are at different temperatures. - Thermochemistry is the study of heat change in chemical reactions. - The heat exchange occurs between the system (part of universe of interest) and the surroundings (the rest of the universe). - Calorimetry is the measurement of heat flow. Calorimeter is a device used to measure heat flow. - The heat capacity (C) is the amount of heat required to raise its temperature by 1 K (or 1^o^C). - The heat capacity of one mole of a substance is called its molar heat capacity (C~m~). - The heat capacity of one gram of a substance is called its specific heat capacity or merely its specific heat. - [\$\\text{specific\\ heat\\ }\\left( C\_{s} \\right) = \\ \\frac{\\text{quantity\\ of\\ heat\\ transferred\\ }}{\\text{mass\\ x\\ temperature\\ change}} = \\ \\frac{q}{m\\ x\\ \\mathrm{\\Delta}T}\$]{.math.inline} - Law of Conservation of Energy states that the total quantity of energy in the universe is assumed constant. Illustrative Examples: 1. The temperature of 335 g of water changed from 24.5^o^C to 26.4^o^C. How much heat did this sample absorb? C for water = 4.18 J/g^o^C 2. A hot iron horseshoe (mass = 0.40 kg) which has just been forged, is dropped into 1.6 kg of water in a 0.30 kg iron pot initially at 20°C. If the final equilibrium temperature is 25°C, determine the initial temperature of the hot horseshoe. The specific heat of iron is 450 J/kg·°C and of water is 4182 J/kg.^o^C.. **Thermochemistry** Terminologies: - Thermodynamics is the scientific study of the interconversion of heat and other kinds of energy. - Thermochemistry is the study of heat change in chemical reactions. - Exothermic process is the transfer of thermal energy to the surroundings. Endothermic process is that heat is supplied to the system.  - Enthalpy is the heat content of a chemical system. The enthalpy change is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure. - Thermochemical equations show the enthalpy changes as well as the mass relationships.  - Standard Enthalpy of formation [(*ΔH*~*f*~^*o*^)]{.math.inline} is the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm. - The standard enthalpy change of reaction [(*ΔH*~rxn~^*o*^) ]{.math.inline}can be calculated using the formula below: - Hess's law states that when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.  Illustrative Examples: 1. Given the following: CH~4~ (g) + O~2~ (g) CO~2~(g) + H~2~O (l) ΔH = - 890 kJ a. Write the balanced thermochemical equation. b. Is the reaction exothermic or endothermic? c. How much heat is given off by the reaction of 3.5 moles of CH~4~? d. How much heat is given off by the reaction of 40.0 g of O~2~? e. How many grams of CH~4~ are needed to release 2000 J of heat? 2. **Use a standard enthalpies of formation table to determine the delta H for each of these reactions. State whether each reaction is endothermic or exothermic.** 3. The standard enthalpy of combustion for xylene, C~8~H~10~(*l*), is ‑3908 kJ mol^-1^. Using this data and the standard enthalpies of formation, : H~2~O(*l*) = ‑285.9 kJ mol^-1^; CO~2~(*g*) = ‑393.5 kJ mol^-1^, calcu­late the standard enthalpy of formation of C~8~H~10~(*l*), in kJ mol^-1^. Hint: Write chemical equations for combustion of Xylene. 4. Calculate the ∆H for the following reaction: NO   +   O~ ~  NO~2   ~  ∆H = ? You are given these three equations: **Calorimetry** Definition of Terms: - *Calorimetry* is a method of measuring the [heat transfer](https://www.thoughtco.com/how-does-heat-transfer-2699422) within a chemical reaction or other physical processes, such as a change between different states of matter. A device used to measure heat flow is a calorimeter. - The heat capacity (C) is the amount of heat required to raise its temperature by 1 K (or 1^o^C). - The heat capacity of one mole of a substance is called its molar heat capacity (C~m~). - The heat capacity of one gram of a substance is called its specific heat capacity or merely its specific heat. - [\$\\text{specific\\ heat\\ }\\left( C\_{s} \\right) = \\ \\frac{\\text{quantity\\ of\\ heat\\ transferred\\ }}{\\text{mass\\ x\\ temperature\\ change}} = \\ \\frac{q}{m\\ x\\ \\mathrm{\\Delta}T}\$]{.math.inline}  - Constant Volume Calorimetry. It is used for accurate determination of the enthalpy of combustion for food fuel and other compounds. A bomb calorimeter is used for this purpose. - To calculate the heat of combustion: q~rxn~ = - C~cal~ x ∆T where C~cal~ is the heat capacity of the calorimeter - Constant Pressure Calorimetry. A thermally insulated container with a thermometer and stirrer is used. Generally, a coffee cup calorimeter is used for this purpose. - The heat gained by the solution, q~soln~, is equal in magnitude to heat lost by the reaction, q~rxn~, but opposite in sign: q~soln~ = - q~rxn~ Illustrative Examples: 1. When 50.0 mL of 0.100 M AgNO~3~ and 50.0 mL of 0.100 M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from 22.30^o^C to 23.11^o^C. The temperature increase is caused by the following reaction: AgNO~3~(aq) + HCl(aq) → AgCl(aq) + HNO~3~(aq) 2. A 0.5865-g sample of lactic acid (HC~3~H~5~O~3~) is burned in a calorimeter whose heat capacity is 4.812 kJ/^o^C. The temperature increases from 23.10^o^C to 24.95^o^C. Calculate the heat of combustion of lactic acid (a) per gram and (b) per mole.