Thermochemistry. Potential Energy is stored energy E P = mgh where m = mass, g = constant acceleration of gravity and h = height

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1 Thermochemistry Thermochemistry is the science of relationships between heat and energy, which is one area of thermodynamics - the study of energy and its transformations. Energy is the potential or capacity to move matter. Work = force x distance Heat is the energy that is transferred from one object to another because of a difference in temperature. Kinetic Energy is the energy associated with an object by virtue of its motion. E k = 1/2 mν 2 velocity, ν = distance/ time SI Units for energy = kg x m 2 / s 2 = J (joule) calorie - a non SI Unit of energy commonly used, is defined as the energy needed to raise the temperature of one gram of water by one degree Celsius. calorie = J Example: A pitcher throws a baseball on avergage 60 to 90 miles/hr. A regulation baseball weighing 143 g is traveling 75 mile/ hr. What is its kinetic energy in joules and calories? Potential Energy is stored energy E P = mgh where m = mass, g = constant acceleration of gravity and h = height

2 Total Energy is the sum of the kinetic and potential energies plus its internal energy. E tot = E k + E p + U Internal Energy, U = total energy = E E = E final - E inital + E then E f > E i energy is gained from the surroundings - E then E f < E i energy is lost to the surroundings Law of Conservation of Energy, 1st Law of Thermodynamics, says that the energy may converted from one form to another, but the total quantity if energy remains constant Systems and Surroundings System is the substance or substances that are under study in which a change occurs. Surroundings are everything else in the vicinity Heat, q, is energy that flows in and out of a system because of a difference in temperature between system and surroundings. +q = heat is absorbed by the system -q = heat is evolved or released by the system The Heat of Reaction (at a given temperature) is the value of q required to return a system to the given temperature at the completion of the reaction.

3 Exothermic process is a chemical reaction or a physical change in which heat is evolved, q = - Endothermic process is a chemical reaction or a physical change in which heat is absorbed, q = + Example: Ammonia burns in the presence of a palatium catalyst to give nitric oxide, NO 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (l) In an experiment, 4 moles of NH 3 is burned and evolves 1170 kj of heat. Is the reaction endothermic or exothermic? What is the value of q? Endothermic q = + The reaction produces a cold solution Exothermic q = - The reaction produces a hot solution and dissipates into the surroundings

4 Enthalpy (H) is an extensive property of a substance that can be used to obtain the heat absorbed or evolved in a chemical reaction under constant pressure. Enthalpy is a state function, a property of a system that depends only on its present state, which is determined by variables such as temperature and pressure, and is independent of any previous history of the system. H = q p heat added or lost by the system when the process occurs under constant pressure H = H final - H initial + H value - H final > H initial gained heat - H value - H final < H initial lost heat to surroundings The H for a reaction is equal in magnitude but opposite in sign to the H for the reverse reaction. CO 2(g) + 2H 2 O (g) CH 4(g) + 2O 2(g) H = 802 kj CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (g) H = -802 kj The H for a reaction depends on the state of the reactants and products H = U + PV where U is the internal energy, P is the pressure, and V is volume. A Thermochemical Equation is the chemical equation for a reaction (including phase labels) in which the equation is given a molar interpretation, and the enthalpy of reaction for these molar amounts is written directly after the equation. N 2 (g) + 3H 2(g) 2NH 3(g) H = -91.8kJ 2N 2 (g) + 6H 2(g) 4NH 3(g) H = -184 kj 2NH 3(g) N 2 (g) + 3H 2(g) H = +91.8kJ

5 Calculating the Heat from Stoichiometry the following equation? Assume the reaction happens at constant pressure. N 2 (g) + 3H 2(g) 2NH 3(g) H = -91.8kJ How much heat evolves when 10.0g of hydrazine reacts according to the following reaction 2N 2 H 4(l) + N 2 O 4(l) 3N 2(g) + 4H 2 O (g) H = kj Heat is required to raise the temperature of a given amount of a substance, and the quantity of heat depends on the temperature change. Heat Capacity (C) is the quantity of heat needed to raise the temperature of the sample of substance one degree Celsius. q = C t where t = t f - t i A piece of iron requires 6.70 J of heat to raise the temperature by one degree Celsius. What is the quantity of heat required to raise the temperature of the piece of iron from 25.0ºC to 35.0ºC?

6 Specific Heat is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius at constant pressure. q = s x m x t Calculate the heat absorbed by 15.0g of water to raise its temperature from 20.0ºC to 50.0ºC (at constant pressure) Iron metal has a specific heat of J/(g. ºC). How much heat is transferred to a 5.00 g piece of iron, initially at 20.0ºC, when it is placed in a pot of boiling water? Assume that the temperature of the water is 100.0ºC and that the water remains at this temperature, which is the final temperature of the iron. Suppose g of graphite is placed in a calorimeter with an excess of oxygen at 25.00ºC and 1 atm pressure. Excess O 2 ensures that all the carbon burns to form CO 2. The graphite is ignited and it burns according to the equation C (graphite) + O 2(g) CO 2(g)

7 On reaction, the calorimeter temperature rises from 25.00ºC to 25.89ºC. The heat capacity of the calorimeter and its contents was determined in a separate experiment to be 20.7 kj/ºc. What is the heat of reaction at 25.00ºC and 1 atm? Express the answer in a thermochemical equation. Hess's Law of Heat of Summation staes that for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps. Example: Suppose you would like to find the enthalpy change for the following reaction: 2C (graphite) + O 2(g) CO (g) The direct determination for this change in enthalpy is very difficult. Applying Hess's law gives us the option to use more than equation to determine this value.

8 Standard Enthalpy of Formation - because Hess's Law relates the enthalpy changes of some reactions to the enthalpy changes of others, we only need to tabulate the enthalpy changes of certain types of reactions. We also list enthalpy changes in a standard state Standard State refers to the standard thermodynamic conditions chosen for substances when listing or comparing thermodynamic data: 1 atm pressure and a specific temperature of 25ºC. Hº = standard enthalpy of reaction H f º = standard enthalpy of formation H f º for diamond equals the enthalpy change from the stablest form of carbon (graphite) to diamond C (graphite) C (diamond) H f º = 1.9 kj H f º fro graphite = 0, the reference forms have zero values In general, we can calculate the Hº for a reaction by the equation Hº = Σ n H f º (products) - Σ m H f º(reactants) Example: Calculate the heat of vaporization, H vap º of water, using standard enthalpies of formation (Table 6.2) Calculate the enthalpy change for the following reaction: 3NO 2(g) + H 2 O (l) 2HNO 3(aq) + NO (g)

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