Thermochemistry

Thermochemistry

ENERGY - "the capacity to do work" • Unlike matter, energy is known and recognized by its effects. • it cannot be seen, touched, smelled or weighed • types of energy • radiant (solar energy) energy from the sun • thermal the energy associated with the random motion of atoms and molecules (measured by temperature) • chemical energy stored within the structural units of chemical substances • potential energy available by virtue of an object's position

Conservation of Energy • the total quantity of energy in the universe is assumed constant • Energy cannot be created or destroyed • When one form of energy disappears, some other form of energy (of equal magnitude) must appear • The above statements are all saying the same thing.

All forms of energy can theoretically convert to other forms of energy... • Chemical reactions are often utilized in our day-to-day lives not for the products that they produce, but for the converted energy that results (i.e. fuels like natural gas - a chemical reaction results in the transfer of Chemical energy to thermal energy) • We do not use the products of the reaction: • Carbon Dioxide • Water Hence, why there is so much pollution...

THERMOCHEMISTRY • Thermochemistry - the study of heat change in chemical reactions • Heat - is the transfer of thermal energy between two bodies that are at different temperatures.

Systems and Surroundings (parameters) • When looking at chemical reactions, we have to define the "system" • 3 types of systems of Thermochemistry: • open system • closed system • isolated system • the rest of the universe (outside the "system" is called the "surroundings")

open systems can exchange mass and energy • closed systems allow the transfer of energy (heat) but not mass • isolated systems do not allow transfer of either mass or energy

HYDROGEN EXPLOSION - combustion • Is this reaction taking place in an open, closed or isolated system? • the reacting mixture is the system (hydrogen, oxygen, and water molecules)... ...everything else is the surroundings • This is exothermic because it is a combustion reaction (thermal energy leaves during this and all other exothermic reaction -- a lot of thermal energy is released in this example...） open system

Liberating Hydrogen from HCl • Is this reaction taking place in an open, closed or isolated system? The gas (hydrogen) is collected and therefore never leaves the system. All the matter remains in the system. This is a closed system. This system is both exothermic and endothermic. Heat is released to the surroundings as Zinc and Chlorine bond together, but as hydrogen disassociates from the Chlorine, energy is taken in.

What's an example of a chemical reaction occurring in a closed system? • Examples of Closed Systems;

Equations PRACTICE QUESTION The work done to compress a gas is 74 J. As a result, 26 J of heat is given off to the surroundings. Calculate the change in energy of the gas. Which equation should you use to solve? ANSWER: 48 J

Enthalpy (H) Most reactions are constant-pressure process. Therefore, so we can equate the heat change to the change in H.

CHANGE IN H

Practice problem: Determine the amount of heat (in kJ) given off when 1.26 x 10^4 g of NO2 are produced according to the equation: 114.6 kJ/mol -1.57 x 10^4 kJ

Practice Problem (for change in Energy) • If 2.0 moles of H2O(g) are converted to H2(g) and O2(g) against a pressure of 1.0 atm at 125 degrees celsius, what is the change in Energy?

Calorimetry - "the measurement of heat change" • a "calorimeter" is an insulated, closed container that creates an ISOLATED SYSTEM • a specific quantity of water surrounds a system carrying out a reaction • during the reaction, heat leaves the system and is absorbed by the surrounding water • by recording temperature change, the heat generated by a reaction can be recorded

Calorimetry • Either heat capacity or specific heat must be applied to find a substances change in heat. • Remember... heat capacity specific heat (C) = m x (s)

Heat Capacity • heat needed to raise a certain quanity of a substance 1 degree (celsius) • Specific Heat • the heat needed to raise a 1 gram of a specific substance 1 degree (celsius)

Calorimeter #1 - Constant-Volume Calorimiter (The "BOMB" Calorimeter) • Purpose: to collect data on combustible reactions • When adding heat to a substance in a Constant-Volume Calorimeters, the pressure will increase because the volume has remained the same. • The substance inside cannot expand

Calorimeter #1 - Constant-Volume Calorimiter (The "BOMB" Calorimeter) • Change of heat of a reaction is qrxn . • Since the a calorimeter is an isolated system, the change in heat of the calorimeter plus water is equal but opposite to the change of heat in the reaction qsystem = qcal + qrxn = 0 Meaning... qcal = -qrxn

Practice Question • A quantity of 1.435 naphthalene (C10H8) is burned in a calorimeter. The water temp. rises from 20.28 to 25.95 degrees celsius. If the heat capacity (C) is 10.17 kJ/celsius, calculate the molar heat combustion of C10H8. • What is being asked here: • How much heat per mole of napthalene is released into the calorimeter? To do: • Need to figure out how much heat is generated by the combustion • Need to convert this total heat to q per mole (kJ/mol)

Practice Question • 1.922g of methanol (CH3OH) was burned in a calorimeter. The temp. of water rose by 4.2 degrees celsius. The heat capacity of the system (calorimeter plus water) is 10.4 kJ/celsius. Calculate the molar heat of the combustion of methanol.

Calorimeter #2 - Constant-Pressure Calorimeter Specific Heat will be used in calculations. Water's specific heat is 4.184 J/g(celsius) • Measures heat changes for non-combustible reaction (i.e. acid-base reactions) • Since the pressure remains the same, change in enthalpy equals that to the change in heat • Also isolated and filled with water

Thermochemistry