Thermochemistry

Chemical reactions involve changes in energy Breaking bonds releases energy Forming bonds requires energy The energy involved in chemistry is real and generally a measurable value Energy units are numerous, but we will concentrate on the Joule (SI base unit) and the calorie (little c, big C is the food Calorie or a kilocalorie) 1 calorie = Joules INTRO TO THERMOCHEMISTRY

Thermochemistry The study of energy changes that occur during chemical reactions and changes of state.

The Universe Can be divided into 2 “parts” System- the part you are investigating Surroundings- the rest of the universe In a thermo-chemical experiments the region in immediate vicinity of the system are the surroundings.

Chemical Potential Energy the energy stored in the chemical bonds of a substance the kinds of atoms and their arrangement in the substance determine the amount of energy stored in the substance.

There are three methods used to transfer heat energy. Conduction – transfer of heat through direct contact Convection – transfer of heat through a medium like air or water Radiant – transfer of heat by electromagnetic radiation

Heat Symbol = q energy that transfers from one object to another because of temperature difference Heat always flows from a warmer object to a cooler object. An Ice Cold Spoon A Hot Spoon energy transfer

AKA: HEAT If 2 objects remain in contact, heat will flow from a warmer object to the cooler object until the temperatures are equal.

WHAT IS HEAT? Hot & cold, are automatically associated with the words heat and temperature Heat & temperature are NOT synonyms The temperature of a substance is directly related to the energy of its particles, specifically its: Kinetic Energy defines the temperature Particles vibrating fast = hot Particles vibrating slow = cold

Different type of materials may have the same temp, same mass, but different conductivity. They are affected by the potential energy stored in chemical bonds or the IMFs holding molecules together It is possible to be at same temp (same KE) but have very different thermal energies. The different abilities to hold onto or release energy is referred to as the substance’s heat capacity

Unit for Measuring Heat Flow calorie (cal) = heat needed to raise the temperature of 1 g of pure water by 1º C 1 Calorie = 1 kilocalorie = 1000 calories calorie = c, dietary calorie = C 1 Calorie = energy in food a doughnut has 500 Calories or 500,000 cal

Unit for Measuring Heat Flow Joule, the SI unit of energy One joule of heat raises the temperature of 1 g of pure water º C 1 J = cal J = 1 cal

Units for Measuring Heat The Joule is the SI system unit for measuring heat: The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

Heat DEFINITION THE FLOW OF THERMAL ENERGY FROM SOMETHING WITH A HIGHER TEMP TO SOMETHING WITH A LOWER TEMP UNITSMEASURED IN JOULES OR CALORIES TYPES THROUGH WATER OR AIR = CONVECTION THROUGH SOLIDS = CONDUCTION TRANSFERRED ENERGY BY COLLISION WITH PHOTON = RADIANT ENERGY

The measure of how well a material absorbs or releases heat energy is its heat capacity Physical property unique to a particular material The heat capacity depends on both its mass and its chemical composition. The greater the mass; the greater the heat capacity. It can be thought of as a reservoir to hold heat, how much it holds before it overflows is its capacity Water takes 1 calorie of energy to raise temp 1 °C Steel takes only 0.1 calorie of energy to raise temp 1 °C Heat Capacity

Specific Heat The amount of heat required to raise the temperature of one gram of substance by one degree Celsius.

q = mC  T

Calculations Involving Specific Heat c = Specific Heat q = Heat lost or gained  T = Temperature change OR m = Mass

Table of Specific Heats

In any chemical or physical process, energy is neither created or destroyed If energy of the system decreases the energy of the surroundings must increase by the same amount so that the total energy of the universe remains unchanged. Law of Conservation of Energy

Endothermic Process a reaction in which heat energy is absorbed process in which energy is absorbed as it proceeds, and surroundings become colder the system gains heat as the surroundings cool down typically feels cooler the longer the reaction proceeds q is positive

Endothermic Reactions

Exothermic Process a reaction in which heat energy is generated process in which energy is released as it proceeds, and surroundings become warmer typically feel warm as the reaction proceeds the system loses heat as the reaction progresses q is negative

Exothermic Reactions

HEATHEATHEATHEATHEATHEATHEATHEAT

Calorimetry The precise measurement of heat flow into or out of a system for chemical and physical purposes The heat released by the system is equal to the heat absorbed by its surroundings. Conversely, the heat absorbed by a system is equal to the heat released by it’s surroundings.

Calorimeter

Key Points Δ = Change q= heat capacity KE = ½ mv 2 Heat Capacity 1 J = cal J = 1 cal q=mcΔT q= heat m = mass c = specific heat ΔT = change in temperature Calorimetry

Enthalpy H The heat content of a system at a constant pressure. The heat released or absorbed by a reaction at constant pressure is the change in enthalpy. Heat and Enthalpy are used interchangeably. So…… q = ΔH

CHANGE IN HEAT ENERGY (ENTHALPY) The energy used or produced in a chemical reaction is called the enthalpy of the reaction Burning a 15 gram piece of paper produces a particular amount of heat energy or a particular amount of enthalpy Enthalpy is a value that also contains a component of direction (energy in or energy out) Heat lost is the out-of direction; ie exo-

CHANGE IN HEAT ENERGY (ENTHALPY) The energy used or produced in a chemical reaction is called the enthalpy of the reaction Burning a 15 gram piece of paper produces a particular amount of heat energy or a particular amount of enthalpy Enthalpy is a value that also contains a component of direction (energy in or energy out) Heat lost is the out-of direction; ie exo- Heat gained is the in-to direction; ie endo-

Exothermic and Endothermic Processes

Enthalpy Change

Thermochemical Equations The enthalpy change for a reaction can be written as a reactant or product. The Heat of Reaction is the enthalpy change for the chemical equation exactly as it is written. CaO(s) + H 2 (l)  Ca(OH) 2 (s) kJ

Heat of Combustion The heat of reaction for the complete burning of one mole of a substance.