Thermochemistry Chapter 16

Vocabulary Thermochemistry: the study of the transfers of energy as heat that accompany chemical reactions and physical changes Calorimeter: a device used to measure the energy as heat absorbed or released in a chemical or physical change

More Vocabulary Temperature: a measure of the average kinetic energy of the particles in a sample of matter Joule: the SI unit of heat as well as all other forms of energy, “J” Heat: the energy transferred between samples of matter because of a difference in their temperatures

Specific Heat Specific heat: the amount of energy required to raise the temperature of one gram of a substance by one Celsius degree (or one kelvin) Units are J/(g °C) or J/(g K) Usually measured under constant pressure

More about Specific Heat Symbol for specific heat is cp “q” represents energy lost or gained “m” is the mass of the sample “ΔT” is the difference between the initial and final temperatures or q = cp  m  ΔT

Problem A 4.0 g sample of glass was heated from 274 K to 314 K and was found to have absorbed 32 J of energy as heat. What is the specific heat of this type of glass? How much energy will the same glass sample gain when it is heated from 314 K to 344 K? 0.20 J/gK Practice problems 1-2 on page 535 in school textbook 24 J

Enthalpy of Reaction “H” is the symbol for enthalpy Enthalpy is measured as a change “ΔH” is the symbol for the energy absorbed as heat during a chemical reaction at constant pressure Enthalpy change: the amount of energy released or absorbed by a system as heat during a process at constant pressure

Enthalpy of Reaction Enthalpy of reaction: the quantity of energy transferred as heat during a chemical reaction Enthalpy change for a reaction is calculated with the following: ΔH = Hproducts – Hreactants Is the difference between the stored energy of the reactants and products

Thermochemical Equation Thermochemical equation: an equation that includes the quantity of energy released or absorbed as heat during the reaction The coefficients are numbers of moles, never molecules (so can be a fraction)

Exothermic Reaction Thermochemical equations are usually written by designating the value of ΔH ΔH = -483.6 kJ Sometimes they are written showing the energy as a product or reactant

Endothermic Reaction Energy is absorbed for the reaction to occur ΔH = +483.6 kJ Written with energy as a reactant:

Entropy Entropy: a measure of the degree of randomness of the particles, such as molecules, in a system Is given the symbol “S” Compare a solid and a gas at the same temperature; the gas has greater entropy because its particles are moving rapidly in all directions