Thermochemistry Part 1 Chapter 17 Sections 17.1 and 17.2

Thermochemistry The study of energy changes that occur during chemical reactions (chemical change) AND changes in state (physical change). Did you know? Chemical energy is stored in chemical bonds of a substance. This is called chemical potential energy. Some substances have more energy stored in their bonds than others. Example: Batteries, Fuel

Chemical Reactions When a chemical reaction occurs… A substance is transformed into another substance with a different amount of chemical potential energy. Gasoline: Buying energy to be burned and propel your car What happens when it burns? What is produced?

Heat! It is the amount of energy transferred between one object to another. We know heat has been transferred by observing the change in temperature. How does heat flow? Temperature DOES NOT EQUAL heat. Let’s compare the two… Make a T-Chart.

Heat Versus temperature Definition Heat is energy that is transferred from one body to another as the result of a difference in temperature. Temperature is a measure of hotness or coldness expressed in terms of any of several arbitrary scales like Celsius and Fahrenheit. Symbol Q T Unit Joules Kelvin, Celsius or Fahrenheit SI unit Joule Kelvin Particles Heat is a measure of how many atoms there are in a substance multiplied by how much energy each atom possesses. Temperature is related to how fast the atoms within a substance are moving. The ‘temperature’ of an object is like the water level – it determines the direction in which ‘heat’ will flow. Ability to do work Heat has the ability to do work. Temperature can only be used to measure the degree of heat.

Endothermic and exothermic System: a specific portion of the universe being studied. Surroundings: the rest of the universe that interacts with the system. Goal of Thermochemistry: to examine the flow of heat between the system and its surroundings. Two types of heat flow: 1) Exothermic: a system releases heat to its surroundings 2) Endothermic: a system gains heat from its surroundings

Law of conservation of Energy Energy cannot be created or destroyed. It can be transferred: endothermically or exothermically! Practice:

Heat and its units Units: Measuring energy in foods: 4.184 J = 1 Cal Heat is represented by q Units: The calorie (cal) The Joule (J) Measuring energy in foods: 4.184 J = 1 Cal

Heat capacity The amount of heat needed to increase the temperature of an object exactly 1 degree Celsius. Heat capacity depends on: 1) Mass of the object The greater the mass of an object, the greater its heat capacity. Examples: Small Steel Nail VS Large steel girder 1 drop of water VS 1 cup of water 2) Chemical composition of the object Different substances have different heat capacities: Puddle of water vs. Iron sewer cover on a hot summer day

Specific Heat capacity Specific heat = The amount of heat it takes to raise the temperature of 1g of the substance by 1 degree Celsius. Specific heat is represented by C (capital C) Unit: J/g C

Practice

Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes. In calorimetry, 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 its surroundings.

Thermochemical equations The heat content of a system at a constant pressure is the same as the property called Enthalpy (H). The heat absorbed or released during a chemical reaction at constant pressure is symbolized as ∆H. In other words, Heat and Enthalpy Change are the same thing. q = ∆H

Thermochemical equation A chemical equation that includes the enthalpy change is called a thermochemical equation. Example: It is a product in this example.

Thermochemical equations The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. It can be positive or negative. Negative: Exothermic, releases heat as a product. Positive: Endothermic, absorbs heat as a reactant.

Exothermic Reaction Energy Transfer Potential energy of the reactants are higher than the products.

Endothermic reaction energy transfer Potential energy of the products are higher than the reactants.

Enthalpy Change calculations The amount of heat released or absorbed during a reaction depends on the number of moles of the reactants involved. Example: The decomposition of 2 moles of sodium bicarbonate requires 129kJ of heat. The decomposition of 4 moles of sodium bicarbonate is twice as many moles, and therefore, requires twice as much energy o decompose. (258 kJ)

Do your own

Heat of Combustion Combustion = flames! Heat of Combustion: the heat of reaction for the COMPLETE burning of 1 mole of a substance.

Looking ahead Homework: Page 510 # 3,5-8,10,11 Page 516-517 # 14,16-20