Topic 16: Energy and Chemical Change Table of Contents Topic 16: Energy and Chemical Change

Chemical potential energy Energy and Chemical Change: Basic Concepts Chemical potential energy Energy is the ability to do work or produce heat. Heat (q) is energy that is in the process of flowing from a warmer object to a cooler object.

First Law of Thermodynamics Energy and Chemical Change: Basic Concepts First Law of Thermodynamics The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed.

joule (J): unit of heat and energy 1 J = 0.2390 calories Energy and Chemical Change: Basic Concepts Measuring heat calorie (cal): the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius (1°C) joule (J): unit of heat and energy 1 J = 0.2390 calories 1 calorie = 4.184 J

Each substance has its own specific heat. Energy and Chemical Change: Basic Concepts Specific Heat Specific heat: amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. Each substance has its own specific heat. 4.184 J/(g∙°C) is the specific heat (c) of water.

Al requires 2x as much heat as Fe to heat up Specific Heat Al requires 2x as much heat as Fe to heat up But it will also lose its heat faster than aluminum

Calculating heat evolved and absorbed Energy and Chemical Change: Basic Concepts Calculating heat evolved and absorbed The heat absorbed or released by a substance depends: specific heat of the substance (c) mass of the substance (m) amount by which the temperature changes = Tfinal – Tinitial)

Energy and Chemical Change: Basic Concepts Measuring Heat A calorimeter is an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process A known mass of water is placed in an insulated chamber to absorb the energy released or provide the energy absorbed by the system

Chemical Energy and the Universe Energy and Chemical Change: Basic Concepts Chemical Energy and the Universe Thermochemistry: study of heat changes that accompany chemical reactions and phase changes. The system is the specific part of the universe that contains the reaction or process you wish to study.

universe = system + surroundings Energy and Chemical Change: Basic Concepts Chemical Energy and the Universe Everything in the universe other than the system is considered the surroundings. Therefore, the universe is defined as the system plus the surroundings. universe = system + surroundings

Enthalpy and enthalpy changes Energy and Chemical Change: Basic Concepts Enthalpy and enthalpy changes For many reactions, the amount of energy lost or gained can be measured conveniently in a calorimeter at constant pressure. Enthalpy (H) is the heat content of a system at constant pressure.

Enthalpy and enthalpy changes Energy and Chemical Change: Basic Concepts Enthalpy and enthalpy changes You cannot measure the actual energy or enthalpy of a substance you can measure the heat absorbed or released in a chemical reaction enthalpy (heat) of reaction (∆Hrxn)

We can only measure a change in enthalpy for a system

Change in enthalpy is calculated using Hess’s law The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps of the process.

Thermochemical Equations 2H2(g) + O2(g)  2H2O + 483.6 kJ 2H2(g) + O2(g)  2H2O ∆H = -483.6 kJ 2H2O + 483.6 kJ  2H2(g) + O2(g) 2H2O  2H2(g) + O2(g) ∆H = +483.6 kJ

Enthalpy and enthalpy changes Energy and Chemical Change: Basic Concepts Enthalpy and enthalpy changes Endothermic Change Physical or chemical change in which a system absorbs heat from its surroundings KE  PE (heat seems to disappear) PE of system increases and it becomes less stable

Enthalpy and enthalpy changes Energy and Chemical Change: Basic Concepts Enthalpy and enthalpy changes Exothermic Change Physical or chemical change in which a system gives off heat to its surroundings PE  KE (heat seems to appear from nowhere) PE of system decreases and it becomes more stable