Energy and Chemical ReactionsMs. Grobsky

So far, we have discussed the various types of chemical reactions, the driving forces behind them, and how to quantitatively predict amounts of reactants and products using stoichiometry

In order to properly understand chemistry, we must understand the energy changes that accompany these chemical reactions

The study of thermodynamics is concerned with the question:

Can a Reaction Occur?

Introduction to Thermodynamics

Unlike matter, energy does not have mass and cannot be held in our hands However, its effects can be observed and measured

When discussing energy, one must assign the following labels: System

Area of the universe we are focusing on (i.e., the experiment)

Surroundings

Everything else outside the system

System and Surroundings

Open

Mass & EnergyExchange:

Closed

Energy

Isolated

Nothing

SYSTEMSURROUNDINGS

Defined as the capacity to do work or to produce heat Units are calorie (cal) or Joule (J)

Before we can make use of this definition, we must understand the concepts of work and heat

Work is the energy used to cause an object to move against a force

Heat is the energy used to cause the temperature of an object to increase

Before we examine these definitions more closely, let’s first consider the ways in which matter can possess energy and how that energy can be transferred from one piece of matter to another

So, What is Energy?

• There are many different forms of energy:

• Thermal energy is the energy associated with the random motion of atoms and molecules

• Kinetic energy is the energy due to motion

• Potential energy is the energy available by virtue of an object’s position

• Chemical energy is the energy stored within the bonds of chemical substances

• Type of potential energy

• All forms of energy are able to interconvert!

Types of Energy

Total Internal Energy = Kinetic Energy + Potential Energy

E = EK + EP

Ball thrown upwards slows &

loses kinetic energy but gains potential energy

The reverse happens as it

falls back to the ground

Example of Energy Conversion –Kinetic Energy and Potential Energy

• Energy can also be transferred back and forth between a system and its surroundings in the forms of work and heat

Why Can Energy Interconvert?

All of these conversions and transfers proceed in accord with one of the most important observations in science:

Energy can neither be created not destroyed – it is conserved! This important observation is known as the First

Law of Thermodynamics

What Does Energy Have to Do with Chemical Reactions?

Introducing Thermochemistry!

During both parts of the lab, you felt striking temperature differences between yourself (the surroundings) and the baggie (the system)

Well, what do you think caused this temperature difference? It was due to the exchange of heat!

What exactly is heat? It is the exchange of thermal energy between two systems at

different temperatures that are in contact with one another

Heat flows from a warmer object to a cooler object

Heat is represented by the letter “q”

But, temperature is NOT a measure of thermal energy It just reflects the kinetic energy (motion) of the particles

Tying in the Citric Acid/Sodium Bicarbonate Lab

Come to find out, many physical changes involve release and absorption of energy in the form of heat

Dissolving of salts

Changes in state

Compression/expansion of gases

Energy and Physical/Chemical Changes

Energy Changes in Chemical Reactions

• Almost every chemical change either releases or absorbs energy in the form as heat as chemical bonds are broken and formed during the course of a reaction

• Energy is consumed when bonds are broken• Energy is released as new bonds are formed

• Net total energy change for a reaction will either be a negative value with the release of heat (exothermic) or a positive value with the absorption of heat (endothermic)

• Thermochemistry is the study of heat exchange in chemical reactions

• An exothermic process is any process in which energy (heat) flows out of the system into the surroundings

• “q” is negative

• Energy of reactants is greater than that of the products

• Energy can be thought of as a product in a chemical reaction because it is “given off”

• An endothermic process is any process in which energy (heat) flows into the system from the surroundings

• “q” is positive

• Energy of products is greater than that of the reactants

• Energy can be thought of as a reactant in a chemical reaction because it is being absorbed

H2O (g) H2O (l) + Energy

Energy + 2HgO (s) 2Hg (l) + O2 (g)

Heat Exchange between System and the Surroundings

Endothermic vs. Exothermic Reactions