2.3 Energy Changes During Chemical Reactions

Kinetic and Potential Energy Definitions

"Kinetic" means "moving." Hence, kinetic energy is the energy due to movement. A thrown ball, a rolling log, a flying atom, and the electrons orbiting the nucleus of an atom all have kinetic energy.

Potential Energy

A rock poised at the top of a hill has potential energy. Give it a push and it rolls downhill converting its potential energy to kinetic. The potential energy in the gasoline burned in an automobile engine is transformed partly into the motion of the car. Because the internal combustion engine is notoriously inefficient, much of the potential energy of the gasoline is converted into wasted heat energy, which is a form of kinetic energy.

Bond Breaking and Bond forming

For a reaction to occur, reactant particles must collide, creating an opportunity for old bonds to break and new bonds to form. Any one collision may be successful or unsuccessful depending on two factors. These factors are the geometry of the collision (head on versus side swipes for example) and the energy of the collision (faster moving molecules with more kinetic energy are more likely to react).

Each chemical reaction has an activation energy or minimum energy required for the reaction to occur. If the colliding atoms or molecules generate sufficient activation energy (kinetic energy is converted to potential energy in the collision) then bonds in reactant molecules will break and atoms can re-arrange and form new bonds in the products that are formed. If collisions that do not generate this activation energy the reactant atoms and molecules will repel each other with no reaction taking place.

The bonds within a chemical compound are a source of potential energy. The process of breaking bonds initially requires energy and then releases energy. If more energy is absorbed than released, the reaction is endothermic. If less energy is absorbed than released it is exothermic. The process of forming bonds absorbs energy. An analogy for a bond is a partly stretched elastic band (PE is present). Energy must be applied to break the bond, but once broken, a different amount of energy is released.

The changes that occur in a chemical reaction typically involve breaking of the bonds that exist in the reactant molecules. This process requires energy. New bonds are formed, as new chemical substances are formed as the products in the reaction. This process releases energy.

Exothermic Reactions

These are reactions which release energy. Energy in chemical reactions is measured in kilojoules (kJ). Consider the following hypothetical reaction:

x₂ + y₂ → 2xy

50 kJ of energy are required to break the bonds that hold the x's and y's together in their respective molecules, but 100 kJ is released when xy forms.

50 kJ + x₂ + y₂ → 2xy + 100 kJ

simplified

x₂ + y₂ → 2xy + 50 kJ

The net result is that 50 kJ of energy are released.

Reactions of this sort which produce energy are called exothermic reactions. "Exo" means "out" and "therm" refers to heat. So, "exothermic" means that heat is coming out, or being produced.

Reactants → Products + energy

An example of an exothermic reaction occurs during cellular respiration in animals and plants. The reaction involves "burning" sugar (glucose C₆H₁₂O₆ (s)) to produce energy. Oxygen from the air is required.

C₆H₁₂O₆(s) + 6 O₂(g) → 6 CO₂(g) + 6 H₂O(l) + 2803 kJ

Endothermic Reactions

There is a net requirement or absorption of energy in some chemical reactions. The bond breaking process in the reactants requires more energy than is released in total. Energy from an external source must be absorbed.

energy + Reactants ? Products

Such reactions which absorb energy are called endothermic reactions. ("Endo" means "into.")

Plants have the ability to use the sun's energy to convert carbon dioxide and water into glucose and oxygen. This is the opposite process of cellular respiration and is called photosynthesis:

6 CO₂(g) + 6 H₂O(l) + 2803 kJ (from the sun) ? C₆H₁₂O₆(s) + 6 O₂(g)

Heat of Reaction

Exothermic		Endothermic
1) Products have less potential energy than reactants. The potential energy lost during the reaction is changed into heat.		1) Products have more potential energy than reactants. The potential energy gained during the reaction is absorbed from the surroundings.
2) Surroundings will feel hot. Be aware! If your hand feels hot when you are holding a test tube containing a reaction, then heat is leaving the reaction in the test tube and is entering your hand. Understand the difference between your personal point of view as the observer and the point of view of the particles participating in the reaction in the test tube.		2) Surroundings will feel cold.
3) There are two ways to show the heat effect. a) The heat term is included in the reaction on the right side. Eg. H2 + Cl2 2HCl + 184 kJ b) Use ?H notation: The reaction is written, then followed by a negative value for ?H that shows the "change in heat". Eg. H2 + Cl2 2HCl, ?H= -184 kJ		3) There are two ways to show the heat effect a) The heat term is included in the reaction on the left side. Eg. 2HCl + 184 kJ H2 + Cl2 b) Use ?H notation: The reaction is written, then followed by a positive value for ?H that shows the "change in heat". Eg. 2HCl H2 + Cl2, ?H= +184 kJ