Keeping Track of Energy During Chemical Changes

A Review of Energy Interaction Energy (E i ) – energy due to the arrangement of particles; based on phase Thermal or Kinetic Energy (E th ) – energy due to the speed of the individual particles; based on temperature Chemical Potential Energy (E ch ) – energy stored in a particle based on the chemical structure/bonds of that particle (will only change if the substance itself changes – bonds being broken or formed)

Modeling Approach to Energy in Chemical Change Uses energy bar diagrams (LOLs) to represent the different energy “accounts” at various stages of the reaction. Provide a mechanism for the change. Connect E Th and E ch Focus on what is happening during the course of the reaction

Which side of the balanced equation do you need to write energy? ENDOTHERMIC REACTION = REACTANTS Energy is supplied to the reactants or the products store more energy than the reactants do (being heated, feels cold) EXOTHERMIC REACTION = PRODUCTS Energy flows from the system to the surroundings or the products store less energy than the reactants (burning, feels hot)

Endothermic Reactions ENERGY IS ABSORBED! Products store more energy than reactants, so ENERGY is written with the reactants in a balanced equation. Two possible types of endothermic reactions 1. Heating a substance 2. Spontaneous: makes surroundings feel cold Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide. ENERGY + 1 CaCO 3 (s)  1 CaO (s) + 1 CO 2 (g)

Endothermic Reactions This is the traditional/standard energy diagram found in most texts This shows that the products store more energy, but it doesn’t tell you the whole story.

Energy Bar Charts Shows the transfer of energy between the surroundings and the system Also allows you to consider thermal (kinetic) energy as well Here is an overview of what happens in the decomposition of calcium carbonate.

A More Detailed Look: The Role of Thermal Energy in Decomposition E th = temperature changes (speed of molecules). The description of the reaction mentions that the reactants must be heated, so we show heat energy coming in from the surroundings.

With Heating, the Reaction Proceeds During collisions, particles trade E th for E ch as the products are formed. After transferring some of their kinetic energy, the resulting particles move more slowly (lower E th ).

The Whole Process Heating the system increases the E th of the reactant molecules. Energy is then transferred from E th to E ch which is stored within the new arrangement of the products

A Spontaneous Endothermic Process Unlike the typical endothermic reaction, that requires an input of heat, there are some processes that take in energy from their surroundings spontaneously, which causes the resulting solution to feel much colder

What causes the E th to decrease? When ammonium chloride dissolves in water, the resulting solution gets much colder. Some of the E th of the water was required to separate the ions in the crystal lattice. Resulting solution has a greater E ch than before The system trades E th for E ch ; eventually energy enters the cooler system from the warmer surroundings (you)

Exothermic Reaction ENERGY IS RELEASED! If energy flows from the system out to the surroundings, the products must store less E ch than the reactants. For example: When methane burns in air, it produces carbon dioxide and water. CH O 2  CO H 2 O + ENERGY

Exothermic Reactions This is the traditional/standard energy diagram found in most texts This shows that the products store less energy, but it still doesn’t tell you the whole story.

Energy Bar Charts Shows the transfer of energy between the surroundings and the system Also allows you to consider thermal (kinetic) energy as well Here is an overview of what happens in the burning of methane (CH 4 ).

Consider the Role of E th in the Combustion Reaction Remember, E th is responsible for temperature changes (speed of molecules). When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly. For now, we are ignoring the energy needed to light the match – just consider the energy involved in the process of burning.

Energy Flows Out of the System With burning, the system is now hotter than the surroundings; energy flows out of the system until thermal equilibrium is re-established.

The Whole Process The decrease in E ch results in increased E th The system is now hotter than the surroundings Energy eventually moves from systems to surroundings via heating