Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons
This is done by chemical bonding, where old bonds are broken and new bonds are formed
The process involves the transfer of thermal energy into and out of reaction mixtures
The terms used to describe this are the system (what happens in the chemical reaction) and the surroundings (anything other than the chemicals reacting)
The energy within the system comes from the chemical bonds themselves, which could be considered as tiny stores of chemical energy
Exothermic Reactions
In exothermic reactions, thermal energy is transferred to the surroundings, increasing the temperature of the surroundings
This energy is transferred from the chemical energy store of the chemical system to the surroundings, causing the energy of the system to decrease - resulting in a negative energy change
The overall transfer is from the system to the surroundings
Combustion, oxidation, and neutralisation reactions are typical exothermic reactions
Hand warmers and self-heating cans of food and drinks use exothermic reactions
Endothermic Reactions
In endothermic reactions, thermal energy is taken in from the surroundings, decreasing the temperature of the surroundings
This energy is transferred to the chemical energy store of the chemical system, causing the energy of the system to increase - resulting in a positive energy change
The overall transfer is from the surroundings to the system
Electrolysis, thermal decomposition reactions, and the first stages of photosynthesis are typical endothermic reactions
Sports injury treatments often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling
Minimum energy colliding particles must have to react
The greater the initial rise, the more energy is required to get the reaction going
Identify a reaction pathway diagram for an exothermic reaction where the energy of the product is lower than the reactants
Identify a reaction pathway diagram for an endothermic reaction where the energy of the product is higher than the reactants
In exothermic reactions, heat exits the system and in endothermic reactions heat enters the system
Exothermic reactions always give off heat and feel hot, whereas endothermic reactions take heat in and feel cold
Enthalpy change, ΔH, of the reaction is either a positive or a negative value depending on whether the reaction is exothermic or endothermic
Reactions with higher activation energies require more energy to start than those with lower activation energies
The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, of the reaction
Reaction pathway diagrams can be interpreted to determine if a reaction is exothermic or endothermic
For a chemical system to react, particles must first come into contact with each other in a collision
A minimum amount of energy is required for the collision to be successful, known as the activation energy (Ea)
Different reactions have different activation energies depending on the chemical identities involved
Factors such as energy, orientation, and number of collisions per second play a role in analyzing collisions
The change in energy is negative in exothermic reactions since the products have less energy than the reactants
Exothermic reactions release more energy when new bonds are formed than the energy required to break the bonds in the reactants
The reaction pathway diagram for an exothermic reaction shows the energy level changes
If more energy is released than absorbed, the reaction is exothermic
The change in energy is positive in endothermic reactions since the products have more energy than the reactants
Endothermic reactions absorb more energy to break bonds than is released to form new bonds
The reaction pathway diagram for endothermic reactions displays the energy level changes
You must be able to draw and label pathway diagrams for reactions, including reactants, products, enthalpy change (ΔH), and activation energy (Ea)
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed