5.1 Exothermic & Endothermic Reactions

Cards (58)

  • 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