Chemical Energetics

• 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
  • This process involves the transfer of thermal energy into and out of reaction mixtures
• The terms used to describe chemical reactions are:

  • System: the reacting chemicals
  • 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
• In exothermic reactions, thermal energy is transferred from the chemical energy store of the chemical system to the surroundings

  • The energy of the system decreases, which means that the energy change is negative
  • The temperature of the surroundings increases because thermal energy is given out / released
• The overall transfer in exothermic reactions is from the system to the surroundings

  A) hotter than surroundings

  
• Typical examples of exothermic reactions include:

  • Combustion
  • Oxidation
  • Neutralisation
• Hand warmers used in the winter time are based on the release of heat from an exothermic reaction
• Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers
• In endothermic reactions, thermal energy is transferred from the surroundings system to the system surroundings

  • The energy of the system increases, which means that the energy change is positive
  • The temperature of the surroundings decreases because thermal energy is taken in / absorbed
• The overall transfer in endothermic reactions is from the surroundings to the system

  A) cooler than surroundings

  
• Endothermic reactions are less common than exothermic reactions
• Typical examples of endothermic reactions include:

  • Electrolysis
  • Thermal decomposition
  • The first stages of photosynthesis
• Cold packs for sports injuries are based on endothermic reactions, designed to take heat away from a recently injured area to prevent swelling
• Reaction pathway diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions
• On a reaction pathway diagram:

  • Progress of the reaction is shown on the x-axis
  • Energy is shown on the y-axis
  • The difference in height between the energy of reactants and products is the overall energy change of a reaction
• Reaction pathway diagram of an exothermic reaction and an endothermic reaction

  A) reactants

  B) released

  C) products

  D) reactants

  E) absorbed

  F) products

  
• In exothermic reactions:

  • Energy is given out to the surroundings
  • The energy of the products will therefore be lower than the energy of the reactants
  • The overall energy change is negative
  • This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants
• In endothermic reactions:

  • Energy is taken in from the surroundings
  • The energy of the products will be higher than the energy of the reactants
  • The overall energy change is positive
  • This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants
• To help you remember whether a chemical system is exothermic or endothermic:

  • In EXothermic reactions heat Exits the system and in ENdothermic reactions heat ENters the system.
  • Exothermic reactions always give off heat and they feel hot
  • Endothermic reactions always take heat in and they feel cold.
• Endothermic reaction - takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
• Exothermic reaction - transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings
• Enthalpy change - transfer of thermal energy during a rection aka the change in H
• We must supply energy to break bonds

  • activation energy is the minimum energy that colliding particles must have to react
• Exothermic reactions - change in H is negative
• Endothermic reactions - change in H is positive
• When a reaction occurs, bonds have to be broken

  • this requires energy
• Bond energy - the amount of energy in kj/mol associated with the breaking or making of one mole of bonds in an element or compound
• For atoms or particles to react with each other in a chemical system they must collide together
• A number of factors affect the success of a collision:

  • Energy
  • Orientation
  • Number of collisions per second - the frequency of collisions
• In terms of the energy of collisions, there is a minimum amount of energy required for a successful collision

  • A successful collision is where the particles in the reactant(s) are rearranged to form the products
  • This minimum amount of energy is called the activation energy, Ea
• Different reactions have different activation energies, depending on the chemical identities involved

  • 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.
• ΔH is:

  • Positive for en endothermic reaction
  • Negative for an exothermic reaction
• A reaction is exothermic when more energy is released forming new bonds for the products than absorbed breaking the bonds in the reactants

  • So, the products have less energy than the reactants
  • This means that the change in energy is negative
  • Therefore, an exothermic reaction has a negative value for enthalpy, ΔH
• The reaction pathway diagram for an exothermic reaction is:

  A) reactants

  B) transition state

  C) negative

  D) lower

  E) products

  
• A reaction is endothermic when more energy is absorbed breaking the bonds in the reactants than released forming new bonds for the products

  • So, the products have more energy than the reactants
  • This means that the change in energy is positive
  • Therefore, an endothermic reaction has a positive value for enthalpy, ΔH
• The reaction pathway diagram for an endothermic reaction is:

  A) transition state

  B) products

  C) positive

  D) reactants

  E) higher

  
• 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
• Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds
• Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed