1.4 Energetics

• Enthalpy is the change in energy that occurs when a chemical reaction occurs.
• The entropy change is the change in energy due to a reaction.
• The heat change in reaction at a constant pressure is denoted as Delta H and has the units of kilojoules per mole.
• Endothermic reactions absorb energy from the surroundings and feel colder when touched.
• Exothermic reactions release energy to the surroundings and feel warmer when touched.
• In endothermic reactions, the reactants are lower in energy than the products, and the Delta H change is always positive.
• In exothermic reactions, the products are lower in energy than the reactants, and the Delta H change is always negative.
• Bond breaking and making occurs during a reaction, and if more energy is needed to break the bonds than is given out when the bonds are formed, the reaction will be endothermic.
• Bond density is the amount of energy required to break a bond or the amount of energy given out when the bond is formed, which is the energy that's stored in that bond.
• The total energy needed to break four CH bonds in methane is 1662 kilojoules.
• In calorimetry, the fuel is weighed beforehand and the amount of fuel that has been burnt is measured.
• The energy change for the combustion of methane is exothermic, meaning more energy is given out when the bonds are formed than is needed to break them.
• Bonds of the same type don't all have the same amount of energy, which is why scientists use a mean bond enthalpy to calculate the energy change for bond formation or breakage.
• Calorimetry is a method in which the enthalpy change of combustion of a fuel can be determined, which is useful for determining which fuel is the best.
• The equation for enthalpy changes using mean bond enthalpy is the total energy needed to break the bonds minus the total energy released from forming them.
• The mean bond enthalpy of a CH bond is not the value of a single bond, but an average of all bonds of the same type.
• If more energy is given out when the bonds are formed than is needed to break the initial bonds, the reaction will be exothermic.
• The bond enthalpies can be used to calculate the mean bond energy, which is the difference between the enthalpies of formation of the products and the reactants.
• The reaction profile shows that more energy is released when the bonds are formed to make the products than what was needed to break the initial bonds, indicating that the reaction will always be exothermic.
• The enthalpy of formation of elements is also zero when the change in oxidation state is negative, for example, when Cesium changes from Cs to Cs+.
• The enthalpy change of the reaction is calculated by dividing the energy by the number of moles.
• The moles of HCL are calculated by multiplying the concentration (one) by the volume (25 mL).
• The enthalpy of formation can be calculated using the elements in their standard states and the formula ΔsH = Σ(Ei - Ef), where Ei and Ef are the energy of the elements in their initial and final states respectively.
• The enthalpy of formation of elements is zero when the change in oxidation state is zero, for example, when Oxygen changes from O to OH - there is no change in oxidation state.
• The enthalpy of formation of elements is also zero when the change in oxidation state is positive, for example, when Rubidium changes from Rb to Rb+.
• The enthalpy of formation of elements is also zero when the change in oxidation state is negative, for example, when Sodium changes from Na to Na+.
• The enthalpy of formation of elements is also zero when the change in oxidation state is zero, for example, when Potassium changes from K to K+.
• The enthalpy of formation of elements is also zero when the change in oxidation state is negative, for example, when Cesium changes from Cs to Cs+
• The enthalpy of formation of elements is also zero when the change in oxidation state is negative, for example, when Sulfur changes from S to S-.
• The enthalpy of formation of elements is also zero when the change in oxidation state is zero, for example, when Iodine changes from I to I-.
• Hess's law states that the total entropy change of a reaction is independent of the route taken.
• The enthalpy of combustion, ΔcH, is the change in energy when a substance burns completely in oxygen to make carbon dioxide and water.
• The enthalpy of formation of elements is also zero when the change in oxidation state is positive, for example, when Lithium changes from Li to Li+.
• The enthalpy of formation of elements is also zero when the change in oxidation state is positive, for example, when Fluorine changes from F to F-.
• The enthalpy of formation, ΔsH, is the change in energy when the elements that make up the reactants and products are in their standard states.
• The enthalpy of formation of elements is also zero when the change in oxidation state is zero, for example, when Chlorine changes from Cl to Cl-.
• The total entropy change of a reaction can be found by using Hess's law and the data from an experiment.
• The enthalpy of combustion is calculated by taking the total enthalpy of formation and subtracting the enthalpy of formation of the elements in their standard states.
• The total enthalpy of combustion is the sum of the
• The total enthalpy of formation can be calculated by adding the enthalpy of formation of the elements in their standard states.