Energetics

• Enthalpy Change (∆H o ): In a reaction, bonds are broken and then made, with energy being taken in from the surroundings when bonds are broken and given out when bonds are formed. The overall energy change of the reaction depends on how much energy is transferred in these processes, with a positive enthalpy change when energy is taken in and a negative enthalpy change when energy is released.
• Measuring Enthalpy Change: Enthalpy is measured under standard conditions of 100 kPa pressure and a temperature of 298 K.
• Enthalpy of Formation (∆ f H o ): The enthalpy change when one mole of a substance is produced from its elements under standard conditions.
• Enthalpy change is the energy released or absorbed when one mole of a substance is burned completely in oxygen under standard conditions.
• Different covalent bonds have different bond enthalpies, which can be experimentally determined using calorimetry methods.
• The experimentally calculated bond enthalpies may differ from the values in data books, as they are averaged values and vary in each situation.
• Calorimetry is an experimental method used to find enthalpy change by measuring temperature change over time.
• The change in temperature observed and plotted on a graph can be extrapolated to determine an accurate value for the change in temperature.
• The change in temperature is proportional to the energy change and can be calculated using the equation q = m * c * ∆T, where q is the energy change, m is the mass, c is the specific heat capacity, and ∆T is the temperature change.
• Capacity is defined as the energy required to raise 1g of substance by 1K without a change of state.
• Enthalpy change per mole of substance can be calculated using the energy value (q).
• ∆H values found using calorimetry are never completely accurate due to energy loss from the system and inaccuracies in measuring temperatures.
• Hess's Law states that energy in a reaction system must be conserved, and the overall enthalpy change for a reaction is the same regardless of the route taken.
• Enthalpies of formation can be calculated using Hess's Law.
• Arrows in the triangular diagram point out from the central product when A and B are formed from the elements at C.
• Arrows in the triangular diagram point towards the central product (H2O and CO2) when A and B burn to form the products at C.
• Bond enthalpy data represents the energy required to break one mole of a bond in a gaseous state under standard conditions.
• Bond enthalpies can be used alongside Hess's Law to find the overall enthalpy change for a reaction.