Chemistry⚗️💉🔬

• In endothermic reactions, more energy is needed to break bonds than make new ones, resulting in a positive ∆H and heat being taken in from the surroundings
• Overall enthalpy change calculation

  Can be calculated based on the energy transferred in bond breaking and bond forming processes
• Reaction

  Bonds are broken, energy is taken in from the surroundings. Bonds are formed, energy is given out. The overall energy change of the reaction depends on how much energy is transferred in these processes
• Enthalpy Change of Neutralisation (∆neutHo)

  The enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water
• Enthalpy Change of Reaction (∆rHo)

  The enthalpy change when quantities of substances in standard states react completely under standard conditions
• Enthalpy changes in a reaction

  Can be shown on an energy level diagram, indicating if the reaction is endothermic or exothermic
• Calorimetry


  An experimental method for finding enthalpy change by measuring temperature change over time. Data can be extrapolated to give an accurate value for the change in temperature at the beginning of the reaction
• Enthalpy Change of Combustion (∆cHo)

  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• Enthalpy change

  Heat energy change represented by the symbol ∆Ho, measured under standard conditions of 100 kPa pressure and a specified temperature, generally 298 K
• Negative enthalpy change


  Occurs when energy is released to the surroundings
• In exothermic reactions, more energy is needed to make new bonds than break existing ones, resulting in heat being given out and a negative ∆H
• Enthalpy Change of Atomisation (∆atHo)

  The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state
• Positive enthalpy change


  Occurs when energy is taken in from the surroundings
• Enthalpy Change of Formation (∆fHo)

  The enthalpy change when one mole of a substance is produced from its elements under standard conditions
• Calorimetry


  • Graph showing the extrapolated line of best fit
• Energy in a reaction system must be conserved according to Hess’s Law
• Heat loss to the surroundings can be reduced by putting a lid on the calorimeter and insulating the outsides using an insulator like polystyrene
• The specific heat capacity of the solution is often taken to be 4.18 kJ mol​-1​, which is the value for water and not the actual solution, leading to inaccuracies in the calculation
• Energy change equation

  q = energy change (J), m = mass (g), c = specific heat capacity (J g​-1​ ℃​-1​), ∆T = temperature change (℃)
• ∆H values found using calorimetry are never completely accurate due to energy loss from the system
• Change in temperature (∆T)

  Proportional to the energy change
• The overall enthalpy change for a reaction is the same regardless of the route taken, as per Hess’s Law
• The specific heat capacity of the calorimeter is not taken into account, causing inaccuracies in the calculation
• Calorimetry

  1. Finding enthalpy change by measuring temperature change over time

  2. Data can be extrapolated to give an accurate value for the change in temperature at the beginning of the reaction
• Specific heat capacity

  The energy required to raise 1g of the substance by 1K without a change of state
• Hess’s Law is used to determine enthalpy changes for reactions that cannot be found directly using an experimental method
• Enthalpies of Formation can be calculated using Hess’s Law and enthalpies of formation
• Sources of error in calorimetry: Heat loss can occur due to conduction, convection, or inaccuracies in measuring temperatures
• Enthalpies of Combustion can be calculated using Hess’s Law and enthalpies of combustion
• Enthalpy change per mole

  Calculated using the energy value (q)
• The direction of the arrows in Hess’s Law calculations indicates whether values should be added or taken away
• Different covalent bonds require different amounts of energy to be broken. Values can be found experimentally using calorimetry methods
• Bond enthalpy calculations


  Mean bond enthalpy values can be used to calculate the overall enthalpy change for a reaction
• Mean bond enthalpy values can be used to infer which bonds might break first in a chemical reaction. Bonds with lower bond enthalpy values would be expected to break first
• If a reaction requires the breaking of many strong bonds or the reaction is endothermic overall, then it is likely that the reaction will occur slowly at room temperature
• Mean bond enthalpy values are often tabulated. You need to identify the bonds broken and formed during the reaction in order to calculate the overall enthalpy change
• Mean bond enthalpy values tell you how much energy is required to break a particular bond, averaged out across the range of compounds containing that bond
• Bond enthalpy data


  • Represents the energy required to break one mole of the stated bond in a gaseous state, under standard conditions
• Bond enthalpy values calculated using calorimetry methods often differ from the data book values as they are not exact and vary in each situation
• The enthalpy change for a reaction is equal to the sum of the bond enthalpies for the bonds formed in the reaction, minus the sum of the bond enthalpies for the bonds broken in the reaction