chemical energetics

• The total chemical energy inside a substance is called the enthalpy (or heat content).
• When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes.
• An enthalpy change is represented by the symbol Δ H (Δ = change; H = enthalpy).
• An enthalpy change can be positive or negative.
• A reaction is exothermic when the products have less energy than the reactants.
• Heat energy is given off by the reaction to the surroundings.
• The temperature of the environment increases as a result of exothermic reactions.
• The temperature of the system decreases during exothermic reactions.
• Δ H relates to the equation: ½ H (g) → H(g).
• Enthalpy changes of atomisation and combustion or formation can be used to calculate average C-H bond energy.
• Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy.
• Enthalpy changes of atomisation and combustion or formation can be used to calculate average bond energies.
• Since there are 2 moles of NaHCO (s), the Δ H value is multiplied by 2.
• There are 4 C-H bonds in methane, so the average C-H bond energy can be calculated by dividing the total Δ H value by 4.
• The enthalpy change of atomisation (Δ H) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.
• There is an enthalpy decrease during the reaction so Δ H is negative.
• Exothermic reactions are thermodynamically possible because the enthalpy of the reactants is higher than that of the products.
• The rate of an exothermic reaction may be too slow to observe any appreciable reaction, in which case the reaction is kinetically controlled.
• The enthalpy change during an exothermic reaction is represented by the symbol Δ H.
• Hess’s Law states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.
• The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route) according to Hess’s Law.
• The combustion products can be formed directly from elements to combustion products = Δ H or the combustion products can be formed indirectly from elements to compound to combustion products = Δ H + Δ H.
• The enthalpy change going from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route) according to Hess’s Law.
• The enthalpy change of the reaction can be calculated using Hess’s Law by writing the equation for enthalpy change of formation at the top and adding oxygen on both sides.
• Hess Cycles are used to calculate enthalpy changes which can’t be found experimentally using calorimetry, for example, the enthalpy change of the reaction 3C (s) + 4H (g) → C H (g) can’t be found experimentally as hydrogen and carbon don’t react under standard conditions.
• A reaction is endothermic when the products have more energy than the reactants.
• Heat energy is absorbed by the reaction from the surroundings.
• In reality, only some bonds in the reactants are broken and then new ones are formed.
• Bond dissociation energy (E) is also known as exact bond energy or bond enthalpy.
• If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic.
• The formula of calculating the standard enthalpy change of reaction using bond energies is: Δ H = enthalpy change for bonds broken + enthalpy change for bonds formed.
• The enthalpy of combustion is the enthalpy change when one mole of a substance reacts in excess oxygen to produce water and carbon dioxide.
• The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy.
• The chemical reaction should be simplified such that only one mole of ethyne reacts in excess oxygen: H-C=C-H + 2 ½ O=O → H-O-H + 2O=C=O.
• The type of bond broken is put in brackets after E.
• If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic.
• Bond energies are affected by other atoms in the molecule (the environment), therefore, an average of a number of the same type of bond but in different environments is calculated.
• Bond energies are used to find the ΔH of a reaction when this cannot be done experimentally.
• The temperature of the environment decreases as a result of endothermic reactions.
• ΔH = enthalpy change for bonds broken + enthalpy change for bonds formed = (+2912 kJ mol) + (- 4142 kJ mol) = -1230 kJ mol.