1.4 Energetics
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Enthalpy (H)
The heat energy that is stored in a chemical system, can’t be measured
Enthalpy change (ΔH)
The heat energy change at constant pressure
Units for enthalpy change
kJ/mol
Exothermic reactions
ΔH is negative - chemical system loses energy to the surroundings
Example of exothermic reactions
• Combustion • Respiration
Endothermic reactions
ΔH is positive - chemical system gains energy
Example of endothermic reactions
• Thermal decomposition of calcium carbonate • Photosynthesis
Activation energy
The minimum energy required to start a reaction by the breaking of bonds
Bond enthalpy
The heat energy required to break one mole of a given covalent bond in the molecules in a gaseous state
Bond enthalpy is always _______ because…
Positive, energy is required to break bonds
What do bond enthalpies indicate about a bond?
Its strength
Mean bond enthalpy
The heat energy required to break one mole of a covalent bond, averaged for that type of bond in a range of different compounds in the gas state.
Energy is required to _____ bonds
break
Energy is released when bonds are _____
formed
How is enthalpy change calculated from bond energies?
ΔH = Σ(energy required to break bonds) - Σ(energy released when a new bond forms)
Why is using bond enthalpies less accurate than using Hess’s law for calculating enthalpy change?
• Bond enthalpies are an average over a series of different compounds • Bond enthapies only apply to reactions in the gas phase
What are the standard condtions?
298K (25°C) 100kPa (1atm)
Standard enthalpy of formation, ΔfHᶿ
The enthalpy change that occurs when one mole of a compound is formed from its constituent elements with all reactants and products in their standard states
Standard enthalpy of formation of an element
0 (Unless it is not in its standard state, in which case an enthalpy change is given for the change in state)
Standard enthalpy of combustion, ΔcHᶿ
The enthalpy change that occurs when one mole of a compound reacts completly in oxygen with all the reactants and products in their standard states.
Equation for specific heat capacity
q = m c ΔT q - energy / J m - mass / g c - specific heat capacity / JK⁻¹cm⁻¹ ΔT - temperature change / K
How is heat loss accounted for in reactions in solutions
Cooling curves
First law of thermodynamincs
Energy cannot be created of destroyed but it can be changed from one form to another
Hess’ law
The enthalpy change for a chemical reaction is independent of the route taken