1.4 Energetics Flashcards
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
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
