Energetics
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what is ΔH
overall enthalpy change energy to break bonds - energy to make bonds
ΔH is negative when…
energy is released to the surroundings, meaning it is an exothermic reaction
ΔH is positive when…
energy is taken in, meaning it is an endothermic reaction
What are the standard conditions that enthalpy change is measured under?
100kPa and 298K
define standard enthalpy of combustion (∆cHƟ)
the enthalpy change when one mole of a substance is burned in oxygen under standard conditions
define standard enthalpy of formation (∆fHƟ).
the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions
equation for heat change
q = mc∆T
equation for enthalpy change per mole
∆H = q / moles
What is Hess’s law?
the energy change that occurs when a substance is changed into a product is the same, regardless of the route taken.
What is mean bond enthalpy?
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 This bond energy is known as the average bond energy Since bond energies cannot be determined directly, enthalpy cycles are used to calculate the average bond energy
What are bond enthalpies?
amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy
explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law.
Define enthalpy change
Change in heat energy at constant pressure