Energetics
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Define the term enthalpy change
the energy change of a reaction per mole at a constant pressure
What are the standard conditions for an enthalpy change to be measured under?
298 K, 100kPa
Define the term standard enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states (under standard conditions)
Define the term standard enthalpy change of combustion
the enthalpy change when one mole of a substance is completely burned in oxygen
Define the term mean bond (dissociation) enthalpy
Then enthalpy change when one mole of covalent bonds in the gaseous state are completely broken, averaged over a range of compounds
Why are calculations from mean bond enthalpies often different from those calculated from Hess’ Law (theoretical values)
Bond enthalpies are a mean average taken over a range of compounds
How is enthalpy change (H) calculated from enthalpy change of formation data? (H f )
ΔH = ΣΔH f (products) - ΔH f (reactants)
How is enthalpy change (H) calculated from enthalpy change of combustion data? (H c )
ΔH = ΣΔHc reactants − ΣΔHc products
How is enthalpy change (H) calculated from bond enthalpy data?
ΔH = Δbond enthalpy (reactants) - bond enthalpy (products)
Calculate energy change from a calorimetry experiment
q = mcΔT q = energy (in J), m = mass of water (in g), c = specific heat capacity ΔT = temperature change ( in o C or K, don’t need to convert)
Calculate enthalpy change from a calorimetry experiment
ΔH = q / moles reacted (smallest number of moles present)
Explain why calculations from calorimetry experiments are often lower than theoretical values
1) heat lost to the surroundings 2) incomplete combustion (of fuels) 3) evaporation (of fuels)
How to improve accuracy of calorimetry experiments due to heat loss?
1) better insulation 2) heat shield (for fuels)
How should temperature change for a calorimetry experiment for an aqueous reaction be calculated?
1) measure start temperature every minute for 4 minutes 2) mix reactants 3) record temperature every minute for 10 minutes 4) plot a graph and regress line of temperature change to find T
Outline a method that could be used to determine the enthalpy change of combustion for an alcohol
• Weigh the spirit burner (alcohol) before and after combustion • Water in a calorimeter / beaker • Measure volume of water (or mass) • Burn the alcohol to heat the water • Measure temperature rise in water
What is Hess’ law?
The enthalpy change of a reaction is the same, regardless of the route taken
What is bond enthalpy data?
an averaged value representing the energy required to break one mole of the bond stated in a gaseous state under standard conditions
Why is the value for the O=O bond enthalpy not a mean?
O2 only substance with O=O bond