Energetics Flashcards
What is enthalpy?
It’s a measure of the heat content of a substance.
What is enthalpy change (ΔH)?
Change in heat energy at a constant pressure.
What are standard conditions (ΔH⦵)?
100KPa, 298K.
Define standard enthalpy change of formation (ΔHf⦵). Units?
Enthalpy CHANGE when 1 MOLE of a substance is formed from its constituent elements with all reactants and products in their standard states at standard conditions. KJmol-1.
What is the equation for the standard enthalpy change pf formation for C2H5OH(l)?
2C(s) + 3H2(g) + 1/2O2(g) ——ΔHf⦵—–> C2H5OH(l)
What is the ΔHf⦵ of an element in its standard state?
0.
When writing equations of standard enthalpy change of formation equations what must you always do?
Write state symbols.
Define standard enthalpy change of combustion (ΔHc⦵).
Enthalpy CHANGE when 1 MOLE of a substance is COMPLETELY BURNED IN OXYGEN with all reactants and products in their standard states under standard conditions.
What is the equation for the standard enthalpy change of combustion for CH4(g)?
CH4(g) + 2O2(g) ——-ΔHf⦵——> CO2(g) + 2H2O(l)
What is the equation for standard enthalpy of combustion the same as?
Complete combustion.
What is the equation for the standard enthalpy of combustion for H2? Why?
H2(g) + 1/2O2(g) -> H2O(l). No carbon in equations so doesn’t produce CO2.
Define standard enthalpy change of neutralisation (ΔH⦵neutralisation).
Enthalpy CHANGE when 1 MOLE of water is formed in a reaction between an acid and an alkali under standard conditions.
State Hess’ Law.
The enthalpy change for a reaction is independent of the path taken.
What is the method for finding an unknown value using a Hess’ Law cycle?
Clockwise arrows = anticlockwise arrows.
What is the quick way to find the unknown value without drawing a Hess’ Law cycle? What does this only work for?
ΔH = ΣΔHf⦵(products) - ΣΔHf⦵(reactants)
Only works for formation.
