Chapter 9 Flashcards
What is the definition of enthalpy?
Is the measure of heat energy present in a chemical system (H)
What is the definition of enthalpy change?
Is the amount of energy released/taken in during a reaction (ΔH)
Are the energy changes +ive or -ive for exo and endothermic reactions?
-ive change is exothermic, +ive change is endothermic
What are standard conditions for enthalpy changes?
100KPa, 298K (25°C), 1moldm^-3, and standard state (the physical state of a substance under standard conditions)
Definition of standard enthalpy change of a reaction.
The energy change associated with a chemical reaction in the molar quantities under standard conditions.
Definition of standard enthalpy change of formation.
The energy change associated with the formation of 1 mole of a compound from its elements, under standard conditions. (The ΔfH of an element is 0)
Definition of standard enthalpy change of combustion.
The energy change associated with the complete combustion of 1 mole of a substance, under standard conditions
Definition of standard enthalpy change of neutralisation.
The energy change associated with the formation of 1 mole of water from a neutralisation reaction between and acid and base, under standard conditions. (Always ΔneutH for acid+base = -57mol^-1)
What are possible accuracy issues for ΔcH?
Heat loss to the surroundings, incomplete combustion, evaporation of fuel, not under standard conditions
Definition of average bond enthalpy.
The energy required to break one mole of a specified type of bond in a gaseous molecule (higher the value, stronger the bond)
Why would a reaction be exothermic?
Making the bonds releases more energy than was required for breaking the bonds.
What does Hess’ Law state?
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total energy enthalpy change is the same
