3.1.4 Energetics Flashcards
Define the term standard enthalpy of formation and give its equation.
The enthalpy change when 1 mole of a substance is formed from its constituent elements, where all substances are in their standard states under standard conditions.
∆H = products - reactants
Define the term standard enthalpy of combustion
The enthalpy change when 1 mole of a substance is completely burned in oxygen, where all substances are in their standard states under standard conditions.
∆H = reactants - products
Define the term standard enthalpy of reaction and give its equation.
The enthalpy change between the total energy absorbed to break reactant bonds and the total energy released from the formation of product bonds.
∆H = reactants - products
State Hess’s Law.
The enthalpy change of a reaction is independent of the reaction pathway (route) taken.
Suggest one reason for why an experimental value may be very different from the data book value for the enthalpy of combustion of a fuel.
Suggest one way that you can reduce this.
Heat loss.
Use a copper calorimeter rather than a Pyrex beaker to insulate the reaction more so that less heat is lost to the surroundings.
How would the data book value for the standard enthalpy of combustion of a fuel, referring to the formation of carbon dioxide (g) and water (g), differ if it referred to the formation of water in the liquid state?
The value would be more exothermic (more negative) because more energy is released when water vapour condenses due to the formation of new bonds.
Refer to one source of error that may occur in a students experiment for the enthalpy of combustion of a fuel/hydrocarbon. Do not include heat loss, apparatus error or student error.
Suggest two ways that the student can reduce this error.
Incomplete combustion.
The student can either allow a longer reaction time or use a more volatile (more flammable), shorter-chained fuel/hydrocarbon.
State the standard conditions.
- Pressure of 1 atmosphere (101kPa).
- Temperature of 25ºC (298K).
