Enthalpy of Formation Cycles - Thermodynamics Pt B (3.1) Flashcards
What does Hess’s Law state?
- total enthalpy change in a chemical reaction is independent of the route by which it takes place
- as long as the initial and final conditions are the same
Fewer bursts of lots of energy = ?
- more bursts of less energy
Why do enthalpy reactions occur?
To convert a reactant to a product (vice versa)
What is the enthalpy of formation?
- enthalpy change when 1 mole = formed from its elements under standard conditions
What are the standard conditions for pressure?
10^5Pa, 100kPa and 1atm
What is the standard temperature for enthalpy reactions?
298k (25+273)
Carbon occurs naturally as graphite. True or False?
True - rather than diamond
In enthalpy of formation, what can a reactant be turned into?
Products (enthalpy of formation of products) or Elements (enthalpy of formation of reactants)
What are the steps in drawing a formation cycle?
- write a balanced symbol equation at the top with state symbols
- write elements in their standard states in a box at the top
- draw arrows in the correct direction (elements to compounds)
- label each arrow with the data given in question
- multiply by the number of moles (per mole)
What is the enthalpy change of elements already in their standard states? e.g. O2
0
When you go in one direction (forward) with an arrow, which action do you perform?
Addition
When you go against the direction of arrow, which action do you perform?
Subtraction
What are some problems with enthalpy of formation data? - give some examples
- elements often do not react to make required compounds (graphite will not react with H2 to make methane)
- elements make more than one compound (enthalpy of formation of CO)
