Energetics Flashcards
In an EXO reaction, how does heat energy move between a system and the surroundings?
Moves FROM the system TO the surroundings
In an ENDO reaction how does the heat energy move between the system and the surroundings?
Moves TO the system FROM the surroundings
Describe and explain the position of products on a reaction profile graph for an EXO reaction
- The products are BELOW the reactants
- the products are more energetically stable than the reactants
Describe and explain the position of products on a reaction profile graph for an ENDO reaction
- The products are ABOVE the reactants
- the products are less energetically stable than the reactants
Define enthalpy change
A heat energy change, measured at a constant pressure
Define Standard enthalpy change
A heat energy change, measured under standard conditions
-298K
-100KPa
With the substances at their standard states
Define standard state
The physical state of a substance at 298K and 100KPa
Define standard enthalpy of formation
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions, with all the reactants and products in standard states.
OR
the enthalpy change for this reaction:
C(s) + 2H2(g) —-> CH4(g)
Define standard enthalpy change of combustion
The enthalpy change when 1 mole of a substance is burned completely in oxygen, under standard conditions, with all the reactants and products in standard states.
OR
The enthalpy change of the reaction:
C(s) +O2(g) ——> CO2(g)
State Hess’ Law
The total enthalpy change for a reaction is the same, regardless of the route or path taken
State the 2 equations to work out the enthalpy change of a reaction AND when you use each
If all of the reactants are ELEMENTS
🔺H=
sum of (🔺Hc)(reactants) - sum of (🔺Hc) (products)
If NOT ALL of the reactants are elements
🔺H=
sum of (🔺Hf)(products) - sum of (🔺Hf)(reactants)
Which bonds are ‘bond enthalpy’ calculations used in?
Only applies to COVALENT BONDS
Define Mean bond enthalpy
Energy required to break a covalent bond, averaged over several different compounds
Why is the data resulting from mean bond enthalpy calculations not as accurate as Hess’ law calculations
- Calculations are only approximations because the data are averages over several different compounds containing that bond
- Hess’s law uses data that are specific to the compounds involved
What is the equation for enthalpy change?
🔺H= bonds broken - bonds formed
