Year 12- Energetics (Physical Chemistry) Flashcards
What is enthalpy change?
The amount of heat energy taken in or given out during any change in a system given the pressure is constant
If an enthalpy change occurs, where does energy transfer between?
Between the system and surroundings.
How does an energy profile appear in an endothermic enthalpy change?
The products have more energy than the reactants
How does an energy profile appear in an exothermic enthalpy change?
The reactants have more energy than the products
What is standard enthalpy change of formation?
The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions. All reactants and products being in their standard states.
What are standard conditions?
T= 298K
P=100kPa
What is standard enthalpy of combustion?
The standard enthalpy of combustion of a substance is defined as
the enthalpy change that occurs when one mole of a substance is
combusted completely in oxygen under standard conditions. All reactants and products being in their standard states.
Equation to measure the enthalpy change of a reaction in solution
q=mcx /\T
- q=J
- m= g
- c= J g-1 K-1
- T= K
What is an issue if the reaction is slow in a calorimetry experiment? Explain how this is counteracted
The exact temp rise is hard to obtain as cooling occurs simultaneously
- Take readings are regular time intervals and extrapolate the temp curve/line to the time the reactants were added
- Or take temp of reactants few minutes before added together
What are issues with calorimetry method?
- Energy transfer to surroundings (loss)
- Approximation of specific heat cap. of solution. Assumes its the same as water.
- Density of solution taken as same as water
- Neglect specific heat cap. of calorimeter and energy it absorbs
- Reaction/dissolving may be slow or incomplete
General method of calculating enthalpy change of reaction from experimental data
1- use q=mc/\t to calculate energy change of quantities used
2- work out moles of reactants
3- divide q by no. of moles of the reactant not in excess to get /\H
4- add sign and unit
-if need be, divide by a thousand to convert Jmol-1 to kJmol-1
What do you assume about the solutions?
- In any reaction dissolved in water, assume the heat cap. is the same as water which is 4.18
- assume they also have the same density as water which is 1g cm-3 e.g. 25cm3 will weight 25g
What does Hess’ law state?
States that total enthalpy change for a reaction is
independent of the route by which the chemical change takes place
Hess’ law equation for enthalpy changes of formation
/\H= /\ products- /\ reactants
Hess’ law equation foe enthalpy changes of combustion
/\H= /\ reactants- /\ products
