Energetics-Enthalpy ( AS ) Flashcards
What energy change is breaking bonds associated with?
Energy is taken in to break bonds—> Endothermic reaction
What energy change is making bonds associated with?
Energy is released to make new bonds—> exothermic reaction
What is an endothermic reaction?
One with an overall positive enthalpy change—> energy in breaking bonds>energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change—> energy in breaking bonds
If a reversible reaction is endothermic one way, what type of reaction is the other way?
Exothermic
Give 2 examples of exothermic reactions
Combustion of fuels
Neutralisation
Give an example of an endothermic reaction
Thermal decomposition
Define enthalpy change
Change in heat energy at a constant pressure
What are standard conditions?
100kPa/1atm pressure
298K/25°C
A concentration of 1 mol dm-3
What does “in standard state” mean?
The state an element/compound exists at in standard conditions
Define standard enthalpy of formation
The enthalpy changes for when one mole of a compound is formed from its constituent elements in their standard state
Give an example of an equation that represents standard enthalpy of formation
H2(g)+½O2(g)—>H2O(l)
Define standard enthalpy of combustion
The enthalpy change for when one mole of a fuel burns completely in oxygen under standard conditions
Give an example of an equation that represents standard enthalpy or combustion
C(s)+O2(g)—>CO2(g)
How can you calculate enthalpy change from experimental data?
Using the equation Q=mcΔT
m= mass of substance
c= specific heat capacity
ΔT= change in temperature
How would you measure the enthalpy change for a reaction occurring in (aq)?
Use an expanded polystyrene cup as a calorimeter(good insulator—> reduces heat loss). Heat is generated in the solution; measure this temperature change. Take heat capacity of solution to be 4.18 and density of solution=1gcm-3
What can you use to make experimental determination changes of a reaction more accurate?
Cooling curves
What is Hess’s Law?
The enthalpy change for a reaction is independent of the route taken
Define bond dissociation enthalpy
The enthalpy change required to break a covalent bond, with all species in the gaseous state
Define mean bond enthalpy
Average value for bond dissociation enthalpy of a given bond
Why may experimental methods for enthalpy determination not be very accurate?
Heat is lost to the surrounding
Not in standard conditions
Reaction may not go to completion
