Energetics Flashcards
Describe an endothermic reaction
More energy is taken in to break bonds such as heat from the surrounding
+ve enthalpy change
breaking bonds > making bonds
Describe an exothermic reaction
More energy is used to make bonds and heat is released to the surroundings as a product
-ve enthalpy change
making bonds > breaking bonds
Give 2 examples of exothermic reactions
Neutralisation
Combustion of fuels
Give an example of an endothermic reaction
Thermal decomposition
Define enthalpy change
Energy change of a system at constant pressure
What does standard enthalpy change mean?
Enthalpy change occurs in the standard conditions:
298K / 25C
100kPa / 1atm
Which type of reaction has a larger activation energy on a graph?
Endothermic
Define standard enthalpy of formation
Enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions
Define standard enthalpy of combustion
Enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions
How do you calculate enthalpy change when you know the mass and temperature change?
q = mc(/\T)
enthalpy change = mass x S.H.C. x change in temp
How would you measure enthalpy change for a reaction occurring in (aq)?
Use a polystyrene cup as a calorimeter (good insulator / reduces heat loss)
Measure temp change
Take heat capacity as 4.18
Density of solution is 1g/cm3
What is Hess’s Law?
Enthalpy change for a reaction is the same regardless of the route taken
Define bond dissociation enthalpy
Enthalpy change required to break a covalent bond (all species in a gaseous state)
Define mean bond enthalpy
Average value for bond dissociation enthalpy of a given bond
