Energetics Flashcards
Enthalpy change is
The heat energy change at constant pressure
Endo and exothermic reactions
•Exothermic means heat energy is given out so temp of surroundings rises. Has negative delta H
•Endothermic means heat energy is absorbed so temp of surroundings falls. Has positive delta H
Activation energy is
The minimum energy required to start a reaction by the BREAKING OF BONDS.
Enthalpy profile diagrams
Exothermic diagrams have higher reactant and lower product.
Endothermic diagrams have lower reactant and higher product.
Define bond enthalpy and state why the values are always positive
The heat energy required to break one mole of a given covalent bond in the molecules gaseous state.
The bond enthalpy values are always positive because energy is required to break bonds.
Evaluate bond enthalpys
Higher bond enthalpy = more heat energy = stronger bond to break
Mean bond enthalpy is
The heat energy required to break 1 mole of a covalent bond averaged for that type of bond in a range of different compounds.
How to calculate enthalpy change for the formation
Sum of all the energy required to break bonds - Sum of all the energy required to form bonds.
Limitations of bond energy calculations
•The bond enthalpies used are mean values and taken from a range of different compounds
•Bond enthalpies only apply to reactions in the gaseous state
What is the standard enthalpy of formation
The enthalpy change that occurs when 1 mole of a compound is formed from its constituent elements with all reactants and products in their standard states.
•NEED STATE SYMBOLS
What is true about standard enthalpy of formation
•The standard enthalpy of formation of an element in its standard state is 0
•The more negative it is, the more stable is the compound
Standard enthalpy of combustion is
The enthalpy change that occurs when 1 mole of a compound reacts completely with oxygen with all reactants and products in their standard states.
•NEED STATE SYMBOLS
Formula to calculate heat produced
q = (delta H)(n)
•gives the heat produced after a compound undergoes complete combustion in KJ
q=(m)(SHC)(temp change)
•gives heat energy released or absorbed in J
•m is the mass of water in grams which is equal to the volume in cm^3
Reasons why you dont get the right value for enthalpy change
•heat loss to surroundings
•incomplete combustion may have occured
•reaction not done under standard conditions
Hess’s law is
the enthalpy change for a chemical reaction is Independent of the route taken
•First law of thermodynamics - energy cant be created or destroyed but can be changed from one to another. Hess’s law applies this to chemical reactions
Reasons why you cant measure the enthalpy change of a reaction directly
•reaction may be too slow
•activation energy may be too high
How to calculate all 3 enthalpy changes in the hess cycle
If we know 2 values we can calculate the third as:
Route 1 = Route 2
•So the straight line is equal to the bent line
•If you go against the arrows, the enthalpy value becomes the opposite i.e, -200 would become +200
