DF - Enthalpy Changes Flashcards
Use ideas about the enthalpy changes that take place during bond breaking and bind making to explain why some reactions are exothermic
Bonds breaking absorbs energy
Bring breaking releases energy
More energy released than absorbed
Suggest 2 reasons why the standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values
Heat released to the surroundings
Non-standard conditions
Define the term enthalpy change of combustion
The enthalpy change for the complete combustion of one mole
Under standard conditions
What are standard conditions?
Pressure is 101KPa
temperature is 298K
It is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why.
Reaction is too slow
What type of number is enthalpy change for combustion?
Negative
Define the term enthalpy change of formation
enthalpy change when 1 mole of substance is formed from its elements
Under standard conditions
State one important manufacturing process in which hydrogen is used
Ammonia production
Write the equation that accompanies the enthalpy change of formation of an alkane
C + H2 -> alkane
Then balance
What type of value is bond enthalpy of bonds like C-H and C-C?
Positive
What is meant by the term exothermic?
Releases energy
How do you work out enthalpy change of formation?
Sum of products - sum of reactants
Define the term bond enthalpy
Energy need to break one mole of bonds in the substance
In the gaseous state
Suggest a reason why hydrazine is used as a rocket fuel and ammonia is not
Hydrazine is liquid
Write an equation including state symbols for the bond enthalpy of I-I
I-I (g) -> 2I (g)
