8 Flashcards
What is the enthalpy change
The heat energy change of a system measured at constant pressure
What happens in an exothermic reaction
.There is an overall transfer of energy from the system to the surroundings
.The enthalpy change is negative
Give an example of an exothermic reaction
The combustion of carbon
What happens in an endothermic reaction
.There is an overall transfer of energy to the system from the surroundings
.The enthalpy change is positive
Give an example of an endothermic reaction
The reaction between carbon and carbon dioxide
What are standard conditions
.100Kpa pressure
.A stated temperature, usually 298K
.1moldm-3
Give one difference between exothermic and endothermic reactions
Exothermic reactions transfer energy to the surroundings, and the temperature increase
Endothermic reactions transfer energy from the surrounding
What is the standard enthalpy change of neutralisation
.The enthalpy change measured at 100kPa and a specific temperature, usually 298K
.When one mole of water is produced by the neutralisation of an acid with an alkali
.With all solution at 1 moldm-3
How is the energy transfer calculated
Q =MC4T
Q=energy transferred n j
m=mass of substance in g
c=speciifc heat capacity in Jg-1K-1
4T=Temperature change in K
What does Hess’s law state
That the enthalpy change for a reaction is independent of the pathway taken
What reactions does Hess law apply to
Reactions in which the initial and final conditions are the same for each pathway
What is the standard enthalpy change of formation
The enthalpy charged measured at 100kPa and a specific temperature, usually 298K when one mole of a substance is formed from its element in their standard states.
Standard enthalpy of formation for an element in its standard state is zero
In a Hess cycle, what way does the arrow face when its:
a) standard enthalpy of formation
b) standard enthalpy of combustion
a) arrow face upwards
b) arrow face downwards
What is meant by bond enthalpy
The enthalpy change when one mole of a bond in the gaseous state is broken
Using XY, show the enthalpy change for a reaction in a diatomic molecule
XY(g) –>X(g) +Y(g)
