Physical Chapter 4: Energetics Flashcards
What is Enthalpy change defined as?
The heat energy transferred in a reaction at a constant pressure.
(weird theta symbol means standard conditions & standard states)
What is the value for delta H if a reaction is endothermic?
positive
What is the value for delta H if a reaction is exothermic?
Negative
What type of bond enthalpy do you use for calculations?
Mean bond enthalpy
What is standard enthalpy of formation defined as?
The enthalpy change when 1 mole of a compound is formed frfom its elements in their standard states under standard conditions
What is standard enthalpy of combustion defined as?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
What is calorimetry?
You burn a fuel or undertake a neutralisation reaction, and measure the temperature change, using this value to calculate the enthalpy change.
What is a good method to obtain an accurate temperature change from calorimetry?
Draw a graph and extrapolate from the beginning of the reaction
WHy can you not just use starting temperature and end temperature in calorimetry to find temperature change?
Heat will have been lost to the surroundings - this will not be 100% accurate
What formula do you use to find emthalpy changes?
Q = mc(delta)T
Q - heat lost (joules)
m = mass in g
c = specific heat capacity
T = temperature (Kelvin)
What is Hess’s law?
Total enthalpy change is the same regardless of the route taken
How can we use Hess’s Law?
We can find out enthalpy changes frfom enthalpy of formation or combustion to find alternative routes
