Module 3.2 - Physical Chemistry Flashcards
Describe what happens in an endothermic reaction.
- energy level of products is higher than that of reactants meaning heat energy has been taken in
- more bonds broken
- temperature decrease
- e.g. photosynthesis, thermal decomposition
- ∆H is positive
Describe what happens in an exothermic reaction.
- energy level of products is lower than that of the reactants meaning heat energy is given out
- more bonds made
- temperature increase
- e.g. respiration, combustion
- ∆H is negative
If the enthalpy change of reaction is negative, is the reaction endothermic or exothermic?
exothermic
What is the equation for the enthalpy change of combustion of ethane?
C2H6(g) + 3 1/2 O2 (g) –> 2CO2 (g) + 3 H2O (l)
What is the equation for the enthalpy change of formation of water?
H2 (g) + 1/2 O2 (g) –> H2O (l)
What is the equation for the enthalpy change of reaction of calcium oxide with hydrochloric acid?
CaO (s) + 2HCl (aq) –> CaCl2(aq) + H2O (l)
What is the equation for the enthalpy change?
(sum of the bond enthalpies of bonds broken) - (sum of bond enthalpies of bonds made)
How do you measure enthalpy changes?
-using a calorimeter for a combustion reaction or their reactions give the ∆rH directly
-bond enthalpies can also be used to estimate ∆rH
(-may not always be possible to directly measure the enthalpy change of a reaction)
What problems may be encountered when measuring bond enthalpy?
- high activation energy
- slow reaction rate
- more than one reaction taking place (e.g. ∆fH of hydrocarbons unlikely to only make one hydrocarbon, virtually impossible to measure directly)
What is Hess’s Law?
- if a reaction takes place by more than 1 route + the initial + final conditions are the same the total enthalpy change is the same
- ∆H (route A) = ∆H (route B) - ∆H (route C)
What is the equation to work out energy change?
Q=mc∆T
