Enthalpy Flashcards
which is more energetically stable?
graphite or diamond?
graphite
definition of Hess law
the total enthalpy change of a chemical reaction is independent of the pathway between the initial and final states
can combustion continues spontaneously whenever it started?
can.
the energy released provides activation energy to the unreacted chemicals
why experimental and actual enthalpy change is different
Not performing the experiment in standard condition
Heat transfer to the surrounding
The heat capacity of the container is neglected.
state the standard condition
1atm 298K
in standard state
can the standard enthalpy change of formation of NH3 be determined directly from experiment
No
all reactants and products are gas so its hard to measure the temperature change
also it is not a must for the reactant to form NH3
why the combustion is exothermic in terms of the breaking and forming of covalent bond
energy released in the bond forming process is larger than the energy absorbed in the bond breaking process
what is an exothermic reaction
a reaction which release heat to the surrounding
why the enthalpy change of neutralization involving weak acid or base is lower
some heat is absorbed for their ionization
ionization involves breaking bond so its endothermic
when will the heat change be considered as enthalpy change
constant pressure
species lower in enthalpy is more energetically stable
T/F
T
so exothermic reaction tends to be spontaneous (not a must)
define standard enthalpy change of neutralisation
standard enthalpy change of neutralization is the enthalpy change when 1 mole of water is formed from the neutralization of an acid and base under standard condition
why less energy will be released if the neutralisation reaction involves a weak acid or a weak alkali
some energy is absorbed for ionisation of the weak acid or weak alkali
define standard enthalpy change of combustion
standard enthalpy change of combustion is the enthalpy change when 1 mole of the substance is completely burnt in oxygen under standard conditions
define standard enthalpy change of formation
standard enthalpy change of formation is the enthalpy change for the formation of 1 mole of substance from its elements in their standard states under standard conditions
how can enthalpy change of a reaction be determined?
by experiments
using Hess’s law
why should we use a calorimeter instead of a beaker to carry out the calorimetric experiment
to prevent heat loss to the surrounding
why should zinc powder instead of granule be used when determining the heat change in the experiment?
faster reaction rate
reduce the error due to the heat loss to the surrounding
suggest why the metal can holding the water should be very thin and light in the experiment of determining the enthalpy change of combustion
to minimize the heat absorbed by the metal can
why the enthalpy changes of some reactions cannot be determined directly by experiment under standard condition
explosive reaction (K+HCl)
reaction rates are too slow (rusting)
formation of side products (C+ O2)
what is the major apparatus needed for determining the enthalpy change through experiment
A well-insulated expanded polystyrene cup with a lid, a thermometer , a stirrer and an electric balance
why is it difficult to determine the enthalpy change of formation of MgO from its element in school laboratory directly
the extent of reaction cannot be controlled
the direct combustion of magnesium can be violent
side reaction may occur
compound of Al toxic?
no
it is non-toxic
so it can be used to contain food
state one assumption while calculating enthalpy change
the Hess’ law is followed
need to rmb wt when drawing enthalpy change cycle
use stateeeeeesssss
rmb to write the O2 add
C ‘s state should be graphite not (s)
