Energetics Flashcards
define enthalpy change
Enthalpy change is the amount of heat energy
taken in or given out during any change in a system provided the pressure is constant.
what happens in exothermic reactions?
In an exothermic change energy is transferred from
the system (chemicals) to the surroundings.
The products have less energy than the reactants.
NOTE delta H is negative
what happens in an endothermic reaction?
In an endothermic change, energy is transferred
from the surroundings to the system (chemicals).
They require an input of heat energy e.g. thermal
decomposition of calcium carbonate.
The products have more energy than the reactants
NOTE delta H is positive
define standard enthalpy change of formation
The standard enthalpy change of formation of a compound is the enthalpy change under standard conditions when 1 mole of the compound is formed from its constituent elements all in their standard states
define standard enthalpy change of combustion
the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions.
(298K and 100kPa), all reactants and products being in their
standard
what are the “standard” conditions?
298K and 1000kPa
RQP method
General method
washes the equipment (cup and pipettes etc) with the solutions to be used
dry the cup after washing
put polystyrene cup in a beaker for insulation and support Measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup
clamp thermometer into place making sure the thermometer bulb is immersed in solution
measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance. If using a solid reagent then use ‘before and after’ weighing method
stirs mixture (ensures that all of the solution is at the same temperature) Record temperature every minute after addition for several minutes
why is a temp rise difficult to obtain in a slow reaction?
how can we improve this?
If the reaction is slow then the exact temperature rise can be
difficult to obtain as ** cooling occurs simultaneously with the reaction.**
To counteract this we take readings at regular time intervals
and extrapolate the temperature curve/line back to the time
the reactants were added together.
We also** take the temperature of the reactants for a few
minutes before they are added together to get a better
average temperature**. If the two reactants are solutions then
the temperature of both solutions need to be measured
before addition and an average temperature is used.
what are some errors in the method?
- energy transfer from surroundings (usually loss)
- approximation in specific heat capacity of solution. The method assumes all
solutions have the heat capacity of water. - neglecting the specific heat capacity of the calorimeter- we ignore any
energy absorbed by the apparatus. - reaction or dissolving may be incomplete or slow.
- density of solution is taken to be the same as water.
what does Hess’s law state?
Hess’s law states that total enthalpy change for a reaction is
independent of the route by which the chemical change takes place
define mean bond energies
The mean bond energy is the enthalpy needed to
break the covalent bond into gaseous atoms, averaged over
different molecules.
whats the general formula for bond energy if all atoms are gaseaous?
In general (if all substances are gases)
H = Σ bond energies broken - Σ bond energies made
why is using mean bond energy to calculate delta H be less accurate?
H values calculated using this method will be less accurate
than using formation or combustion data because the mean
bond energies are not exact
