enthalpy determination - main one Flashcards
method - neutralisation but same
Using a measuring cylinder set volume of HCl into the polystyrene cup
Using a measuring cylinder set volume of sodium hydroxide solution into a separate polystyrene cup
Put a thermometer in each cup, stir + record the temperature every half minute for 2½ minutes
At precisely 3 minutes, add the contents of the two cups together (DO NOT RECORD THE TEMPERATURE AT 3 MINUTES)
Continue stirring and record the temperature for an additional 6 minutes
assumptions
That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1
That the density of the solution is the same as pure water, i.e. 1 g cm-3
The specific heat capacity of the container is ignored
The reaction is complete
There are negligible heat losses
how accurate?
experimental value will generally be LESS exothermic
why is the method not that accurate
heat loss to the surroundings other than water - includes beaker
incomplete combustion
evaporation from the wick
non standrad conditions
how to improve the method
Place a lid on the calorimeter
(To reduce evaporation of the water/heat loss)
pour water into a copper can
(copper better thermal conductor than glass/lower specific heat capacity)
fit the spirit burner with a cap
(reduces loss of methanol before burning)
arrange for less distance between top of flame and bottom of can/beaker
(less heat transferred/’lost’ to surroundings)
stir water throughout heating
(ensures even distribution of heat)
burner weighed asap after extinguishing flame
(prevent evaporation off the wick)
whats the problem with reactions that arent instantaneous
may be a delay before the maximum temperature is reached
During that delay the substances themselves may be losing heat to the surroundings, so that the true maximum temperature is never actually reached
how to overcome this
use graphical analysis to determine the maximum enthalpy change - use a temperature correction curve
how to make a temperature correction curve
Take a temperature reading before adding the reactants for a few minutes to get a steady value
Add the second reactant and continue recording the temperature and time
Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was adde
whats different about the neutralisation reaction
the initial temperature of the acid and the base may be the same or different
what to do if there the same
they can both be plotted as the same line, the graph can be extrapolated to determine the change in temperature
what to do if there different
the initial temperature of the acid is plotted as one line and the initial temperature of the base are plotted as a separate line on the same graph.
The temperature of the reaction mixture can be plotted and then you will do two extrapolations for the acid and base
After completing the subsequent q=mct for both, you calculate the average of those two values to get the overall enthalpy change of neutralisation
example of cooling curve / temperature correction curve
what is calorimetry
process used to find the amount of energy released / used in a chemical reaction
why styrofoam over glass beaker
styrofoam is a better thermal insulator - less heat will be lost to the surroundings
