Energetics Flashcards
Define enthalpy change and give units used
is the heat energy transferred in a reaction at constant pressure
the units are kJ mol-1
What happens in an exothermic reaction?
give out energy
enthalpy change is -
temp increases
What happens in an endothermic reaction?
absorb energy
enthalpy change is +
temp decreases
Define mean bond enthalpy
the average energy needed to break a certain type of bond
they are not exact
Breaking bonds is always endothermic, so mean bond enthalpies are always _______
positive
How to calculate enthalpy change of reaction?
total energy absorbed - total energy released
Break - make
Define standard enthalpy of formation
the enthalpy change when 1 mole of a compound is formed from its elements under standard states and conditions
Define standard enthalpy of combustion
the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard states and conditions
How to find enthalpy changes using calorimetry?
By measuring temperature change
To find enthalpy of combustion of a flammable liquid , you burn it inside a colorimeter
As the fuel burns it heats the water, you can find the heat energy that has been absorbed by the water if you know the mass of water, temperature change and the SHC of the water
How is calorimetry used to measure enthalpy changes in solution (example neutralisation)?
1) to find the enthalpy change for a neutralisation reaction, add a known volume of acid to an insulates container and measure the temperature
2) then add a known volume of alkaline and record the temperature of the mixture at regular intervals over a period of time
3) find the temperature change for the experiment and use it to calculate the enthalpy change of the reaction
you should assume all solutions have the same density as water so 1cm3 of water is 1g
How to use a graph to find an accurate temperature change?
During the experiment, record the temperature at regular intervals, beginning a couple of minutes before you start the reaction
Plot a graph of your results and draw two lines of best fit
Extend both lines past the time when the reaction started
The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction
What is the equation to calculate the enthalpy change?
q = mc change in T
q= joules
m= grams
c = SHC of water 4.18
change in temp = kelvins
In a labortary experiment, 1.16g of an organic liquid fuel were completely burned in oxygen. The heat formed during this combustion raised the temperature of 100g of water from 295 K to 358K.
its Mr is 58
Calculate the standard enthalpy of combustion
1) calculate amount of heat given out
q = 100 x 4.18 x (358-295) = 26334 J
2) standard enthalpies of combustion are always given in units kJ mol-1 so change unites to kJ
26334 /1000 = 26.334 kJ
3) standard enthalpy of combustion is for 1 mole of fuel so find how many moles of fuel produced this
1.16/ 58 = 0.0200
4) so the heat produced by one mol is
-26.334 / 0.0200 = -1316.7 kJ mol-1
Its - as it raised the temperature of the water
Define Hess’ law
The total enthalpy change of a reaction is independent of the route taken
How to calculate enthalpy change of formation using a cycle ?
Products - reactants
