B5 Energetics 2 Flashcards
Define Lattice Enthalpy
The enthalpy change when 1 mole of an ionic compound is formed from its ions in their gaseous state
under standard conditions
is lattice enthalpy exo or endo
exo
state and explain whether Nacl or Mgcl2 will have the highest melting point
what are the 2 ways of calculating lattice enthalpy
Experimental values - born harder cycles - using known energy changes from experiments
Theoretical changes - calucted using physics caluculations- assume ions are perfect spheres
Draw a Born- Harber cycle
standard enthalpy of formation
enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
standard enthalpy of atomisation
enthalpy change when one mole of a gaseous atoms forms from its element in its standard state under standard conditions
is standard enthalpy of atomisation exo or endo
always endothermic
standard enthalpy of atomisation equations
Atomisation equation for iodine
First ionisation enthalpy
enthalpy change when one electron is removed from each atom in one mole of gaseous atoms to form +1 ions
First ionisation enthalpy equation
second ionisation enthalpy
enthalpy change when one electron is removed from each +1 ion in one mole of gaseous +1 ions to form +2 ions
second ionisation enthalpy equation
first electron affinity
enthalpy change when one electron is added to one mole of gaseous atoms to form -1 ions
First electron affinity of chlorine equation
-ve
is second electron affinity exo or endo
endo
Born harber cycle of sodium chloride
write an equation for the standard enthalpy change of atomisation of bromine
What must you remember when using born harber cycle
state symbols
definitions
write the equation of the second electron affinity of oxygen - state and explain whether this will be endothermic or exothermic
endothermic
When must data be multiplied by 2 in a Born-Haber Cycle
When the product has a molar mass of two
When calculating lattice enthalpy from a Born-Haber Cycle, how should the equation be structured
With enthalpy of formation on the left-hand side of the equation, and the route on the right
Going clockwise round a Born-Haber cycle, what is the order of the processes
Atomisations, Ionisations, Electron Affinities, Lattice Enthalpy
What must be remembered with electrons in Born-Haber Cycles
The electrons produced by ionisation energies are used in the affinities
what happens when we put ionic compounds into solution
ions are separated from the lattice and surrounded by water molecules
write an equation to show the lattice enthalpy of sodium chloride
the charge on water molecules are attracted to the charge on the ions, these interactions are known as
ion - dipole interactions
when ions are separated from the lattice and surrounded by water molecules they are considered to be
in solution (aq) and hydrated
Define Enthalpy Change of Solution
enthalpy change when one mole of a solute dissolves in water to give a very dilute solution - avoids ions interacting
Define Enthalpy Change of Hydration
enthalpy change for forming a solution of ions from 1 mol of gaseous ions
Na + (g) + aq —> Na+ (aq)
is enthalpy change of hydration exo or endo
exo -ve
Enthalphy of hydration will depend on
ionic radius
charge
- charge density
more negative
more exothermic
How do we calculate Enthalphy change of solution
Draw a solution enthalpy cycle, with general terms
Draw a solution enthalpy cycle, for sodium chloride
explain this entalphy level diagram
explain this entalphy level diagram
explain this enthalpy level diagram
what about non polar solvents
describe an experiment you could conduct to determine the enthalpy change of solution of NACl
factors that effect standard lattice enthalpy
charge size,
ionic radius
- charge density
factors that effect standard lattice enthalpy key words
charge
ionic radius
lattice entalphy
attraction
Design an experiment to determine the enthalpy change of solution of magnesium chloride (MgCl2) - you should include appropriate measurements to use and names of all equipment used. Include details on how to make your experiment accurate and how to conduct the calculation required. (6 marks)
a)
1. measure water with named appropriate apparatus (measuring cylinder)
2. suitable volume/mass in the range 10 - 200cm3/g
3. into insulated container/polystyrene cup
4. add known mass of MgCl2(s)
5. use of ‘before and after’ weighing method
b)
1. record initial temperature (min 2 measurements)
2. record T at regular timed intervals for 5+ mins/until trend seen
3. plot T vs time
c)
1. extrapolate lines to when solid added (to find initial and final T)
2. Tfinal - Tinitial
3. Q=mc change in T
4. amount = mass/Mr
5. change Hsoln = -q/8.39x10-3
What is the effect on deltaT, and the enthalpy change of solution, when the volume of water in an experiment is doubled
deltaT will half, as the same amount of energy is transferred to double the volume of water. Enthalpy of solution stays the same as the ratio of energy releases: moles is the same, and so the same energy is released per mole.
What is the effect on deltaT, and the enthalpy change of solution, when the mass of the solid is doubled
deltaT will double, as double the amount of energy is being transferred to the same volume of water. Enthalpy of solution stays the same, as the ratio of energy released: moles is the same, and so the same amount of energy is released per mole.
How does a greater ionic radius affect the attraction to ions
Weaker attraction to ions
How does a smaller ionic radius affect the attraction to ions
Stronger attraction to ions
How does a greater ionic charge affect the attraction to ions
Stronger attraction to ions
How does a smaller ionic charge affect the attraction to ions
Weaker attraction to ions
Explanation of comparing two lattice enthalpies with different ionic radii
Lattice enthalpy of X is more exothermic than Y. As ion X is smaller, so has a smaller ionic radius. Therefore attraction to Z- ions is stronger. So more energy is needed to break ionic bonds between ions.
Explanation of comparing two lattice enthalpies with different ionic charge
Lattice enthalpy of X is more exothermic than Y. As X has a 2+ charge, compared to Y which has a 1+ charge. X has a greater ionic charge. Therefore attraction is stronger to Z- ions. So more energy is needed to break ionic bonds between ions.
Dissolving energetically favourable
if Ah soln is negative dissolving is energetically favourable
Define entropy
the number of different ways of arranging molecules and distributing energy
- more random/ disordered –> higher entropy
Describe and explain how entropy changes from (s) to (l) [or (l) to (g)]
Entropy increases as disorder increases. As liquid particles have more disorder than solid particles.
Describe and explain how entropy changes from (l) to (s) [or (g) to (l)]
Entropy decreases as disorder decreases. As solid particles have less disorder than liquid particles.
Describe and explain how entropy changes when there are more moles of gas as products
Entropy increases as disorder increases. As more gaseous molecules are produced.
Describe and explain what happens to entropy when there are less moles of gas in the products
Entropy decreases as disorder decreases. As less gaseous molecules are produced.
Equation to calculate deltaS
Unit for entropy
J K-1 mol -1
remember to divide by 1000
symbol for entropy
if a system changes to become more random, energy can be spread out more
entropy changes
positive entropy change
solid - gas
less moles to more moles
in equations
Units for deltaS
J K(-1) mol (-1)
Free energy
overall energy change in a chemical reaction
- The enthalpy change (ΔH) - This is the energy transfer between the chemical system and the surroundings
The entropy change at the temperature of reaction (TΔS) - This is the dispersal of energy within the system at a given temperature
Gibb’s Free Energy Equation
Gibb’s Free Energy Equation labeled
What is crucial to do when calculating Gibb’s Free Energy
Always divide deltaS by 1000, so that it is in kJ
What must be done when calculating Gibb’s free energy with the units
Divide deltaS by 1000 so it is in kJ
Why must deltaS be divided by 1000 when using it in Gibb’s Free Energy equation
As deltaG is in kJ, and deltaS is in J
Units for deltaG
kJ mol(-1)
When is a reaction feasible
When deltaG is equal to/below zero
When does the feasibility of a reaction change
When deltaG = 0
and find T
Describe and explain how increasing temperature affects feasibility
Feasibility increases as temperature increases. As -TdeltaS becomes more negative than deltaH, so deltaG decreases.
What is the gradient when Gibb’s Free Energy is graphical
- deltaS
Explain how Gibb’s Free Energy is related graphically
shown graphically
Draw a graph and annotate where:
* Point where feasibility changes
* deltaH
* What the gradient represents
2nd electron affinity is
end0
AG - for it be feasible
must be -ve
why does a feasible reaction not get observed
kinetics are not taken into account, so that activation energy might be too high for it to actually occur
if temp increase
-ve –> exothermci
