Unit 4 entropy Flashcards
What is a spontaneous process?
A SPONTANEOUS PROCESS (e.g. diffusion) will proceed on its
own without any external influence.
What is the driving force behind most spontaneous processes?
They are exothermic meaning they produce more energetically stable products.
What is entropy?
Entropy is a description of the number of
ways atoms can share quanta of energy.
If number of ways of arranging the energy
(W) is high, then system is disordered
and entropy (S) is high
What chemical forms tend to have lower entropy’s?
Elements, simple compounds and pure substances.
What chemical forms tend to have higher entropy’s?
compounds, complex compounds and mixtures.
As a substance goes from solid to liquid to gas, at which stage will there be a bigger increase in system entropy?
When going from a liquid to a gas.
How by looking at the equation for a chemical reaction can the magnitude of the entropy of the system be predicted?
It will be a positive change if the no. of moles of molecules increases and things go from solids to liquids or gases.
Do elements in their standard states have entropy?
Yes. Only perfect crystals at absolute zero
(T = 0 K) will have zero entropy:
How is the entropy of the system found in a chemical reaction?
∆S˚system = Σ S˚products - ΣS˚reactants
Look up the standard entropy’s per mole of the compounds used and use the above equation.
How is the magnitude of the entropy of the surroundings found ?
If the reaction is endothermic, energy will have been taken from the surroundings meaning a negative entropy.
If the reaction is exothermic, then the surroundings gain energy and entropy is positive.
What is the equation used to find the entropy of surroundings ?
∆S surrounding = (- ∆H reaction) / T
Convert the enthaply to joules by multiplying by 1000.
Convert T to K by adding 273.
How is the total entropy change found?
∆S total = ∆S system + ∆S surrounding
What must be true for a reaction to be spontaneous?
∆S total must be positive.
How is the temperature when a reaction will become feasible found?
∆S total must be positive or 0 so make it 0 in ∆S total = ∆S system + ∆S surrounding
Rearrange it to get T= ∆H / ∆S system
What is the ∆S Total for a change in state?
0.
How would you find the temperature a substance melts if you had the enthaply of the melting and the entropy of the system?
Use ∆ S total = ∆S system + ∆S surrounding and make the total 0.
Get T= ∆H / ∆S system and figure it out.
If a reaction involves an increase in the entropy of the system what affect will increasing the temperature have?
If the reaction involves an increase in entropy of the system
(∆S system is +) then increasing Temperature will make the
-∆H/T have a smaller magnitude and it will more likely that
∆S total will be positive and more likely that the reaction
occurs
If a reaction involves a decrease in the entropy of the system what affect will increasing the temperature have?
If the reaction involves an decrease in entropy
(∆S system is -ve) then increasing Temperature will
make it more less likely that ∆Stotal will be positive
because the -∆H/T will have a smaller magnitude and
will be less likely to compensate for the negative ∆S system
It will be less likely for the reaction to occur
If a reaction involves a system entropy close to 0, what affect will increasing the temperature have?
If the reaction has a ∆H close to zero then
temperature will not have a large effect on the
feasibility of the reaction as - ∆H/T will be
small and ∆ Stotal won’t change much
What is the enthaply of lattice formation?
The Enthalpy of lattice formation is the standard enthalpy
change when 1 mole of an ionic crystal lattice is formed
from its constituent ions in gaseous form.
What is the enthaply of solution
The enthalpy of solution is the standard enthalpy change
when one mole of an ionic solid dissolves in an large
enough amount of water to ensure that the dissolved ions
are well separated and do not interact with one another
What is the enthaply of hydration?
Enthalpy change when one mole of gaseous ions
become hydrated such that further dilution causes no
further heat change
This always gives out energy because bonds are always made between the ions and the water molecules.
What trends does the strength of the enthaply of lattice formation depend on?
The sizes of the ions:
The larger the ions, the less negative the enthalpies of lattice
formation (i.e. a weaker lattice). As the ions are larger the charges
become further apart and so have a weaker attractive force
between them.
The charges on the ion:
The bigger the charge of the ion, the greater the attraction between
the ions so the stronger the lattice enthalpy (more negative values).
What happens, in terms of bonds, when an ionic compound dissolves in water?
When an ionic lattice dissolves in water it involves breaking up the bonds in the lattice and forming new
bonds between the metal ions and water molecules.
Calculate the enthalpy of solution of NaCl
given that the lattice enthalpy of formation of
NaCl is -771 kJmol and the enthalpies of
hydration of sodium and chloride ions are -406
and -364 kJmol respectively
∆H sol = - ∆ HLatt formation + Σ∆Hhyd
= - (-771) + (-406-364)
= + 1 kJmol
What are the bonds made between ions and water molecules?
The negative ions are attracted to the δ+ hydrogens on
the polar water molecules and the positive ions are
attracted to the δ
- oxygen on the polar water molecules.
When an ionic lattice is dissolved in water, how do the bonds form?
The negative ions are attracted to the positive hydrogen and the positive ions are attracted to the negative oxygen.
What affect does the charge density of an ion dissolved in water have on hydration enthaply?
It is more negative as the bonds formed are stronger.
If the enthaply of solution is not very exo or endothermic, what does that mean?
the hydration enthalpy is about the same as lattice
enthalpy
In general the substance is more likely to be soluble if
the ΔH solution is exothermic.
If a substance is insoluble it is often because
the lattice enthalpy is much larger than the hydration enthalpy and
it is not energetically favourable to break up the lattice, making ΔH solution endothermic.
When a lattice dissolves in water what does that mean for the system entropy and what can that mean for the total entropy ?
The ions will get more disorganized meaning that the system entropy will increase and be positive. Thus it may be able to off set an endothermic enthaply of hydration.
What is the symbol for the entropy change?
∆s
What is the symbol for enthalpy change
∆H
If you are finding the enthalpy for a process by measuring temperature change, how could you improve the accuracy?
Use larger amounts of reactants to give a larger change in temperature.
