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Terms in this set (81)
What is entropy?
Entropy is a measure of the disorder of a system.
What do we mean by order?
Order means few arrangements of matter or energy, so disorder means many arrangements of matter or energy.
Can entropy be measured?
What is the symbol for entropy?
What are the units for entropy?
T/F: Perfect crystals at 0 K have 0 entropy.
True. This is the 3rd law of thermodynamics.
When does the entropy of a system increase?
When the matter or energy in the system becomes more random in its arrangement.
Why does the entropy increase when a solid melts to become a liquid.
Regular arrangement of particles in a solid goes to less regular arrangement of particles in a liquid.
Does the entropy increase when a liquid vaporizes to become a gas, if so why?
Entropy increases. Irregular arrangement goes to a random arrangement in gas.
Does the entropy increase when a substance dissolves in liquid? If so, why?
Particles in solid are highly ordered, in solution they are randomly dispersed so the entropy increases.
Does the entropy increase when a substance is heated with no change of state? If so, why?
Yes. Particles gain more energy, therefore they vibrate more, therefore they are more disordered.
What is the system?
The system is the vessel in which the reaction takes place.
What is the surroundings?
The air in the room is the surroundings.
T/F: Entropy changes take place in both the system and surroundings.
Fill in the gap, the entropy change of the surroundings is ... for all exothermic reactions (enthalpy change of the system is negative)
Fill in the gap: the entropy change of the surroundings is ... for all endothermic reactions (enthalpy change of the system is positive)
What is the equation relating Delta S surroundigns, enthalpy change and temperature?
Delta S surroundings = -Delta H / T. N.B. Temperature is in KELVIN.
Delta S total = ??
Delta S total = Delta S system + Delta S surroundings
For a spontaneous reaction to occur...?
Delta S must be positive.
True or false, for a spontaneous reaction to occur, Delta S must be positive?
True. This is the 2nd law of thermodynamics.
How do you calculate Delta S system?
Delta S system = Sigma S products - Sigma S reactants.
entropy change positive for both system and surroundings
entropy change negative for both system and
entropy change of system = negative and entropy change of surroundings is positive (exothermic reaction)
Could be feasible, if the magnitude of DS surroundings is greater than the magnitude of DS system. More likely at low temperatures
entropy change for system is positive and entropy change for surroundings is negative (endothermic reaction)
Maybe if magnitude of DS system is greater than the magnitude of DS surroundings so more likely at high temperatures.
What is the most important factor in determining whether a reaction will go or not?
DS surroundings. That is why we find that so many reactionsn that go are exothermic and endothermic reactions that go are rare.
An increase in temperature results in negligible change to the value of DS system, why is this?
Because the entropies of reactants and products change by similar amounts.
T/F: Reactions are thermodynamically feasible if the total entropy change is positive.
True. The reaction will therefore occur providing the kinetics of change are favourable.
T/F: Thermodynamics gives no information about reaction rate.
True. A thermodynamically feasible reaction might have such a high activation energy that it does not proceed. Here the reactants are thermodynamically unstable with respect to the products but because the activation energy is so high the rate is so slow that the reaction does not proceed. The reactants are kinetically stable with respect to the products.
T/F: When a crystal lattice is formed from the gaseous ions a large amount of heat is liberated due to strong electrostatic attractions between the ions.
Define the lattice enthalpy.
The lattice enthalpy (Delta H lattice) is defiend as the enthalpy released when 1 mole of the solid crystal lattice is formed from its constituent gaseous ions that start infinitely far apart. Hence Delta H l is always exothermic (enthalpy change is negative).
What does the vale of the lattice enthalpy depend on?
1. The charge on the ions.
2. The internuclear distance.
3. The type of crystal lattice.
The charge on the ions
The more highly charged the ions the stronger the electrostatic attraction between them and therefore the more exothermic is the lattice enthalpy.
e.g. the lattice enthalpy for an M^2+x^2- compound is 2x2 = 4 times that for an M+ x1 compound (assuming that both compounds have the same internuclear distance).
The internuclear distance
The larger the internuclear distance the weaker the electrostatic attraction between the ions and therefore the less exothermic is the lattice enthalpy.
The type of crystal lattice
i.e. the arrangement of the ions in the crystal lattice e.g. hexagonal close packed, face centred cubic, body centre cubic (this is the least important factor
Define the enthalpy of Hydrtation (Delta H h of an ion)
The enthalpy change (exothermic) when one mole of the gaseous ions is dissolved in water to form a dilute solution.
Why are hydration enthalpies exothermic?
Because the ions bond strongly to water molecules, electrostatic forces holding the particles together.
What does the magnitude of a hydration enthalpy depend on?
1. The charge on the ion.
2. The size of the ion
The charge on the ion.
The higher the charge the stronger the electrostatic attraction and the more exothermic the hydration enthalpy.
The size of the ion.
The smaller the ion the stronger the electrostatic attraction and the more exothermic the hydration enthalpy.
What can you deduce from this?
Ions with high charge densities (high charge and small size) have very exothermic hydration enthalpies.
Define the enthalpy of solution (delta H soln)
The enthalpy change when one mole of a solid is dissolved in water to form an infinitely dilute solution.
Delta H solution =???
- Delta H lattice + Delta H hydration
T/F: Theory shows that the change of diamond to graphite is spontaneous.
True. However, the change is very, very slow.
T/F: Diffusion is a spontaneous change
Chemists sometimes describe entropy as a measure of disorder or randomness. These descriptions have to be interpreted with care, why?
Because the disorder refers not only to the arrangement of the particles in space but much more significantly to the numbers of ways of distributing the energy of the system across all the available energy levels.
DS total = -ve, activation energy high
No reaction - reactants stable relative to products.
DS total = +ve, high Ea
No reaction - reactants unstable relative to products but kinetically inert.
DS total = -ve, low Ea
No reaction, reactants stable relative to products
DS total - +ve, low Ea
Fast reaction, reactants unstable relative to products.
Define the standard enthalpy change of solution
The enthalpy change when one mole of a compound dissolves to form a solution containing 1 mol/dm^3 under standard conditions. Enthalpy changes of solution are often calculated and quoted for 1 mole of a compound dissolving in specified amounts of water.`
As the radius of an ion increases...
Its charge density decreases. This results in a weaker attraction for oppositely charged ions and for d+ and d- charges on polar molecules such as water.
What happens in a more concentrated solution?
In a more concentrated solution the ions are closer together and there are fewer free water molecules to hydrate the ions. These changes modify the values of the hydration energies and the entropies of the ions in solution. Overall this meas that once a certain amount of a salt has dissolved, the processes ceases to be spontaneous and no more solid dissolved. At this point the solution is saturated. The salt crystals and the saturated solution are in equilibrium.
A chemical or mixture of chemicals is thermodynamically stable if...?
If there is no tendency for a reaction. If the total entropy change, DS total is negative, this indicates that the reaction does not tend to occur.
Kinetic inertness is a term used when a reaction does not go even though it appears to be feasible. The reaction tends to go as indicated by a positive entropy change, yet nothing happens. There is no change because the rate of reaction is too slow to be noticeable. There is a barrier preventing change - usually a high Ea. The compound or mixture is inert.
When is a chemical inert?
A chemical is inert if it has no tendency to react even when the reaction is feasible.
What is a spontaneous or feasible reaction?
A spontaneous, or feasible reaction is one that naturally tends to happen, even if it is very slow because the activation energy is high.
The entropy of a chemical rises as the temperature rises. Increasing the temperature raises the number of energy quanta to share between the atoms and molecules.
Also there is an increase in the number of ways of arranging or molecules as a solid changes to a liquid and then to a gas.
C diamond vs. C graphite
graphite has a higher entropy value. Diamond has a hard, rigid structure, graphite is softer and atoms are more disordered.
Mg vs. Pb
Pb has a higher value. Mg with smaller atoms has a harder, more rigid structure than soft Pb.
Mgo vs. MgCO3
MgCO3 has a higher value. MgO is simpler than MgCO3 so has a lower entropy value.
Mgo (s) vs H2o (l) vs. O2 (g)
o2 highest, then h2o. In solids, thermal motion of the atoms is restricted, entropy is small; for liquids and gases there is much more disordered motion and entropy values are larger.
positive DS total
Reactants are thermodynamically unstable relative to products
negative DS total
Reactants are thermodynamically stable relative to products
Reactants are kinetically unstable relative to products
Reactants are kinetically stable relative to products
T/F: For a given quantity of energy, the increase in entropy is greater when the surroundings are cool than when they are hot.
True: Adding energy to molecules in a cool system has a proportionately greater effect on the number of ways of distributing matter and energy than adding the same quantity of energy to a system that is already very hot.
Why do most exothermic reactions tend to go?
Because at about room temperature, the value of -DH/T is much larger and more positive than DS system.
A reaction that does not tend to go at room temperature may become feasible as the temperature rises because DS surroundings decreases in magnitude as T increases.
Group 2 sulfates
DS system is negative and gets less negative down the group as the ions get bigger in radius. The change is DS system going from magnesium sulfate to barium sulfate is 108J/K/mol less negative. The enthalpies of solution change from -91kJ/mol for MgSO4 to +10kJ/mol for BaSO4, so DSsurr changes from +305 to -64J/K/mol. This is a decrease of 369J/K/mol, and is a much larger decreases than the change in DS system. This causes DS total to change from a positive value (soluble) for magnesium sulfate to a negative value (insoluble) for barium sulfate
do not assume that DS system for dissolving is always positive - it can be negative.
Group 2 hydroxides
DS sytem is negative and gets less negative down the group as the ions get bigger in radius. DH solution goes from being slightly positive for magnesium to negative (exothermi) for barium hydroxide. Both these changes increase the solubility.
The solubility of hydroxides increases dow nthe group.
Thermodynamics of dissolving
When an ionic solid dissolves in water, the entropy of the solute increases as it changes from an ordered solid to a more random solution. If the ionic solid is anhydrous it becomes hydrated on dissolving. Several water molecules, usually six, use the lone pair of electrons on the oxygen atom to bond exothermically with the cation. This makes the hydrogen atoms much more delta + than in isolated water molecules and causes other water molecules to be joined on in a second sphere. Even more water molecules hydrogen bond to the second sphere, causing the water to become more ordered than in the pure state. The extent to which this happens depends on the charge density of the cation. Small, highly charged ions such as Al3+ or Mg2+ cause a considerable orsering of the water molecules, which outweighs the increase in the randommness of the dissolved solute.
As a guide the following may be assumed (singly positive cations)
For singly positive cations, such as group 1, NH4+ and Ag+, the ordering is not very large and is considerably outweighed by the increase in disorder of the solute. Thus DS system for dissolving is positive.
For anhydrous salts containing doubly or triply charged cations, such as group 2 and 3 and many transition metal ions...
The ordering is significant and can outweigh the increase in randomnees or disorder of the solute. Thus Delta S system for dissolving is sometimes negative.
Whether an ionic solid dissolves in water is determined by what?
The sign of DELTA S total. If this is positive, the solid dissolves. If negatie, it is either insoluble or only slightly soluble.
The value of Delta S total depends on the values of Delta S system and Delta S surrounedeings:
delta S total = Delta S system - DH/T
Enthalpy of solution refers to substances, whereas hydration enthalpy refers to ions.
Factors determing hydration enthalpy - Ionic charge
The greater the charge, the stronger is the ion/dipole force of attraction between the ion and the water molecules. Therefore, the hydration energy is more exothermic.
Group 1 substances:
DS system is positive. Even if DH solution is positive (endothermic), -DH/T is smaller in magneitdue than DS system, so DS totla is opstiive and all group 1 solids are water-soluble.
Group 2 sulfates:
DS system is negative and gets less negative down the group as the ions get bigger in radius. The change in DS system going from magnesium sulfate to barium sulfate is less negative.
This causes DS total to change from a positive value (solube) for magnesium sulfate to a negative value (insoluble) for barisium sulfate.
Group 2 hydroxide
DS system is negative and gets less negative down the group as the ions get bigger in radius. DH solution goes from being slightly positive for magnesium hydroxide to negative (exothermic) for barium. Both of these changes increases the solubility.
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