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Entropy

 

This is considered to be the degree of disorder of a system. Gas particles have random motion  have high entropy values. Liquids have much lower entropies and solids lower still. Solutions have a greater entropy than pure liquids as the particles in a solution are more separated and solvent molecules separate the solute particles.

Solids   Liquids < Aqueous Solutions <  Gases

If the number of moles of gas increases in a chemical reaction (as shown by the stoichiometry of the reaction) then the entropy also increases.

dinitrogen tetroxide nitrogen dioxide

N2O4(g) 2NO2(g)

In this example there are two moles of gas on the right hand side and only one mole of gas on the left hand side - the entropy increases going from left to right (reactants to products).

Disorder can also arise by mixing different components. For instance a mixture of two different gases will have greater entropy than the sum of the entropies of the two gases alone.

The entropy change of a reaction is given the symbol ΔS, units when used are J/mol K (not kJ).

 

***Entropy increases will be observed when any one or more of the following occur:

  1. A reaction breaks up a larger molecule into smaller molecular fragments
  2. A reaction occurs in which there is an increase in the moles of gas in the product.
  3. A process where a solid changes to a liquid or gas or a liquid changes to a gas

 Examples-Highlight the boxes to reveal the answer and reasoning.

Reaction Change in Entropy Reasoning
Ag+(aq) + Cl-(aq) ----> AgCl(s) Decrease Less moles 2 vs. 1 

and aq. becomes a solid

H2O(l) -> 2 H2(g) + O2(g) Increase More moles of particles 2 vs. 3

 and a liquid producing gases

 

CO(g) + 3 H2(g)-> CH4(g) + H2O(g) Decreases 4 moles become 2 moles
2 NaHCO3(s) ---> Na2CO3(s) + H2O(g) + CO2(g) Increase More moles of particles 2 vs. 3

 and a solid producing gases

CO(g) + H2O(g) ----> CO2(g) + H2(g) No Obvious Change Must actually do a calculation
CaCO3(s) ----> CaO(s) + CO2(g) Increase More moles of particles 1 vs. 2

and a solid becomes a gas

2NH3(g) + CO2(g) ----> NH2CONH2(aq) + H2O(l) Decrease Less moles 3 vs. 2 

and gases becomes liquids and solutions

H2O(l) ---> H2O(s) Decrease Liquid becomes a solid
2Na2O2(s) + 2H2O(l) ---> 4NaOH(aq) + O2(g) Increase More moles 4 vs. 5

solid and liquid becomes a solution and gas

C2H5OH(l) + 3O2(g) ---> 2CO2(g) + 3H2O(g) Increase More moles 4 vs. 5

liquid and a gas becomes all gases

September 6, 2000 -Christian Right Lobbies To Overturn Second Law Of Thermodynamics

Past Regents Questions on Entropy

Jan 201044 Given the balanced equation representing a phase change:
C6H4Cl2(s) + energy==>C6H4Cl2(g)
Which statement describes this change?
(1) It is endothermic, and entropy decreases.
(2) It is endothermic, and entropy increases.
(3) It is exothermic, and entropy decreases.
(4) It is exothermic, and entropy increases.

Aug 2009-45 The entropy of a sample of H2O increases as the sample changes from a
(1) gas to a liquid    (3) liquid to a gas
(2) gas to a solid     (4) liquid to a solid

June 2008-45 Which 1-mole sample has the least entropy?
(1) Br2(s) at 266 K     (3) Br2(l) at 332 K
(2) Br2(l) at 266 K      (4) Br2(g) at 332 K

June 2008-17 A thermometer is in a beaker of water. Which statement best explains why the thermometer reading initially increases when LiBr(s) is dissolved in the water?
(1) The entropy of the LiBr(aq) is greater than the entropy of the water.
(2) The entropy of the LiBr(aq) is less than the entropy of the water.
(3) The dissolving of the LiBr(s) in water is an endothermic process.
(4) The dissolving of the LiBr(s) in water is an exothermic process.

Jan 2007-27 In terms of energy and entropy, systems in nature tend to undergo changes toward
(1) higher energy and higher entropy
(2) higher energy and lower entropy
(3) lower energy and higher entropy
(4) lower energy and lower entropy

Aug 2006-49 Which list of the phases of H2O is arranged in order of increasing entropy?
(1) ice, steam, and liquid water
(2) ice, liquid water, and steam
(3) steam, liquid water, and ice
(4) steam, ice, and liquid water

Jan 2006-8 Given the balanced equation:
I2(s) + energy ==> I2(g)
As a sample of I2(s) sublimes to I2(g), the entropy of the sample
(1) increases because the particles are less randomly arranged
(2) increases because the particles are more randomly arranged
(3) decreases because the particles are less randomly arranged
(4) decreases because the particles are more randomly arranged

Aug 2005-46 At STP, a sample of which element has the highest entropy?
(1) Na(s)      (3) Br2(l)
(2) Hg(l)       (4) F2(g)

June 2005-20 Systems in nature tend to undergo changes toward
(1) lower energy and lower entropy
(2) lower energy and higher entropy
(3) higher energy and lower entropy
(4) higher energy and higher entropy

Jan 2005-43 Which of these changes produces the greatest
increase in entropy?
(1) CaCO3(s) --> CaO(s) + CO2(g)
(2) 2 Mg(s) + O2(g) --> 2 MgO(s)
(3) H2O(g) --> H2O(l)
(4) CO2(g) --> CO2(s)

Aug 2004-21 Even though the process is endothermic, snow can sublime. Which tendency in nature accounts for this phase change?

(1) a tendency toward greater entropy

(2) a tendency toward greater energy

(3) a tendency toward less entropy

(4) a tendency toward less energy

Jan 2004-

June 2005-

June 2008-22 Systems in nature tend to undergo changes toward
(1) lower energy and less disorder
(2) lower energy and more disorder
(3) higher energy and less disorder
(4) higher energy and more disorder

 

June 2004-

June 2003-50 As carbon dioxide sublimes, its entropy

(1) decreases (2) increases  (3) remains the same

Jan 2003-41 Which phase change represents a decrease in entropy?

(1) solid to liquid      (3) liquid to gas
(2) gas to liquid       (4) solid to gas

Aug 2002-39 Which sample has the lowest entropy?
(1) 1 mole of KNO3(l)   (3) 1 mole of H2O(l)
(2) 1 mole of KNO3(s)  (4) 1 mole of H2O(g)

June 2002-44 Which process is accompanied by a decrease in entropy?

(1) boiling of water
(2) condensing of water vapor
(3) subliming of iodine
(4) melting of ice

 

 

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