Spontaneous Reactions

In the context of a chemical reaction, “spontaneous” describes a reaction that can “proceed of its own accord without outside or external cause”. Consider the system of water on the earth acting under the influence of gravity. Water flows spontaneously downhill. The direction of flow is from a position of greater PE to one of lesser PE. The change is in the direction of lower energy, thus greater stability.  If possible, most reactions will occur spontaneously in the direction of lower ENTHALPY! (-)ΔH  THUS EXOTHERMIC REACTIONS ARE FAVORED TO BE SPONTANEOUS!

Spontaneous Reactions However, the drive toward lower enthalpy (energy) is NOT the only factor in determining whether a reaction will or will not occur spontaneously. A second factor, called ENTROPY, is involved. ENTROPY is a measure of the number of ways that energy (PE and KE) can be stored by a system. Recall that in a chemical system, PE is stored by bonds and electron shells (PEL's), whereas KE is due to all the types of motion (translational, rotational, and vibrational). Systems that possess great amounts of energy must have many “places” to put the energy and will have a high entropy measure.

Spontaneous Reactions USUALLY, but not always, if the components of a system are arranged randomly or are in a state of great disorder, then the system has a large amount of entropy. THUS a highly used definition for entropy (but not the most complete) is ____________________________________ ______________________________________________  If possible, most reactions will occur spontaneously in the direction of higher ENTROPY! (+)ΔS  THUS REACTIONS THAT INCREASE ENTROPY ARE FAVORED TO BE SPONTANEOUS! that entropy is a measure of the disorder, randomness, or chaos of a system.

Spontaneous Reactions Entropy increases: solid → liquid → aqueous → gas Entropy increases when there are more moles of gaseous products than gaseous reactants. These two factors, (1) a drive towards lower enthalpy and (2) a drive towards higher entropy, are what control all reactions in the universe. Sometimes they “work together” and sometimes they “oppose each other” as we shall eventually see.

Spontaneous Reactions Ex.1) Which change is accompanied by a decrease in entropy? (1) H 2 O (l) → H 2 O (s) (3) H 2 O (s) → H 2 O (g) (2) H 2 O (l) → H 2 O (g) (4) H 2 O (s) → H 2 O (l) Ex.2)Which change is accompanied by a increase in entropy? (1) 6 CO 2(g) + 6 H 2 O (l) → C 6 H 12 O 6(s) + 6 O 2(g) (2) C 6 H 12 O 6(s) → C 6 H 12 O 6(aq) (3) CO 2(g) → CO 2(aq) (4) 2 Mg (s) + O 2(g) → 2 MgO (s)

Spontaneous Reactions Ex.3) At a constant temperature, a system tends to undergo a reaction so that its final state, compared to its initial state, has (1) higher enthalpy and higher entropy (2) lower enthalpy and higher entropy (3) higher enthalpy and lower entropy (4) lower enthalpy and lower entropy Ex.4)In which of the following sequences does the entropy for a substance continually increase? (1) (s)  (l)  (g)  (aq) (2) (s)  (l)  (aq)  (g) (3) (s)  (aq)  (l)  (g) (4) (g)  (aq)  (l)  (s)

Spontaneous Reactions Ex.5)If the  H for a reaction is (-) and the  S is (+) then the reaction will (1) definitely be spontaneous (2) definitely be nonspontaneous (3) may be spontaneous or nonspontaneous (4) is in equilibrium with its reverse reaction Ex.6) If the  H for a reaction is (-) and the  S is (-) then the reaction will (1) definitely be spontaneous (2) definitely be nonspontaneous (3) may be spontaneous or nonspontaneous (4) is in equilibrium with its reverse reaction

Spontaneous Reactions Ex.7) If the  H for a reaction is (+) and the  S is (+) then the reaction will (1) definitely be spontaneous (2) definitely be nonspontaneous (3) may be spontaneous or nonspontaneous (4) is in equilibrium with its reverse reaction Ex.8)If the  H for a reaction is (+) and the  S is (-) then the reaction will (1) definitely be spontaneous (2) definitely be nonspontaneous (3) may be spontaneous or nonspontaneous (4) is in equilibrium with its reverse reaction

Spontaneous Reactions Ex.9)Which chemical reaction will definitely react spontaneously? (1) an endothermic reaction in which there is an increase in entropy (2) an endothermic reaction in which there is a decrease in entropy (3) an exothermic reaction in which there is an increase in entropy (4) an exothermic reaction in which there is a decrease in entropy

Spontaneous Reactions Ex.10)Which chemical reaction will definitely NOT react spontaneously? (1) an endothermic reaction in which there is an increase in entropy (2) an endothermic reaction in which there is a decrease in entropy (3) an exothermic reaction in which there is an increase in entropy (4) an exothermic reaction in which there is a decrease in entropy

Spontaneous Reactions Ex.11)A chemical reaction will most likely react spontaneously if (1)  H is + and  S is + (3)  H is - and  S is + (2)  H is + and  S is - (4)  H is - and  S is – Ex.12)In order for an endothermic reaction to occur spontaneously, the entropy must (1) increase (2) decrease (3) remain the same Ex.13)The difference between the PE of the products and the PE of the reactants is (1)  G (3)  S (2)  H (4)  T

Spontaneous Reactions Ex.14)The difference between a gaseous substance’s condensing point and its solid’s melting point is (1) ΔG (2) ΔH (3) ΔS (4) ΔT Ex.15)Which change is accompanied by an increase in entropy (1) NaOH (aq) + HCl (aq)  H 2 O (l) + NaCl (aq) (2) HCl (aq) + NH 3(aq)  HNO 3(aq) + AgCl (s) (3) HCl (aq) + AgNO 3(aq)  HNO 3(aq) + AgCl (s) (4) FeS (s) + HCl (aq)  FeCl 2(aq) + H 2 S (g)

Spontaneous Reactions Ex.16)NaCl (s) has _______ entropy compared with NaCl (aq) because__________________________________ _________________________________________ less the ions in the aqueous state are separated and free to move around which is more chaotic than the ions in the solid state