53 The enthalpy of the system is like a measure of water in a reservoir. When the reaction releases 208 kJ of heat at constant pressure, the reservoir falls by 208 kJ and the ∆ H = − 208kJ When the reaction absorbs 208 kJ of heat at constant pressure, the reservoir rises by 208 kJ and the ∆ H = +208kJ 54 Relationship of ∆ H and ∆ E Example: Given that the combustion of n-pentane, n- C 5 H 12 , at constant pressure releases 3523 kJ of heat at a temperature of 298K. , ∆ H = -3523 kJ/mol What are the values of the work term and ∆ E for this reaction? ) ( 2 2(g) 2(g) ) 12( 5 O H 6 CO 5 O 8 H C A A + → + 8 mol of gas 5 mol of gas
1 55 Relationship of ∆ H and ∆ E Determine the work done by this reaction. 5 12( ) 2(g) 2(g) 2 ( ) 8 mol gas 5 mol gas gas gas J mol K C H 8 O 5 CO 6 H O H -3523 kJ/mol, T 298 K. w -P V -( n )RT n 5 8 mol -3 mol w -(-3 mol)(8.314 )(298 K) 7433 J 7.433 kJ wor + → + ∆ = = = ∆ = ∆ ∆ = − = = = = A A ´±±±³±±±² ´±±±³±±±² k is done on the system by the surroundings 56 Relationship of ∆ H and ∆ E Now calculate the ∆ E for this reaction from the values of ∆ H and w that we have determined. Remember, work is done on the system by the surroundings; therefore, work is positive. ( ) ( ) 3523 7.422 3523 7.422 3516 E q w H kJ w kJ E kJ kJ kJ ∆ = + ∆ = − = + ∆ = − + + = − 57 Relationship of ∆ H and ∆ E Example: Assuming the gases are ideal, calculate the amount of work done, in joules, for the conversion of 1.00 mole of Ni to Ni(CO) 4 at 75°C in the reaction below. The value of R is 8.314 J/mol•K. Ni(s) + 4CO(g) → Ni(CO) 4 (g) We will calculate this one in class. 58 Relationship of ∆ H and ∆ E The reaction of 1.00 mole of H 2 (g) with 0.500 mole of O 2 (g) to produce 1.00 mole of steam, H 2 O(g), at 100°C and 1.00 atm pressure evolves 242 kJ of heat. Calculate ∆ E per mole of H 2 O(g) produced. The universal gas constant is 8.314 J/mol•K. We will calculate this problem in class. 59 Spontaneous Changes Spontaneous changes happen without any continuing outside influences. A spontaneous change has a natural direction. For example the rusting of iron occurs spontaneously. Have you ever seen rust turn into iron metal without man made interference? The melting of ice at room temperature occurs spontaneously. Will water spontaneously freeze at room temperature? 60 Spontaneous Changes A spontaneous change in a process has a natural tendency to occur. The change does not necessarily occur rapidly A nonspontaneous change in a process can be brought about only by doing work.
1 61 Entropy and Disorder Observations of such events as: A hot block of metal cools as the energy of its vigorous vibrating atoms spreads into the surroundings It is highly improvable that energy will gather from the surroundings and concentrate in a small block of metal The randomly moving molecules of a gas spread throughout their containers It is highly improvable that the random motion of gases will bring them all simultaneously back into one corner of the container.