ENTHALPY CHANGES

Enthalpy changes

            Various enthalpy change measured at standard conditions, ie at a temperature of 25 ° C (298 K) and 1 atm pressure is expressed in units of kJ and molar enthalpy change in units of kJ / mol.

a.       Standard enthalpy change of formation (ΔH˚f)

     Is the enthalpy change that occurs in the formation of one mole of a compound from  its elements at standard conditions.

Example:

1.      Reaction formation of water (H₂O)

         1/2O₂ + H₂ → H₂O                          ΔH˚f = - 270 kJ

2.      Reaction formation of HN3

        1/2N₂ + 3/2H₂ → NH₃                       ΔH˚f = + 300 kJ

Example question:

Given the thermochemical reaction:

1. H₂ + I₂ → 2 HI                                    ΔH = - 650 kJ

2. N₂ + 2 O₂ → 2 NO₂                              ΔH = + 245 kJ

Determine the standard formation reaction of the above reaction?

Answer:

1. H₂ + I₂ → 2 HI                                    ΔH = - 650 kJ

    1/2H₂ + 1/2I₂  → 2 x 1/2 HI                  ΔH˚f = - 6502 kJ= - 325 kJ

    1/2H₂ + 1/2I₂ → HI                              ΔH˚f = - 325 kJ

2. N₂ + 2 O₂ → 2 NO₂                              ΔH =+ 245 kJ

1/2N₂ + 2 x 1/2 O₂ → 2 x 1/2 NO₂             ΔH˚f = 245 x + 1/2 + kJ = 122.5 kJ

1/2N₂ + O₂→ NO₂                                    ΔH˚f = + 122.5 kJ

b.      Standard enthalpy change of decomposition (ΔH˚d)

     Is the enthalpy change that occurs in the decomposition of 1 mole of a compound into

its elements at standard conditions.

Example:

1.      The reaction of decomposition of water (H₂O)

        H₂O → H₂ + 1/2O₂                                   ΔH˚d = + 270 kJ

2.      Reaction of decomposition of NH₃

        NH₃   → 1/2N₂ + 3/2H₂                              ΔH˚d = - 300 kJ

Example question:

Given the thermochemical reaction:

1. 2 HI → H₂ + I₂                                                   ΔH = + 650 kJ

2. 2 NO₂ → N₂ + 2 O₂                                       ΔH = - 245 kJ

Determine the standard decomposition reaction of the above reaction?

Answer:

1. 2 HI → H₂ + I₂                                             ΔH = + 650 kJ

    2 x 1/2 HI → 1/2I₂ + 1/2H₂                            ΔH 1/2x + 650  kJ  = + 325 kJ

    HI → 1/2H₂ + + 1/2I₂                                   ΔH˚d = +325 kJ

2. 2 NO₂ → N₂ + 2 O                                               ΔH = - 245 kJ

   2 x 1/2 NO₂ → 1/2N₂ + 2 x 1/2 O₂     ΔH˚d = - 245 x 1/2 kJ = - 122.5 kJ       NO₂     → O₂ + 1/2N₂                        ΔH˚d= - 122.5 kJ

c.       The standard enthalpy change of combustion (ΔH˚c)

     Is the enthalpy change that occurs in the combustion of one mole of a substance at

standard conditions.

Example:

Determine the standard alcohol combustion thermochemical reactions if ΔH˚c C₂H₅OH =

- 560 kJ!

Answer:

C₂H₅OH + 2 O₂ → 2 CO₂ + 3 H₂O      ΔH˚c = - 560 kJ

Enthalpy Changes Quiz

Lesson Subject            : Chemistry

Class                            : XI/ Science 1

Day/date                     :

Direction

1. Write name, absent number and class at the answer sheet.

2. Try to solve problems clearly.

1. Writing thermochemical equations is true with the rules are ....                            
A. H₂ (g) + 1/2O₂ (g) → H₂O (g) ΔH = -x kJ                                                                 
B. H₂ (g) + O₂ (g) → H₂O (g) ΔH = -x kJ
C. 2H₂ (g) + 2O₂ (g) → 2H₂O (g) ΔH = -x kJ
D. H₂ (g) + O₂ (g) → H₂O (g)
E. H₂ (g) + O₂ (g) + energy → H₂O (g)
2. .   For Reaction
N_2(g) + 2O₂               →          2NO_2(g)                ∆Hrxn = + 68 kJ

What is the value of ∆H˚d for the following reaction ?                                               

2NO_2(g)           →           N_2(g) + 2O_2(g)
A. -68 KJ
B. -34 KJ
C. +68 KJ
D. +34 KJ
E. +136 KJ
3. Combustion of one mole of methane gas the following thermochemical equation:
CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) ΔH = -294 kJ
Total enthalpy change for the reverse reaction is ....                                                        
A. -147 KJ
B. -294 KJ
C. -588 KJ
D. +147 KJ
E. +294 KJ
4. 100 mL gasoline combustion release of 275 kJ. ΔH for the combustion of 1 liter is ....
A. -275 KJ                                                                                                                      
B. -2750 KJ
C. -2.75 KJ
D. +275 KJ
E. +2750 KJ

5. As much as 5.6 L of methane burned at STP conditions. The reaction:                    
CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) ΔH = -294 kJ
The amount of heat released is ....
6. How much heat is evolved when 128 gr of sulfur is burned is excess O₂, given the thermochemical equation below? ( Ar S = 32, Ar O = 16 )                                            

S_(s) + O_2(g)                   →        SO_2(g)               ∆Hrxn = - 296 kJ 

7. For the reaction                                                                                                          

            2H₂O_(l) + 2CO_2(g)             →  C₂H_4(g) + 3O_2(g) ∆Hrxn= 1401 kJ

 How much heat is absorbed when 4,4 gr of carbon dioxide react with excess water?

(Ar C=12, Ar O=16)