If Ba(Oh) 2+Nh4Cl Are Reactants, Then What Will Be The Product?

     
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Demonstrations › Thermodynamics I ›7.1

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Thermodynamics I: Energy, Heat, Enthalpy

7.1 Endothermic Reaction

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Subjects: Thermodynamics – endothermic reactions, enthalpy, entropy, không tính tiền energy

Description: An endothermic reaction involving the combination of two solids is demonstrated. The flask will rapidly become cold (45°C below room temperature) and frozen condensation will be visible on the surface of the flask. The flask can be placed on a block with a pool of water. The flask will freeze to lớn the block.

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Materials:

32 g Barium hydroxide (Ba(OH)2)‡ 11 g NH4Cl (or 17 g NH4NO3 or 16 g NH4SCN)‡ 250 mL Erlenmeyer flask funnel small block of wood (optional) water in wash bottle (optional) thermometer - standalone or Vernier Stainless steel (optional)* Logger Pro or Logger Lite software for Vernier temp probe Go-Link interface *

‡Barium hydroxide và ammonium chloride are located in the general dichvuhaotam.comical storage cabinets. Ammonium nitrate is located on the oxidizer shelf.

*Located in the drawers opposite the bin storage shelves.

Procedure: 1. Using the funnel, place the solids in the flask and gently swirl the contents of the flask to lớn mix. 2. A liquid slurry will form in the bottom of the flask. 3. Wet the small wood block with a few drops of water và set the reaction flask on it. The flask will freeze to the board.

Alternatively, when the reaction is complete, pass the flask around the class.

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Discussion: The reaction for the above demonstration using ammonium chloride is given below:

Ba(OH)2·8H2O(s) + 2 NH4Cl(s) à 2 BaCl2·2H2O(aq) + 2NH3(aq) + 8 H2O(l)

For a reaction to be spontaneous at constant temperature and pressure, the change in không lấy phí energy (∆G) must be negative.

∆G = ∆H - T∆S

In an endothermic reaction, the ∆H is positive corresponding khổng lồ the absorbed heat. Thus this positive ∆H must be offset by a sufficient increase in entropy (∆S). Two solids creating a new solid và an aqueous solution of ammonia represents a large increase in entropy.

The thermodichvuhaotam.comical date for barium hydroxide, ammonium chloride, barium chloride, ammonia & water are given in the table below.

Substance

∆H˚f (kJ/mol)

S˚298(J/mol·K)

∆G˚f (kJ/mol)

Ba(OH)2·8H2O(s)

-3342

427

-2793

NH4Cl(s)

-341.4

94.6

-203

BaCl2·2H2O(s)

-1460.1

203

-1296.5

NH3(aq)

-80.29

111

-26.6

H2O(l)

-285.83

75.291

-237.2

The enthalpy for the reaction is calculated using the following equation: ∆rH˚= ∑∆H˚f (products) - ∑∆H˚f(reactants) ∆rH˚ = 63.5 kJ

The entropy for the reaction is calculated using the following equation: ∆rS˚= ∑∆S˚f (products) - ∑S˚f(reactants) ∆rS˚ = 368 J/K = .368kJ/K

Calculating the không lấy phí energy from the above values at 298K: ∆G˚ = ∆H˚ - T∆S˚ = 63.5-298(.368) = -46.1 kJ

Safety: Avoid inhalation of the ammonia produced by the reaction. Perform in a well-ventilated room. Avoid prolonged contact of the skin with the cold flask. Barium salts are poisonous. These as well as ammonium salts can cause skin irritations.

Disposal: The contents of the flask should be disposed of in an appropriate waste container.

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References: 1. B.Z. Shakhashiri; dichvuhaotam.comical Demonstrations: A Handbook for Teachers of dichvuhaotam.comistry; Wisconsin; 1983; Volume 1;p 10-12 2. L. Summerlin, J. Ealy; dichvuhaotam.comical Demonstrations: A Sourcebook for Teachers. ACS publications. Washington D.C; 1983; p 44

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