赫斯定律验证

用三个反应证明焓是状态函数

Thermochemistry Advanced (University) 60 分钟 ~$15.00

目标

测量三个相关反应的焓变，证明ΔH1+ΔH2=ΔH3，从而验证赫斯定律。

背景

Hess's law states that the total enthalpy change for a reaction is independent of the pathway. This is verified by measuring: (1) NaOH(s) → NaOH(aq), (2) NaOH(aq) + HCl(aq) → NaCl(aq), and (3) NaOH(s) + HCl(aq) → NaCl(aq). The sum ΔH1 + ΔH2 should equal ΔH3.

安全警告

  • NaOH pellets are extremely corrosive and generate heat when dissolved
  • HCl is corrosive
  • Wear goggles and gloves throughout

所需PPE

goggles gloves lab_coat

材料

  • Sodium hydroxide pellets (8 g)
    Anhydrous
  • Hydrochloric acid (HCl) (150 mL)
    1M
  • Sodium hydroxide solution (50 mL)
    1M
  • Distilled water (200 mL)

设备

Polystyrene cup calorimeter (2 cups nested) Thermometer Analytical balance Graduated cylinder Stopwatch Lid with hole

步骤

1

Reaction 1: Add 50 mL distilled water to the calorimeter. Record temperature. Add 2 g NaOH pellets. Stir and record max temperature.

8 分钟 Exothermic — NaOH dissolving
2

Calculate ΔH1 for dissolution. Repeat for accuracy.

5 分钟
3

Reaction 2: Add 50 mL 1M HCl to the calorimeter. Record temperature. Add 50 mL 1M NaOH. Record max temperature.

8 分钟
4

Calculate ΔH2 for neutralization.

5 分钟
5

Reaction 3: Add 50 mL 1M HCl to the calorimeter. Record temperature. Add 2 g NaOH pellets directly. Record max temperature.

8 分钟 Very exothermic
6

Calculate ΔH3 for combined dissolution + neutralization.

5 分钟
7

Compare ΔH1 + ΔH2 with ΔH3. They should be approximately equal, verifying Hess's law.

8 分钟

预期结果

ΔH1 (dissolution) ≈ -44 kJ/mol, ΔH2 (neutralization) ≈ -57 kJ/mol, ΔH3 (combined) ≈ -101 kJ/mol. ΔH1 + ΔH2 should equal ΔH3 within experimental error (±10%).

清理

Neutralize all solutions before disposal. Rinse calorimeter cups. Dry and store equipment.

详情

类别
Thermochemistry
难度
Advanced (University)
时长
60 分钟
预估费用
$15.00
步骤数
7
材料
4

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Frequently Asked Questions

What is the objective of 赫斯定律验证?
测量三个相关反应的焓变，证明ΔH1+ΔH2=ΔH3，从而验证赫斯定律。
How difficult is 赫斯定律验证?
This experiment is rated as Advanced (University). It takes approximately 60 minutes to complete.
What safety precautions are needed for 赫斯定律验证?
Key safety precautions include: NaOH pellets are extremely corrosive and generate heat when dissolved; HCl is corrosive; Wear goggles and gloves throughout.
What materials are needed for 赫斯定律验证?
The main materials required are: Sodium hydroxide pellets, Hydrochloric acid (HCl), Sodium hydroxide solution, Distilled water.
What results should I expect from 赫斯定律验证?
ΔH1 (dissolution) ≈ -44 kJ/mol, ΔH2 (neutralization) ≈ -57 kJ/mol, ΔH3 (combined) ≈ -101 kJ/mol. ΔH1 + ΔH2 should equal ΔH3 within experimental error (±10%).