Crystallization heat
To demonstrate an exchange of heat between a system and its environment — in this case the opposite of the dissolution of a salt.
ما يتعلمه الطلاب
الجانب العلمي وراء هذه التجربة
Changes of state imply a transfer of energy between a substance and its environment. During melting, substances absorb heat and the attraction between the molecules or ions breaks down. This experiment examines the behaviour of a hot concentrated solution as it cools to room temperature.
As already seen in the experiment on the dissolving of a solute in water, a change of state is a thermodynamic matter which involves both enthalpy changes and entropy changes.
When the solute crystallises, there is a decrease in its entropy which contrasts with the decrease in enthalpy but is not enough to prevent the spontaneous crystallization of the melted salt.
المعدات المستخدمة
- Test tube holder — 5423.30
- Spatula — 5460.34
- Thermometer — 2245.15
- Spirit burner — 5514.10
- Iron stand — 5404.51
- Universal clamp — 5416.20
- Right angle clamp — 5401.16
المواد الكيميائية والمواد الكاشفة
- Sodium acetate
ملاحظات السلامة
- Take care when using the burner and hot glassware. Always use a test tube holder when heating test tubes in a flame.
أسئلة للطلاب
- Describe what happens on inserting the thermometer in the liquid.
- Record the temperature during the crystallisation.
- State whether the crystallisation is exothermic or endothermic.
- Will dissolving sodium acetate in water give out or absorb heat?
ما يمكن توقعه
<p>There is a sudden crystallization (stirring or movement of the liquid helps the crystallization). If the crystallization does not happen, inserting some crystals can help the process.</p><p>Heat is evolved and the process is therefore exothermic.</p><p>According to this experiment, dissolving sodium acetate in water should produce a decrease of the temperature. This is easily confirmed by adding sodium acetate to a small quantity of distilled water.</p><p>This is because more energy is necessary to overcome the attraction between the ions than is produced when the ions coordinate with the water molecules.</p>
التخلص
Dispose of sodium acetate in the solid wastes container.
قم بإجراء هذه التجربة في فصلك الدراسي
تُعد هذه التجربة جزءًا من برنامج «مختبر العلوم المتنقل» (ATP) Mobile Lab — وهو مختبر متكامل يحول الفصل الدراسي العادي إلى مختبر علوم فعال، ويضم أكثر من 200 تجربة في مجالات الفيزياء والكيمياء والبيولوجيا والروبوتات والهندسة. ويقوم « Gali » — وهو المعلم الذكي المدمج في موقع ATP Connect — بتوجيه الطلاب خلال كل خطوة والإجابة على أسئلتهم أثناء إجراء التجارب.
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