放热反应和吸热反应之间的区别

化学反应以符号形式表示的化学反应所涉及的化学物质称为化学方程式。这是一种使用所涉及物质的符号和公式来表示化学反应的方法。有三种方法可以使化学方程式更具信息性。

• By describing the reactants’ and products’ physical states. Solids are represented by ‘s,’ liquids by ‘l,’ and gases by ‘g.’ Aqueous solutions are represented by'(aq)’.
• By describing the conditions in which the reaction occurs. If the reaction is taking place in the presence of a catalyst, for example, the catalyst’s symbol or formula is written above or below the arrow sign.
• By indicating the heat changes that are occurring during the reaction. An exothermic reaction, for example, is one in which heat is released. A reaction in which heat is absorbed is known as an endothermic reaction.

下面进一步讨论反应过程中发生的热量变化。在热变化的基础上，有两种类型的反应。这些反应是放热反应和吸热反应。

什么是放热反应？

A chemical reaction that involves the release of energy in the form of heat or light is known as an exothermic reaction. When carbon burns in oxygen to make carbon dioxide, for example, a large amount of heat is produced.

C + O ₂ → CO ₂
(碳) (氧) (二氧化碳)

由于碳在氧气中燃烧时会产生热量，因此它是放热反应。在等式的产品一侧，使用“+Heat”或“+Heat energy”或简单地“+Energy”来表示放热过程。

放热反应的例子

• 甲烷是天然气的主要成分。当天然气在空气中存在氧气的情况下燃烧时，会产生二氧化碳和水蒸气。还会产生大量的热能。以下是如何表达这一点的示例：

CH ₄ + 2O ₂ → CO ₂ + 2H ₂ O + 热能
（甲烷）                         （氧）                        （二氧化碳）            （水）                                                

由于天然气燃烧过程中会产生热量，因此它是放热反应。放热反应是所有燃烧反应的特征。例如，木材、煤、煤油、汽油和柴油等燃料的燃烧都是放热反应，因为它们会产生热能。放热反应包括呼吸过程中我们体内的食物燃烧，例如葡萄糖。这将进一步详细解释。

• 为了生存，我们需要能量。这种能量来自我们食用的食物。食物在消化过程中被分解成更简单的成分。淀粉碳水化合物主要存在于薄饼、面包、米饭和土豆等膳食中。淀粉碳水化合物在消化过程中被分解成一种简单的碳水化合物，称为葡萄糖。在一个称为呼吸的过程中，这种葡萄糖通过与我们身体细胞中的氧气混合而缓慢燃烧以产生能量。除其他外，这种能量还可以使我们的身体保持温暖。在呼吸过程中，葡萄糖与我们身体细胞中的氧气相互作用，产生二氧化碳和水，以及能量。

C ₆ H ₁₂ O ₆ + 6O ₂ → 6CO ₂ + 6H ₂ O + 能量

                   (葡萄糖) (氧气) (二氧化碳) (水)

• 由于在整个呼吸过程中都会产生能量，如上述反应所示，所以它是一个放热过程。
• 由于镁线在空气中燃烧产生氧化镁的过程中会释放热能和光能，这是一种放热反应。
• 此外，蔬菜废物分解成堆肥的过程中会释放热能，这也是一个放热过程。

These reactions are the polar opposites of endothermic reactions, and they can be written as follows in a chemical equation: 

Reactants       →         Products      +       Energy

As a result, it’s easy to see how the net amount of energy required to start an exothermic process is less than the net amount of energy released. The net quantity of heat energy that passes through a calorimeter, which is an instrument used to quantify the heat released by a chemical reaction, is equal to the negative of the total energy change of the system.

However, measuring or even calculating the absolute total of energy in a chemical system is extremely challenging. As a result, the energy change (also known as the enthalpy change and indicated by the letter ΔH) is measured instead. The following equation describes the relationship between the value of ΔH and the reaction’s bond energies.

ΔH = (energy used in the bond formation that yields products) – (energy released when the reactant bonds are broken)

As a result, it’s clear that the change in enthalpy for an exothermic reaction will always be negative., i.e. ΔH < 0.

什么是吸热反应？

吸热反应定义为吸收热量的反应。当氮气和氧气被加热到 3000 ⁰ C 左右时，它们结合产生一氧化氮，并且在这个过程中吸收了大量的热量。

N ₂ + O ₂ → 2NO
(氮气) (氧气) (一氧化氮)

由于在氮和氧之间的反应中消耗热量以生成一氧化氮，所以它是吸热反应。在方程式的反应物一侧，写“+热”和“+热能”或只写“+能量”来表示吸热反应。在汽车发动机内部，空气中的氮和氧发生反应，产生一氧化氮。为了发生，所有分解反应都需要热、光或电形式的能量。因此，所有分解反应都是吸热的。

吸热反应的例子

• 例如，当碳酸钙被加热时，它会分解成氧化钙和二氧化碳。

CaCO ₃ + 热→ CaO + CO ₂
(碳酸钙) (氧化钙) (二氧化碳)

• 由于碳酸钙的分解过程中吸收了热能，所以它是一个吸热反应。
• 光合作用的反应是吸热的。这是因为绿色植物在光合作用过程中吸收了阳光能量。
• 水电解产生氢气和氧气，这是一种吸热反应。这是因为在这个反应过程中，电能被吸收了。从这个讨论中可以明显看出，能量可以以热、光或电的形式在化学反应中释放或吸收。

The endothermic reaction can be written as follows in a chemical equation:

                               Reactants        +       Energy      →         Products 

Endothermic reactions are chemical processes in which the reactants absorb heat from the environment to produce products. These reactions cause a cooling effect by lowering the temperature of the surrounding environment. Ice cubes absorb heat energy from their surroundings and melt to generate liquid water, which is an endothermic process (no chemical bonds are broken or formed).

When a chemical bond is broken, energy is normally released in the process. The creation of chemical bonds, too, necessitates an energy input. The energy that is supplied or released can take many different forms (such as heat, light, and electricity). Endothermic reactions are characterized by the creation of chemical bonds as a result of heat absorption from the environment. Exothermic reactions, on the other hand, involve the release of heat energy generated by the bond breakdown.

吸热反应需要添加外部能量，通常以热的形式，以进行。由于吸热反应从周围环境吸收热量，因此它们倾向于冷却周围环境。

吸热反应产生的产物能量高于反应物，因此它们通常是非自发的。因此，对于吸热反应，焓的变化总是正的。

能量变化（也称为焓变，用字母 ΔH 表示）。由于在吸热反应中反应物转化为产物时向系统添加热量，因此焓的符号为正，即 ΔH > 0。

放热反应和吸热反应之间的区别

“Endo”和“Exo”这两个词分别表示“内”和“外”，有希腊语词根。正如它们的名字所暗示的那样，吸热反应和放热反应之间的主要区别在于前者吸收周围环境的热量，而后者则释放热量。

吸热反应——术语“吸热反应”是指系统从周围环境中以热量的形式吸收能量的过程。光合作用、蒸发液体、融化的冰、干冰、烷烃裂解、热分解、水中的氯化铵和许多其他吸热过程都是例子。

放热反应——与吸热反应相反的是放热反应。它以光或热的形式向周围环境释放能量。中和、燃烧化学物质、燃料反应、干冰沉积、呼吸作用、硫酸水溶液以及更多过程都是示例。

下表给出了吸热反应和放热反应之间的区别：

Exothermic Reaction	Endothermic Reaction
A reaction that releases heat as a type of energy from the system.	A reaction in which the system absorbs energy in the form of heat from its surroundings.
The system’s energy is released into the environment.	The energy from the surroundings is absorbed into the reaction.
Heat, electricity, light, and sound are released as a kind of energy.	Heat is released as a kind of energy.
Examples include rusting iron, settling, chemical bonds, explosions, and nuclear fission.	Examples include ice melting, evaporation, cooking, gas molecules, and photosynthesis.

示例问题

问题 1：是什么使呼吸成为放热反应？

回答：

During respiration, glucose interacts with oxygen in our body’s cells to produce carbon dioxide and water, as well as energy.

                     C₆H₁₂O₆    +       6O₂       →         6CO₂           +     6H₂O           +       Energy

                 (Glucose)             (Oxygen)          (Carbon dioxide)     (Water)

Since energy is produced throughout the process of respiration, as shown in the above reaction, so it is an exothermic process.

问题2：为什么中和反应的温度会升高？

回答：

When the temperature of a system rises due to heat transfer, it is called an exothermic process. Since neutralization processes are often exothermic, this heat is released into the environment, and hence the temperature of the neutralization reaction rises.

问题 3. 我们使用什么标准来确定反应是吸热的还是放热的？

回答：

A chemical reaction that involves the release of energy in the form of heat or light is known as an exothermic reaction. 

Example: When carbon reacts with oxygen, it produces carbon dioxide and heat energy.

C  +  O₂  →  CO₂  +  Heat

A chemical reaction that involves the absorption of energy in the form of heat or light is known as an endothermic reaction. 

Examples: Nitric oxide gas is generated when nitrogen and oxygen are heated to a specific temperature of around 3000⁰C.

N₂  +  O₂  +  Heat  →   2NO

It can be seen that in exothermic reactions, heat is released whereas, in endothermic reactions, heat is required; this helps to quickly determine the type of reaction.

问题4：吸热反应的焓变是正的还是负的。

回答：

Since in an endothermic reaction, the heat is added to the system as reactants convert to products, the sign of enthalpy is positive.

问题 5：说出任何自然发生的吸热反应过程。

回答：

The process of photosynthesis is an endothermic reaction. This is due to the fact that green plants absorb sunlight energy during the photosynthesis process.