A-level化学 | Several types of enthalpy changes, 你都掌握了吗？

在chapter 5 chemical energetics 中提到了多种不同enthalpy change, 很容易会混淆。所以下面把enthalpy change其中的基础知识简单的整理一下，希望可以帮助大家在做题的时候加深理解。

首先我们要先简单介绍一下什么是enthalpy change（反应焓变）：refer to the amount of heat evolved or absorbed in a reaction carried out at constant pressure. 简单来说在不变的气压下，释放或吸收的热量叫做反应焓变。

The standard enthalpy change (标准焓变)ΔH°: refer to reactions done under standard conditions (标准条件), and with everything present in their standard states.单位Unit：KJ/mol

Standard conditions are:

298 K(25°C)

Apressure of 1 bar (100 kPa).

Where solutions are involved, a concentration of 1 mol dm-3

Types of enthalpy changes:

Enthalpy change of solution ΔH°sol溶解热

The enthalpy change when 1 mole of asubstancedissolves in waterto give a solution of infinite dilution. 一摩尔的溶质溶解在水时所发出或吸收的热量

Enthalpy change of hydration ΔH°hyd水合焓

The heat energyreleased when new bondsare made between the ions and water molecules is known as the hydration enthalpy of the ion (always -ve) 电离子和水分子的新合成化学键时所释放出的热量

两个因素可以影响到hydration enthalpy: chargeand sizeof ions离子的电荷和大小. (the attractions are stronger the smaller and more highly charged the ion)

这两个焓变很容易分不清，所以大家在这里要注意一下啦！

Enthalpy change of combustion ΔH°c 燃烧焓变

Itoccurs when one mole of the compound isburnedcompletely in oxygenunder standard conditions. 一摩尔物质在标准状况下完全燃烧时的反应焓变

Enthalpy change of formation ΔH°f 生成焓

It occurs when one mole of the compound is formedfrom its elementsunder standard conditions由元素最稳定的单质生成生成一摩尔纯化合物时的反应焓变

Enthalpy change of atomisation ΔH°at雾化焓

Theenergy required to produce gaseous atoms from an element or compound under standard conditions. 化合物的键断裂并且组分原子被还原为单个原子时的焓变量

ΔH°at = ΔH°m (enthalpy change of fusion) + ΔH°vap (enthalpy change of vaporisation)

Enthalpy change of neutralization ΔH°neut中和焓

It occurs when solutions of an acidand an alkali react together under standard conditions to produce 1 mole of water. 酸碱反应生成一摩尔水时所释放的热量

我们可以用calorimetry (量热法)来测量焓变，经常出现的题型涉及到中和焓，燃烧焓和溶解焓。不要忘记运用它的公式哟~