Enthalpy of Formation · 生成焓
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| enthalpy of formation/enˈθælpi ɒv fɔːˈmeɪʃn/ | 生成焓 | shēng chéng hán |
Building compounds from scratch
- Imagine making a compound straight from its raw elements.
- The heat of that assembly is tabulated for thousands of substances.
- With those values, you can find any reaction's heat.
- No lab needed -- just a table and some subtraction.
从零开始搭建化合物
- 想象直接用原始元素制造一个化合物。
- 那次组装的热量已为成千上万种物质列成表。
- 有了那些数值,你能求出任何反应的热量。
- 无需实验室——只要一张表和一些减法。
The heat of formation
- The enthalpy of formation 生成焓 is the heat to make 1 mol from its elements.
- It is measured with those elements in their standard states.
- A pure element in its standard state has a formation enthalpy of zero.
生成热
- 生成焓是从元素制造 1 mol 该物质所需的热量。
- 它是在那些元素处于标准态时测得的。
- 处于标准态的纯元素,其生成焓为零。
The enthalpy of formation is for making exactly 1 mole of a substance from its elements. · 生成焓是指由元素恰好生成 1 摩尔物质时的焓变。
It is defined per mole of the compound formed. · 它是按生成的化合物的每摩尔定义的。
Products minus reactants
- $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$.
- Add up the products' formation enthalpies, then subtract the reactants'.
- Multiply each one by its coefficient first.
产物减反应物
- $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$。
- 把产物的生成焓相加,再减去反应物的。
- 先把每一个乘以它的系数。
The reaction enthalpy from formation data is... · 利用生成数据计算的反应焓变为...
$\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$.
You must multiply each formation enthalpy by its coefficient before summing. · 在求和之前，必须将每个生成焓乘以其系数。
Coefficients scale the amount of each substance. · 系数按比例缩放每种物质的量。
Why elements count as zero
- An element in its standard form is the starting point, so no heat is needed to make it.
- $\text{O}_2$, $\text{N}_2$, and solid carbon all have $\Delta H_f = 0$.
- This simplifies the arithmetic.
为什么元素算作零
- 处于标准态的元素是起点,所以制造它不需要热量。
- $\text{O}_2$、$\text{N}_2$ 和固体碳的 $\Delta H_f = 0$。
- 这简化了算术。
Enthalpy of formation · 标准生成焓
ΔHf is the energy change when one mole of a compound forms from its elements. · ΔHf 是指由元素生成一摩尔化合物时的能量变化。
What is the standard enthalpy of formation of $\text{O}_2$ gas? · $\text{O}_2$ 气体的标准生成焓是多少？
An element in its standard state has $\Delta H_f = 0$. · 处于标准态的元素具有 $\Delta H_f = 0$。
An element in its standard state has a formation enthalpy of . · 处于标准态的元素的生成焓为。
No formation step is needed, so it is zero. · 不需要生成步骤，因此为零。
Products' total $\Delta H_f = -400$; reactants' total $= -250$.
- $\Delta H_{rxn} = -400 - (-250) = -150\ \text{kJ}$.
- The reaction is exothermic.
产物总 $\Delta H_f = -400$;反应物总 $= -250$。
- $\Delta H_{rxn} = -400 - (-250) = -150\ \text{kJ}$。
- 反应是放热的。
Products' $\sum \Delta H_f = -300$; reactants' $= -100$. Find $\Delta H_{rxn}$ (in kJ). · 产物的 $\sum \Delta H_f = -300$；反应物的 $= -100$。求 $\Delta H_{rxn}$（单位：kJ）。
$\Delta H_{rxn} = -300 - (-100) = -200\ \text{kJ}$.
The formula is products minus reactants -- watch the double negatives when $\Delta H_f$ values are negative. Elements in their standard state contribute zero, so do not look them up. And multiply each formation enthalpy by the equation's coefficient before summing.
公式是产物减反应物——当 $\Delta H_f$ 值为负时,当心双重负号。处于标准态的元素贡献为零,所以不用去查它们。而且在求和之前,先把每个生成焓乘以方程的系数。
The enthalpy of formation is the heat to make 1 mol of a substance from its elements, and it is zero for an element in its standard state. Any reaction's heat is $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$, each scaled by its coefficient.
生成焓是从元素制造 1 mol 物质所需的热量,处于标准态的元素其值为零。任何反应的热量是 $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$,各自按系数缩放。