Introduction to Enthalpy of Reaction · 反应焓引论
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| enthalpy of reaction/enˈθælpi ɒv rɪˈækʃn/ | 反应焓 | fǎn yìng hán |
Putting a number on the heat
- Every reaction gives off or takes in a definite amount of heat.
- We can label it with an exact value, in kilojoules.
- The sign tells you the direction; the size tells you how much.
- That single number is one of chemistry's most useful.
给热量标个数
- 每个反应都放出或吸收一定量的热。
- 我们能用一个精确的数值(单位千焦)给它标记。
- 符号告诉你方向;大小告诉你有多少。
- 那个数是化学中最有用的之一。
The enthalpy of reaction
- The enthalpy of reaction 反应焓 ($\Delta H_{rxn}$) is the heat at constant pressure.
- Negative means exothermic; positive means endothermic.
- It is usually quoted per mole of reaction.
反应焓
- 反应焓($\Delta H_{rxn}$)是恒压下的热量。
- 为负表示放热;为正表示吸热。
- 它通常按每摩尔反应给出。
A reaction with $\Delta H = -200\ \text{kJ}$ is... · 具有$\Delta H = -200\ \text{kJ}$的反应是...
A negative $\Delta H$ releases heat -- exothermic. · 负的$\Delta H$释放热量——这是放热反应。
The enthalpy of reaction is the heat measured at constant ____. · 反应焓是在恒____下测量的热量。
$\Delta H$ is the heat at constant pressure. · $\Delta H$是恒压下的热量。
A positive enthalpy of reaction means the reaction is endothermic. · 正的反应焓意味着该反应是吸热的。
Positive $\Delta H$ absorbs heat -- endothermic. · 正的$\Delta H$吸收热量——这是吸热反应。
Thermochemical equations
- A thermochemical equation writes $\Delta H$ next to the reaction.
- Double the amounts and $\Delta H$ doubles too.
- Reverse the reaction and $\Delta H$ flips its sign.
热化学方程
- 热化学方程在反应旁边写上 $\Delta H$。
- 量加倍,$\Delta H$ 也加倍。
- 把反应逆转,$\Delta H$ 就变号。
Reversing a reaction changes its $\Delta H$ by... · 逆转反应会使其$\Delta H$发生...
The reverse reaction has the opposite $\Delta H$. · 逆反应具有相反的$\Delta H$。
Scaling with amount
- $\Delta H$ scales with how much reacts.
- Half the moles gives half the heat.
- Always match the $\Delta H$ to the equation's coefficients.
随量缩放
- $\Delta H$ 随反应量缩放。
- 摩尔数减半,热量减半。
- 始终让 $\Delta H$ 与方程的系数相匹配。
Enthalpy scales with amount · 焓随物质的量缩放
Enthalpy of reaction is energy per mole, so the total heat is proportional to how much reacts. · 反应焓是每摩尔的能量，因此总热量与反应物的量成正比。
If burning 1 mol releases $500\ \text{kJ}$, how much does burning 3 mol release (in kJ)? · 如果燃烧1 mol释放$500\ \text{kJ}$，那么燃烧3 mol释放多少（单位：kJ）？
$\Delta H$ scales with amount, so $3 \times 500 = 1500\ \text{kJ}$. · $\Delta H$随物质的量缩放，因此是$3 \times 500 = 1500\ \text{kJ}$。
If a reaction of 2 mol releases $600\ \text{kJ}$, how much does 1 mol release (in kJ)? · 如果2 mol的反应释放$600\ \text{kJ}$，那么1 mol释放多少（单位：kJ）？
Half the moles gives half the heat, so $600/2 = 300\ \text{kJ}$. · 物质的量减半，热量也减半，因此是$600/2 = 300\ \text{kJ}$。
$\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}$, with $\Delta H = -890\ \text{kJ}$.
- Burning 1 mol of methane releases $890\ \text{kJ}$.
- Burning 2 mol releases $2 \times 890 = 1780\ \text{kJ}$.
$\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}$,$\Delta H = -890\ \text{kJ}$。
- 燃烧 1 mol 甲烷放出 $890\ \text{kJ}$。
- 燃烧 2 mol 放出 $2 \times 890 = 1780\ \text{kJ}$。
$\Delta H$ scales with amount -- double the moles, double the heat. Reversing a reaction flips the sign of $\Delta H$. And a negative $\Delta H$ means heat is released (exothermic); a common slip is to read the sign backwards.
$\Delta H$ 随量缩放——摩尔数加倍,热量加倍。逆转反应会把 $\Delta H$ 变号。而且负的 $\Delta H$ 意味着热被放出(放热);把符号读反是常见的失误。
The enthalpy of reaction $\Delta H_{rxn}$ is the heat at constant pressure -- negative for exothermic, positive for endothermic, quoted per mole. In a thermochemical equation it scales with the amounts and flips sign when the reaction is reversed.
反应焓 $\Delta H_{rxn}$ 是恒压下的热量——放热为负、吸热为正,按每摩尔给出。在热化学方程里它随量缩放,反应逆转时变号。