📚 Energy from Chemical Reactions | 化学反应中的能量
Chemical reactions do more than make new substances. They also transfer energy between the reacting chemicals and their surroundings. This energy may be released as heat, taken in from the surroundings, or converted into light and electrical energy.
化学反应不仅会生成新物质，还会在反应物与周围环境之间传递能量。这种能量可能以热的形式释放，也可能从周围环境中吸收，或者转化为光能和电能。
1. What Is Chemical Energy? | 什么是化学能？
Chemical energy is stored in the bonds between atoms. When a chemical reaction happens, old bonds break and new bonds form, so energy is transferred between the reacting chemicals and their surroundings.
化学能储存在原子之间的化学键中。当化学反应发生时，旧的化学键断裂并形成新的化学键，因此能量会在反应物与周围环境之间传递。
We usually observe this energy transfer as a temperature change. If the surroundings get warmer, the reaction is giving out heat. If the surroundings get colder, the reaction is taking in heat.
我们通常通过温度变化观察这种能量传递。如果周围环境变热，说明反应在放出热量；如果周围环境变冷，说明反应在吸收热量。
In a chemical reaction, the total amount of energy is always conserved. Energy cannot be created or destroyed, but it can change from chemical energy to heat energy, light energy or electrical energy.
在化学反应中，总能量始终守恒。能量不能被创造或消灭，但可以从化学能转变为热能、光能或电能。
2. Exothermic Reactions | 放热反应
Exothermic reactions transfer energy to the surroundings. The temperature of the surroundings increases, and this can be measured with a thermometer.
放热反应向周围环境释放能量。周围环境的温度会升高，这可以用温度计测量。
Common examples of exothermic reactions include combustion of fuels, neutralisation of acids and alkalis, and respiration in living cells. Rusting of iron is also a slow exothermic reaction.
常见的放热反应包括燃料的燃烧、酸与碱的中和反应以及活细胞中的呼吸作用。铁生锈也是一种缓慢的放热反应。
In an exothermic reaction, more energy is released when new bonds form than is absorbed when old bonds break. The extra energy is transferred to the surroundings as heat.
在放热反应中，形成新化学键时释放的能量大于断裂旧化学键时吸收的能量。多余的能量会以热的形式传递给周围环境。
3. Endothermic Reactions | 吸热反应
Endothermic reactions absorb energy from the surroundings. The temperature of the surroundings decreases, so the reaction vessel feels cold.
吸热反应从周围环境吸收能量。周围环境的温度会下降，因此反应容器摸起来是冷的。
Common examples include thermal decomposition of carbonates, photosynthesis in green plants, and the reaction between citric acid and sodium hydrogen carbonate. Dissolving ammonium nitrate in water is also endothermic.
常见的例子包括碳酸盐的热分解、绿色植物的光合作用以及柠檬酸与碳酸氢钠的反应。硝酸铵溶于水也是吸热过程。
In an endothermic reaction, more energy is absorbed to break bonds than is released when new bonds form. The reaction takes the missing energy from its surroundings, making the temperature fall.
在吸热反应中，断裂化学键吸收的能量大于形成新化学键释放的能量。反应会从周围环境吸收缺少的能量，导致温度下降。
4. Measuring Temperature Changes | 测量温度变化
To measure the energy change of a reaction, we record the starting temperature and the final temperature. The temperature change is calculated using the equation below.
为了测量反应的能量变化，我们记录起始温度和最终温度。温度变化可以用下面的公式计算。
ΔT = final temperature − initial temperature
ΔT = 最终温度 − 起始温度
A larger temperature rise means more heat energy was released to the surroundings. A larger temperature fall means more heat energy was absorbed from the surroundings.
温度上升越大，说明释放到周围环境的热能越多。温度下降越大，说明从周围环境吸收的热能越多。
In a school experiment we often use a polystyrene cup with a lid to reduce heat loss to the air. We stir the mixture and read the thermometer every 30 seconds.
在学校实验中，我们通常使用带盖的聚苯乙烯杯减少向空气的热量散失。我们搅拌混合物，并每 30 秒读取一次温度计。
- Control variables: volume of solution and starting temperature
- 控制变量：溶液体积和起始温度
- Independent variable: type of reaction or concentration
- 自变量：反应类型或浓度
- Dependent variable: temperature change
- 因变量：温度变化
5. Combustion of Fuels | 燃料的燃烧
Fuels such as coal, oil, natural gas and wood store chemical energy. When they burn, they react with oxygen in an exothermic combustion reaction.
煤、石油、天然气和木材等燃料储存着化学能。它们燃烧时与氧气发生放热的燃烧反应。
Complete combustion of methane can be shown by the word equation and balanced symbol equation below.
甲烷完全燃烧的化学方程式如下所示。
methane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion produces carbon dioxide and water. Incomplete combustion can produce carbon monoxide and soot, which are harmful to health and the environment.
完全燃烧生成二氧化碳和水。不完全燃烧会产生一氧化碳和碳烟，这些物质对健康和环境有害。
Burning fuels is useful for heating homes, cooking food and powering vehicles, but it releases greenhouse gases such as carbon dioxide.
燃烧燃料可用于家庭取暖、烹饪食物和驱动车辆，但会释放二氧化碳等温室气体。
6. Neutralisation Reactions | 中和反应
When an acid reacts with an alkali or a base, a neutralisation reaction takes place. This reaction is exothermic, so the temperature of the mixture rises.
当酸与碱或碱性物质反应时，会发生中和反应。该反应是放热的，因此混合物的温度会升高。
The reaction between hydrochloric acid and sodium hydroxide is a common example.
盐酸与氢氧化钠的反应是一个常见例子。
hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl + NaOH → NaCl + H₂O
The heat released during neutralisation can be used in self-heating food cans and heating packs. The reaction is safe if the chemicals are handled correctly.
中和反应释放的热量可用于自热食品罐和加热包。只要正确处理化学药品，该反应是安全的。
7. Thermal Decomposition | 热分解反应
Thermal decomposition means breaking down a compound using heat. These reactions are endothermic: they must keep taking in energy to continue.
热分解是指利用热量使化合物分解。这些反应是吸热的：它们必须持续吸收能量才能继续进行。
Heating calcium carbonate is a classic example. It forms calcium oxide and carbon dioxide gas.
加热碳酸钙是一个典型例子。它会生成氧化钙和二氧化碳气体。
calcium carbonate → calcium oxide + carbon dioxide
CaCO₃ → CaO + CO₂
Heating copper carbonate produces copper oxide and carbon dioxide. The colour changes from green to black, and the gas can be tested with limewater.
加热碳酸铜会生成氧化铜和二氧化碳。颜色从绿色变为黑色，产生的气体可以用石灰水检验。
8. Photosynthesis and Respiration | 光合作用与呼吸作用
Photosynthesis is an endothermic reaction. Plants absorb light energy to convert carbon dioxide and water into glucose and oxygen.
光合作用是一个吸热反应。植物吸收光能，将二氧化碳和水转化为葡萄糖和氧气。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Respiration is an exothermic reaction. Living cells break down glucose with oxygen to release energy for movement, growth and keeping warm.
呼吸作用是一个放热反应。活细胞利用氧气分解葡萄糖，释放能量用于运动、生长和维持体温。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
葡萄糖 + 氧气 → 二氧化碳 + 水 + 能量
These two reactions are a good reminder that living things depend on energy transfers from chemical reactions every second.
这两个反应很好地提醒我们，生物每时每刻都依赖化学反应中的能量传递。
9. Everyday Uses of Energy Changes | 能量变化的日常应用
Exothermic reactions are used in hand warmers. Iron powder reacts slowly with oxygen in the air and releases heat to keep hands warm.
放热反应用于暖手宝。铁粉与空气中的氧气缓慢反应并释放热量，使手保持温暖。
Endothermic reactions are used in instant cold packs. When ammonium nitrate dissolves in water, it absorbs heat from the surroundings, so the pack feels cold.
吸热反应用于速冷冰袋。硝酸铵溶于水时会从周围环境吸收热量，因此冰袋感觉很冷。
Sherbet sweets contain citric acid and sodium hydrogen carbonate. When they dissolve on the tongue, an endothermic reaction produces the familiar cooling fizz.
雪酪糖含有柠檬酸和碳酸氢钠。它们在舌头上溶解时发生吸热反应，产生熟悉的清凉气泡感。
Self-heating drinks cans use the exothermic reaction between calcium oxide and water. This reaction releases enough heat to warm the drink safely.
自热饮料罐利用氧化钙与水之间的放热反应。该反应释放的热量足以安全地加热饮料。
10. Investigating Energy Changes in the Laboratory | 实验室中探究能量变化
To compare the energy released by different fuels, measure a fixed mass of water, record its initial temperature, burn the fuel under the water, and measure the highest temperature reached.
要比较不同燃料释放的能量，可以量取固定质量的水，记录其初始温度，在下方燃烧燃料，并测量达到的最高温度。
Variables: the independent variable is the type of fuel; the dependent variable is the temperature change of the water; control variables include mass of water, distance from flame, and burning time.
变量：自变量是燃料类型；因变量是水的温度变化；控制变量包括水的质量、与火焰的距离以及燃烧时间。
For neutralisation, add an alkali to an acid in a polystyrene cup and record the temperature rise. Repeat with different concentrations to see the effect on energy released.
对于中和反应，在聚苯乙烯杯中将碱加入酸中，记录温度上升。用不同浓度重复实验以观察对释放能量的影响。
Always wear eye protection and follow safety rules when using fuels, acids and alkalis.
使用燃料、酸和碱时，务必佩戴护目镜并遵守安全规则。
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