化学反应

A physical change 物理变化 does not make a new substance. The substance only changes its state or shape, and the change can usually be reversed. Melting ice and dissolving sugar are physical changes.

A chemical change 化学变化 (a chemical reaction) makes one or more new substances and is usually hard to reverse. Signs of a chemical change include a colour change, a gas being given off, an energy change, or a precipitate 沉淀 (a solid) forming.

The rate of reaction 反应速率 tells you how fast the reactants 反应物 change into products 生成物.

Collision theory

Collision theory 碰撞理论 explains what controls the rate. For a reaction to happen, the particles 粒子 must collide 碰撞, and they must collide with enough energy. The least energy they need is the activation energy 活化能 ($E_a$). A faster rate happens when the particles have successful collisions more often.

What changes the rate

Catalysts

A catalyst speeds up a reaction but is not used up — it is unchanged at the end. It works by lowering the activation energy. Enzymes 酶 are biological catalysts (catalysts that work in living things).

Measuring the rate

You can follow a reaction over time in these ways:

• Change in mass: stand the flask on a balance. If a gas escapes, the mass falls. Record the mass at regular times.
• Volume of gas: collect the gas in a gas syringe 注射器 and read its volume at regular times.
• Formation of a precipitate: in a reaction that turns cloudy, time how long it takes for a mark under the flask to disappear.

On a graph of product against time, the line is steepest at the start (fastest rate), becomes less steep as reactants are used up, and goes flat when the reaction has finished.

Some reactions are reversible 可逆反应: the products can react to form the reactants again. We show this with the symbol $\rightleftharpoons$.

Changing the direction

A clear example uses hydrated 水合 and anhydrous 无水 compounds:

• Blue hydrated copper(II) sulfate, when heated, loses its water to become white anhydrous copper(II) sulfate. Adding water turns it blue again.
• Pink hydrated cobalt(II) chloride loses water when heated to become blue anhydrous cobalt(II) chloride. Adding water turns it pink again.

Adding water to the anhydrous solid (and seeing the colour return) is used as a test for water.

Equilibrium

In a closed system 密闭系统 (where no matter enters or leaves, although energy may be transferred), a reversible reaction reaches equilibrium 平衡 when:

• the rate of the forward reaction 正反应 equals the rate of the reverse reaction 逆反应, and
• the concentrations of the reactants and products are no longer changing.

Changing the position of equilibrium

The position of equilibrium 平衡位置 tells you whether there are more reactants or more products. You can move it:

• Temperature: heating moves the equilibrium in the direction that takes in heat (the endothermic 吸热反应 direction); cooling moves it in the direction that gives out heat (the exothermic 放热反应 direction).
• Pressure (gases): increasing pressure by compressing the gases at constant temperature favours the side with fewer gas molecules. If both sides have the same number, this does not shift the equilibrium.
• Concentration: adding more of a substance moves the equilibrium to the other side, to use it up.
• A catalyst does not move the position of equilibrium. It only helps the reaction reach equilibrium faster.

The Haber process

The Haber process 哈伯法 makes ammonia:

• The hydrogen 氢气 comes from methane 甲烷 (natural gas); the nitrogen 氮气 comes from the air.
• Typical conditions: a temperature of $450\,{}^{\circ}\text{C}$, a pressure of about $200$ atm ($20\,000$ kPa), and an iron 铁 catalyst.

The Contact process

The Contact process 接触法 turns sulfur dioxide into sulfur trioxide:

• The sulfur dioxide comes from burning sulfur 硫 (or roasting sulfide ores); the oxygen 氧气 comes from the air.
• Typical conditions: a temperature of $450\,{}^{\circ}\text{C}$, a pressure of about $2$ atm ($200$ kPa), and a vanadium(V) oxide 五氧化二钒 catalyst.

Why these conditions are chosen

The conditions are a compromise 折中:

• Higher pressure favours products in both equations. Haber uses high pressure as a compromise with cost and risk; Contact uses only low pressure because its conversion is already high.
• A lower temperature would give more product (both forward reactions are exothermic), but the reaction would be too slow, so a fairly high temperature is used to keep a good rate.
• The catalyst speeds up the reaction without changing the position of equilibrium, which lowers cost.

(Supplement) Worked example. Explain the effect of raising the pressure on the Haber process. Count the gas molecules on each side of the equation: on the left $1 + 3 = 4$, on the right $2$. A higher pressure moves the equilibrium to the side with fewer gas molecules, which is the right. So more ammonia is made. The same count for the Contact process gives $2 + 1 = 3$ on the left and $2$ on the right, so a higher pressure would also help there. But the yield at $2$ atm is already very high, so a higher pressure is not worth its cost.

Common mistakes.

• Saying a higher temperature gives more ammonia because "the reaction goes faster". A higher temperature makes it reach equilibrium faster, but the equilibrium mixture contains less ammonia, because the forward reaction is exothermic.
• Saying a catalyst gives a higher yield. It gives the same yield, only sooner.
• Counting atoms instead of gas molecules when you predict the effect of pressure.
• Saying the conditions are chosen "to give the highest yield". They are a compromise between yield, rate, cost and safety.

Oxidation 氧化 and reduction 还原 always happen at the same time, in what is called a redox reaction 氧化还原反应. ('Redox' is short for reduction–oxidation.)

There are three ways to describe oxidation and reduction:

• Oxygen: oxidation is the gain of oxygen; reduction is the loss of oxygen.
• Electrons 电子: oxidation is the loss of electrons; reduction is the gain of electrons.
• Oxidation number 氧化数: in oxidation the oxidation number goes up; in reduction it goes down.

A useful memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain — of electrons.

Oxidation number rules

The oxidation number is shown by a Roman numeral, as in iron(II) and iron(III). The rules are:

• An element that is not combined has an oxidation number of $0$.
• A single-atom ion has an oxidation number equal to its charge (so $\text{Na}^{+}$ is $+1$).
• The oxidation numbers in a compound add up to $0$.
• The oxidation numbers in an ion add up to the charge on the ion.

Worked example. Find the oxidation number of manganese in the manganate(VII) ion, $\text{MnO}_4^{-}$. Each oxygen is $-2$, and there are four of them, giving $4 \times (-2) = -8$. The oxidation numbers in an ion must add up to the charge on the ion, which here is $-1$. So if manganese is $x$, then $x + (-8) = -1$, giving $x = +7$ - exactly what the (VII) in the name tells you. Set the total to the ion's charge, not to zero: zero is only for a neutral compound.

Oxidising and reducing agents

• An oxidising agent 氧化剂 oxidises another substance, and is itself reduced.
• A reducing agent 还原剂 reduces another substance, and is itself oxidised.

Some redox reactions show clear colour changes:

• Acidified potassium manganate(VII) 高锰酸钾 is purple. When it acts as an oxidising agent it is reduced, and the purple colour fades to colourless.
• Potassium iodide 碘化钾 is colourless. When it is oxidised, red-brown iodine 碘 is formed.

• Explain a faster rate with collision theory: successful collisions happen more often. Concentration, pressure and surface area all raise how often particles collide; a higher temperature does that and gives more particles enough energy.
• A catalyst lowers the activation energy and speeds the reaction up, but is not used up and does not change $\Delta H$ or the position of equilibrium.
• At equilibrium the forward and reverse rates are equal and the concentrations stop changing — it does not mean the amounts of reactant and product are equal.
• Use Le Chatelier's principle: raising temperature favours the endothermic direction; compressing gases favours the side with fewer gas molecules, if the numbers differ; adding a reacting substance shifts the equilibrium to consume some of it.
• Redox — OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. They always happen together, and an oxidising agent is itself reduced.