OK，前面的单元全是对化学物质的定义和解释，从这一单元开始，才真正开始AP化学的学习。😁

第四单元

Introduction of reactions

• 化学反应是反应物转化为化学组成不同的产物的过程。
• 理解化学反应是化学的基础，可解释物质间相互作用、结合与变化。

Key Concepts​

• Reactants: Starting substances in a chemical reaction
• Products: Substances formed after the reaction
• Chemical Equation: Symbolic representation of a reaction
• Example: $\ce{2H2 + O2 -> 2H2O}$

总而言之，箭头左边的是反应物，箭头右边的是生成物。

Types of Reactions

• Synthesis (Combination): 多合一

  • Two or more substances combine to form a single product
  • $\ce{A + B -> AB}$

• Decomposition: 一分多

  • A single compound breaks down into simpler substances
  • $\ce{AB -> A + B}$

• Single Replacement (Displacement): 单置换反应。反应物和生成物都必须要是单质+化合物。

  • An element replaces another in a compound
  • $\ce{A + BC -> AC + B}$

• Double Replacement (Metathesis): 复分解反应。两个化合物双双互换粒子。

  • Exchange of ions between two compounds
  • $\ce{AB + CD -> AD + CB}$

• Combustion: 燃烧反应

  • A substance reacts with oxygen, producing energy
  • $\ce{ \text{Hydrocarbon} + O2 -> CO2 + H2O}$

Law of Conservation of Mass​

• Matter is neither created nor destroyed in a chemical reaction
• Total mass of reactants = total mass of products
• 反应物质量 = 生成物质量

Net Ionic Equations

• 净离子方程式只显示实际参与化学反应的粒子。
• 省略旁观离子（反应前后不变的离子）。

Key Concepts​

• Spectator ions (没有参与实际反应的离子): Ions present in solution that do not take part in the reaction
• Molecular equation (展示所有粒子): Shows all reactants and products as compounds
• Ionic equation (展示所有可以分开的强电解质离子): Shows all strong electrolytes dissociated into ions
• Net ionic equation (只展示参与反应的离子): Eliminates spectator ions to focus on the chemical change

Steps to write net ionic equations

1. 写出配平的分子方程式
2. 将所有强电解质拆分为离子形式
3. 找出并消去旁观离子
4. 写出最终的净离子方程式

Example

• Molecular equation:
  $$\ce{NaCl(aq) + AgNO3(aq) -> AgCl(s) + NaNO3(aq)}$$
• Ionic equation:
  $$\ce{Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) -> AgCl(s) + Na+(aq) + NO3-(aq)}$$
• Net ionic equation:
  $$\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}$$

Representations of Reactions

其实这一部分没什么要讲的，就是让你知道配平的分子式和离子式还有方程式都能表示化学反应。

Physical and Chemical Changes

在这一部分我们主要是定义一下物理变化和化学变化的实质。

Physical Changes

指物质的状态发生变化，一般都是可逆的。

• Change in state or appearance without altering chemical composition
• Reversible in many cases
• Examples: Melting, boiling, freezing, dissolving, breaking, cutting

Chemical Changes

指产生了新物质具有不同化学性质，变化不可逆。

• Change that produces a new substance with different chemical properties
• Often irreversible under normal conditions
• Indicators: Color change, gas formation, precipitate formation, energy release/absorption
• Examples: Rusting of iron, combustion, baking a cake, digestion

Distinguishing Between Changes​

Stoichiometry

我们可以基于已经平衡了的化学式，来计算出每个反应物和生成物的物质的量（$n$）。

Key concepts

• Mole Ratios: Derived from the coefficients in a balanced equation
• Limiting Reactant: Reactant that determines the maximum amount of product
• Excess Reactant: Reactant left over after the reaction is complete
• Theoretical Yield: Maximum amount of product predicted by stoichiometry
• Actual Yield: Amount of product actually obtained
• Percent Yield: $\frac{\text{Actual Yield} }{ \text{Theoretical Yield} } \times 100\%$

Steps to Solve Stoichiometry Problems

1. 书写并配平化学方程式
2. 将已知量换算为物质的量（摩尔）
3. 利用摩尔比计算目标物质的量
4. 把摩尔数换算回克、升或分子数等单位
5. 核对计算过程与单位一致性

Example

Given:
$$\ce{2H2 + O2 -> 2H2O}$$

• If 4 moles of $\ce{H2}$ react with 2 moles of $\ce{O2}$:
• $\ce{H2}$: $$\ce{4 \text{moles} \times \frac{ 2 \text{moles} H2O }{ 2 \text{moles} H2 } = 4 \text{moles} H2O}$$
• $\ce{O2}$: $$\ce{2 \text{moles} \times \frac{ 2 \text{moles} H2O }{ 1 \text{mole} O2 } = 4 \text{moles} H2O}$$
• Both reactants are perfectly stoichiometric; 4 moles of water form.

Introduction to Titration

这个方法酸碱中和反应中特别爱考。

滴定是用已知浓度溶液与待测溶液反应测定待测溶液的浓度。

但这一部分主要介绍滴定实验如何做，和在什么反应下用滴定可以很好的解决问题。具体可能会在之后的酸碱单元进行深究。

Key Concepts​

• Analyte: 浓度未知。
  The solution whose concentration is unknown.
• Titrant: 浓度已知，需要其与待测物反应。
  The solution of known concentration used to react with the analyte.
• Equivalence Point: 滴定剂恰好完全中和待测物。
  Point where the amount of titrant exactly neutralizes the analyte.
• End Point: 指示剂变色指示。
  Point indicated by a color change of an indicator, close to the equivalence point.

Types of Titrations​

不咋考。

• Acid-Base Titration

  • Determines concentration of an acid or base
  • Common indicators: phenolphthalein, bromothymol blue

• Redox Titration

  • Involves electron transfer reactions
  • Example: Titration of $\ce{Fe^{2+}}$ with $\ce{KMnO4}$

• Complexometric Titration

  • Forms complexes with metal ions
  • Example: EDTA titrations for hardness of water

• Precipitation Titration

  • Formation of a precipitate signals completion
  • Example: $\ce{AgNO3}$ titration for $\ce{Cl-}$ ions

Types of Chemical Reactions

这一部分又开始放水了，主要是和前面讲的化学反应的类型基本一样，就只是再多引入了一个氧化还原反应而已。

Redox Reactions

• Involves transfer of electrons (oxidation-reduction)
• Example: $$\ce{2Na + Cl2 -> 2NaCl}$$

Introduction to Acid-Base Reactions

这一部分也只是主要提前预告一下酸碱反应，第八单元才会重点学习。

Acid-base reactions involve the transfer of protons ($\ce{H+}$ ions) between reactants. These reactions are central in chemistry, biology, and environmental processes.
酸碱反应的本质是反应物之间 $\ce{H+}$ 的转移。

Key concepts

• 酸：作为反应物可以电离出氢离子和共轭碱（供给氢离子）
• 碱：在反应物中接受氢离子并生成共轭酸（消耗氢离子）
• pH：衡量氢离子浓度，计算公式为$\ce{pH = -log[H+]}$
• pOH：衡量氢氧根离子浓度，计算公式为$\ce{pOH = -log[OH-]}$

Types of Acid-Base Reactions​

• Strong Acid + Strong Base: Complete dissociation in water

  • Example: $$\ce{HCl + NaOH -> NaCl + H2O}$$

• Weak Acid + Strong Base: Partial dissociation of weak acid

  • Example: $$\ce{CH3COOH + NaOH -> CH3COONa + H2O}$$

• Strong Acid + Weak Base:

  • Example: $$\ce{HCl + NH3 -> NH4Cl}$$
• Neutralization Reactions: Formation of water and a salt from an acid and a base

Indicators​

常见的酸碱指示剂。

• Substances that change color depending on pH
• Examples: Litmus paper, phenolphthalein, bromothymol blue

Oxidation-Reduction (Redox) Reactions

分子中有原子的化合价在经历了某种反应后会升高/降低，我们把这类反应叫做氧化还原反应。

Key Concepts​

[!NOTE]

发生氧化反应时，反应物为还原剂；发生还原反应时，反应物为氧化剂

• Oxidation: Loss of electrons (increase in oxidation state)
• Reduction: Gain of electrons (decrease in oxidation state)
• Oxidizing agent: Species that is reduced, causes oxidation
• Reducing agent: Species that is oxidized, causes reduction

Identifying Redox Reactions​

1. Assign oxidation numbers to all elements in the reaction.
2. Look for changes in oxidation numbers.
3. Determine which species is oxidized and which is reduced.

[!NOTE]

如何判断其反应类型和某原子得失状况的口诀

升失氧，降得还。

化合价升高，还原剂失去电子，发生氧化反应；后一句反之亦然。

Example​

可以用上面那个口诀进行试验。

$$\ce{Zn(s) + Cu^{2+}(aq) -> Zn^{2+}(aq) + Cu(s)}$$

• $\ce{Zn}$: $\ce{0 -> +2}$ (oxidation)
• $\ce{Cu}$: $\ce{+2 -> 0}$ (reduction)
• $\ce{Zn}$ is the reducing agent; $\ce{Cu^{2+}}$ is the oxidizing agent

Types of Redox Reactions​

在一般情况下，我们认为只要化学反应后出现了化合价升高/降低，无论什么其他的反应类型。都叫其氧化还原反应。

• Combination reactions: Two elements combine, electrons are transferred
• Decomposition reactions: Single compound breaks down, electron transfer occurs
• Displacement reactions: One element replaces another in a compound
• Combustion reactions: Rapid oxidation producing energy