Chemical Reactions

4.1

Recognizing a Chemical Reaction

Syllabus

Learning Objective	Essential Knowledge
4.1.A Identify evidence of chemical and physical changes in matter.	4.1.A.1 A physical change occurs when a substance undergoes a change in properties but not a change in composition. Changes in the phase of a substance (solid, liquid, gas) or formation/separation of mixtures of substances are common physical changes. 4.1.A.2 A chemical change occurs when substances are transformed into new substances, typically with different compositions. Production of heat or light, formation of a gas, formation of a precipitate, and/or color change provide possible evidence that a chemical change has occurred.

Learning Objective

Essential Knowledge

4.1.A
Identify evidence of chemical and physical changes in matter.

4.1.A.1 A physical change occurs when a substance undergoes a change in properties but not a change in composition. Changes in the phase of a substance (solid, liquid, gas) or formation/separation of mixtures of substances are common physical changes.
4.1.A.2 A chemical change occurs when substances are transformed into new substances, typically with different compositions. Production of heat or light, formation of a gas, formation of a precipitate, and/or color change provide possible evidence that a chemical change has occurred.

Source: College Board AP Course and Exam Description

A chemical reaction 化学反应 rearranges atoms into new substances. Signs one has happened: a color change, a gas or precipitate 沉淀 forming, or a temperature change. Atoms are conserved, so an equation must be balanced – the same count of each element on both sides.

Vocabulary Train

English	Chinese	Pinyin
chemical reaction	化学反应	huà xué fǎn yìng
precipitate	沉淀	chén diàn

Exercise sheet

4.2

Net Ionic Equations

Syllabus

Learning Objective	Essential Knowledge
4.2.A Represent changes in matter with a balanced chemical or net ionic equation: i. For physical changes. ii. For given information about the identity of the reactants and/or product. iii. For ions in a given chemical reaction.	4.2.A.1 All physical and chemical processes can be represented symbolically by balanced equations. 4.2.A.2 Chemical equations represent chemical changes. These changes are the result of a rearrangement of atoms into new combinations; thus, any representation of a chemical change must contain equal numbers of atoms of every element before and after the change occurred. Equations thus demonstrate that mass and charge are conserved in chemical reactions. 4.2.A.3 Balanced molecular, complete ionic, and net ionic equations are differing symbolic forms used to represent a chemical reaction. The form used to represent the reaction depends on the context in which it is to be used.

Learning Objective

Essential Knowledge

4.2.A
Represent changes in matter with a balanced chemical or net ionic equation:
i. For physical changes.
ii. For given information about the identity of the reactants and/or product.
iii. For ions in a given chemical reaction.

4.2.A.1 All physical and chemical processes can be represented symbolically by balanced equations.
4.2.A.2 Chemical equations represent chemical changes. These changes are the result of a rearrangement of atoms into new combinations; thus, any representation of a chemical change must contain equal numbers of atoms of every element before and after the change occurred. Equations thus demonstrate that mass and charge are conserved in chemical reactions.
4.2.A.3 Balanced molecular, complete ionic, and net ionic equations are differing symbolic forms used to represent a chemical reaction. The form used to represent the reaction depends on the context in which it is to be used.

Source: College Board AP Course and Exam Description

For reactions in water, ionic compounds split into ions. A net ionic equation 净离子方程式 shows only the species that actually change, leaving out the spectator ions 旁观离子 that appear unchanged on both sides. It captures the real chemistry (e.g. $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}(s)$).

Mixing two solutions can form an insoluble precipitate

Vocabulary Train

English	Chinese	Pinyin
net ionic equation	净离子方程式	jìng lí zi fāng chéng shì
spectator ions	旁观离子	páng guān lí zi

Exercise sheet

4.3

Three Ways to Represent a Reaction

Syllabus

Learning Objective	Essential Knowledge
4.3.A Represent a given chemical reaction or physical process with a consistent particulate model.	4.3.A.1 Balanced chemical equations in their various forms can be translated into symbolic particulate representations.

Source: College Board AP Course and Exam Description

The same reaction can be shown as a symbolic equation, a particulate drawing (atoms and molecules), and a macroscopic observation (what you see). Moving between these levels – connecting the equation to the particles to the beaker – is a core skill.

A balanced equation has the same number of each atom on both sides

Exercise sheet

4.4

Physical Changes versus Chemical Changes

Syllabus

Learning Objective	Essential Knowledge
4.4.A Explain the relationship between macroscopic characteristics and bond interactions for: i. Chemical processes. ii. Physical processes.	4.4.A.1 Processes that involve the breaking and/or formation of chemical bonds are typically classified as chemical processes. Processes that involve only changes in intermolecular interactions, such as phase changes, are typically classified as physical processes. 4.4.A.2 Sometimes physical processes involve the breaking of chemical bonds. For example, plausible arguments could be made for the dissolution of a salt in water, as either a physical or chemical process, involves breaking of ionic bonds, and the formation of ion-dipole interactions between ions and solvent.

Learning Objective

Essential Knowledge

4.4.A
Explain the relationship between macroscopic characteristics and bond interactions for:
i. Chemical processes.
ii. Physical processes.

4.4.A.1 Processes that involve the breaking and/or formation of chemical bonds are typically classified as chemical processes. Processes that involve only changes in intermolecular interactions, such as phase changes, are typically classified as physical processes.
4.4.A.2 Sometimes physical processes involve the breaking of chemical bonds. For example, plausible arguments could be made for the dissolution of a salt in water, as either a physical or chemical process, involves breaking of ionic bonds, and the formation of ion-dipole interactions between ions and solvent.

Source: College Board AP Course and Exam Description

A physical change 物理变化 alters form but not identity (melting, dissolving); a chemical change 化学变化 makes new substances by breaking and forming bonds. Dissolving salt is physical; the salt is unchanged and recoverable.

Vocabulary Train

English	Chinese	Pinyin
physical change	物理变化	wù lǐ biàn huà
chemical change	化学变化	huà xué biàn huà

Exercise sheet

4.5

Stoichiometry

Syllabus

Learning Objective	Essential Knowledge
4.5.A Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.	4.5.A.1 Because atoms must be conserved during a chemical process, it is possible to calculate product amounts by using known reactant amounts, or to calculate reactant amounts given known product amounts. 4.5.A.2 Coefficients of balanced chemical equations contain information regarding the proportionality of the amounts of substances involved in the reaction. These values can be used in chemical calculations involving the mole concept. 4.5.A.3 Stoichiometric calculations can be combined with the ideal gas law and calculations involving molarity to quantitatively study gases and solutions.

Learning Objective

Essential Knowledge

4.5.A
Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.

4.5.A.1 Because atoms must be conserved during a chemical process, it is possible to calculate product amounts by using known reactant amounts, or to calculate reactant amounts given known product amounts.
4.5.A.2 Coefficients of balanced chemical equations contain information regarding the proportionality of the amounts of substances involved in the reaction. These values can be used in chemical calculations involving the mole concept.
4.5.A.3 Stoichiometric calculations can be combined with the ideal gas law and calculations involving molarity to quantitatively study gases and solutions.

Source: College Board AP Course and Exam Description

Stoichiometry 化学计量 uses the balanced equation's mole ratios to relate amounts of reactants and products. The path is always grams → moles → (mole ratio) → moles → grams. The limiting reactant 限量反应物 runs out first and sets the maximum product (the theoretical yield 理论产量); the percent yield compares actual to theoretical.

The limiting reactant runs out first and decides how much product forms

Worked example. How much water forms when $4.0\ \text{g}$ of hydrogen burns in excess oxygen? $2\text{H}_2+\text{O}_2\rightarrow2\text{H}_2\text{O}$. Convert to moles, cross the mole ratio ($2:2=1:1$ here), convert back:

$$n(\text{H}_2)=\frac{4.0}{2.0}=2.0\ \text{mol}\;\Rightarrow\;n(\text{H}_2\text{O})=2.0\ \text{mol}\;\Rightarrow\;m=2.0\times18=36\ \text{g}.$$

Worked example (limiting reactant). $10.0\ \text{g}$ of N$_2$ reacts with $5.0\ \text{g}$ of H$_2$ ($\text{N}_2+3\text{H}_2\rightarrow2\text{NH}_3$). Which runs out? Moles: $n(\text{N}_2)=10.0/28=0.36$, $n(\text{H}_2)=5.0/2=2.5$. The reaction needs $3$ H$_2$ per N$_2$; you have $2.5/0.36=7.0$, far more than $3$, so N$_2$ is limiting. It makes $2\times0.36=0.72\ \text{mol}$ of ammonia, with hydrogen left over.

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Scale reactants and products by the mole ratio

A balanced equation fixes the mole ratio between species. Product amount is proportional to the limiting reactant, scaled by that ratio.

Vocabulary Train

English	Chinese	Pinyin
Stoichiometry	化学计量	huà xué jì liàng
limiting reactant	限量反应物	xiàn liàng fǎn yìng wù
theoretical yield	理论产量	lǐ lùn chǎn liàng

Exercise sheet

4.6

Introduction to Titration

Syllabus

Learning Objective	Essential Knowledge
4.6.A Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.	4.6.A.1 Titrations may be used to determine the amount of an analyte in solution. The titrant has a known concentration of a species that reacts specifically and quantitatively with the analyte. The equivalence point of the titration occurs when the analyte is totally consumed by the reacting species in the titrant. The equivalence point is often indicated by a change in a property (such as color) that occurs when the equivalence point is reached. This observable event is called the endpoint of the titration.

Learning Objective

Essential Knowledge

4.6.A
Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.

4.6.A.1 Titrations may be used to determine the amount of an analyte in solution. The titrant has a known concentration of a species that reacts specifically and quantitatively with the analyte. The equivalence point of the titration occurs when the analyte is totally consumed by the reacting species in the titrant. The equivalence point is often indicated by a change in a property (such as color) that occurs when the equivalence point is reached. This observable event is called the endpoint of the titration.

Source: College Board AP Course and Exam Description

An acid–base titration curve

A titration 滴定 finds an unknown concentration by reacting it with a solution of known concentration until they reach the equivalence point 等当点 (stoichiometrically equal). From the known volume and concentration, use the mole ratio to find the unknown. An indicator or a pH curve signals the endpoint.

Titration apparatus: a burette delivers a known solution into a conical flask

Worked example. $25.0\ \text{mL}$ of hydrochloric acid is exactly neutralized by $30.0\ \text{mL}$ of $0.100\ \text{M}$ NaOH. Find the acid's concentration. The reaction is $1:1$, so the moles match:

$$n(\text{NaOH})=0.0300\times0.100=3.00\times10^{-3}\ \text{mol}=n(\text{HCl}),\qquad [\text{HCl}]=\frac{3.00\times10^{-3}}{0.0250}=0.120\ \text{M}.$$

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Titrate an acid and find the equivalence point

Adding base to acid raises the pH slowly, then sharply at the equivalence point where moles of acid and base are equal. The steep jump locates that volume.

Vocabulary Train

English	Chinese	Pinyin
titration	滴定	dī dìng
equivalence point	等当点	děng dāng diǎn

Exercise sheet

4.7

Types of Chemical Reactions

Syllabus

Learning Objective	Essential Knowledge
4.7.A Identify a reaction as acid-base, oxidation-reduction, or precipitation.	4.7.A.1 Acid-base reactions involve transfer of one or more protons ($\text{H}^+$ ions) between chemical species. 4.7.A.2 Oxidation-reduction (redox) reactions involve transfer of one or more electrons between chemical species, as indicated by changes in oxidation numbers of the involved species. Combustion is an important subclass of oxidation-reduction reactions, in which a species reacts with oxygen gas. In the case of hydrocarbons, carbon dioxide and water are products of complete combustion. 4.7.A.3 In a redox reaction, electrons are transferred from the species that is oxidized to the species that is reduced. Exclusion statement: The meaning of the terms "reducing agent" and "oxidizing agent" will not be assessed on the AP Exam. 4.7.A.4 Oxidation numbers may be assigned to each of the atoms in the reactants and products; this is often an effective way to identify the oxidized and reduced species in a redox reaction. 4.7.A.5 Precipitation reactions frequently involve mixing ions in aqueous solution to produce an insoluble or sparingly soluble ionic compound. All sodium, potassium, ammonium, and nitrate salts are soluble in water. Exclusion statement: Rote memorization of "solubility rules" other than those implied in 4.7.A.5 will not be assessed on the AP Exam.

Learning Objective

Essential Knowledge

4.7.A
Identify a reaction as acid-base, oxidation-reduction, or precipitation.

4.7.A.1 Acid-base reactions involve transfer of one or more protons ($\text{H}^+$ ions) between chemical species.
4.7.A.2 Oxidation-reduction (redox) reactions involve transfer of one or more electrons between chemical species, as indicated by changes in oxidation numbers of the involved species. Combustion is an important subclass of oxidation-reduction reactions, in which a species reacts with oxygen gas. In the case of hydrocarbons, carbon dioxide and water are products of complete combustion.
4.7.A.3 In a redox reaction, electrons are transferred from the species that is oxidized to the species that is reduced.
- Exclusion statement: The meaning of the terms "reducing agent" and "oxidizing agent" will not be assessed on the AP Exam.
4.7.A.4 Oxidation numbers may be assigned to each of the atoms in the reactants and products; this is often an effective way to identify the oxidized and reduced species in a redox reaction.
4.7.A.5 Precipitation reactions frequently involve mixing ions in aqueous solution to produce an insoluble or sparingly soluble ionic compound. All sodium, potassium, ammonium, and nitrate salts are soluble in water.
- Exclusion statement: Rote memorization of "solubility rules" other than those implied in 4.7.A.5 will not be assessed on the AP Exam.

Source: College Board AP Course and Exam Description

Common patterns include synthesis (combining), decomposition (breaking apart), combustion (with oxygen, releasing energy), precipitation (forming an insoluble solid), acid–base (proton transfer), and redox (electron transfer). Recognizing the type helps predict the products.

Exercise sheet

4.8

Acid-Base Reactions

Syllabus

Learning Objective	Essential Knowledge
4.8.A Identify species as Brønsted-Lowry acids, bases, and/or conjugate acid-base pairs, based on proton-transfer involving those species.	4.8.A.1 By definition, a Brønsted-Lowry acid is a proton donor and a Brønsted-Lowry base is a proton acceptor. 4.8.A.2 Only in aqueous solutions, water plays an important role in many acid-base reactions, as its molecular structure allows it to accept protons from and donate protons to dissolved species. 4.8.A.3 When an acid or base ionizes in water, the conjugate acid-base pairs can be identified and their relative strengths compared. Exclusion statement: Lewis acid-base concepts will not be assessed on the AP Exam. The emphasis in AP Chemistry is on reactions in aqueous solution.

Learning Objective

Essential Knowledge

4.8.A
Identify species as Brønsted-Lowry acids, bases, and/or conjugate acid-base pairs, based on proton-transfer involving those species.

4.8.A.1 By definition, a Brønsted-Lowry acid is a proton donor and a Brønsted-Lowry base is a proton acceptor.
4.8.A.2 Only in aqueous solutions, water plays an important role in many acid-base reactions, as its molecular structure allows it to accept protons from and donate protons to dissolved species.
4.8.A.3 When an acid or base ionizes in water, the conjugate acid-base pairs can be identified and their relative strengths compared.
- Exclusion statement: Lewis acid-base concepts will not be assessed on the AP Exam. The emphasis in AP Chemistry is on reactions in aqueous solution.

Source: College Board AP Course and Exam Description

An acid 酸 donates a proton ($\text{H}^+$); a base 碱 accepts one (Brønsted–Lowry). They react to form water and a salt: $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$. Strong acids and bases dissociate completely; weak ones only partly.

Bronsted-Lowry: an acid donates a proton to a base, forming two conjugate pairs

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Move along the pH scale

pH measures how acidic or basic a solution is. Each step of 1 pH is a tenfold change in hydrogen-ion concentration; 7 is neutral, below is acidic, above is basic.

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English	Chinese	Pinyin
acid	酸	suān
base	碱	jiǎn

Exercise sheet

4.9

Oxidation-Reduction (Redox) Reactions

Syllabus

Learning Objective	Essential Knowledge
4.9.A Represent a balanced redox reaction equation using half-reactions.	4.9.A.1 Balanced chemical equations for redox reactions can be constructed from half-reactions.

Source: College Board AP Course and Exam Description

In a redox 氧化还原 reaction, electrons transfer between species. Oxidation 氧化 is loss of electrons (oxidation number rises); reduction 还原 is gain (oxidation number falls). Track changes with oxidation numbers, and remember every oxidation is paired with a reduction – the electrons lost equal the electrons gained.

Redox is electron transfer: the reducing agent is oxidised, the oxidising agent reduced

Worked example. Find the oxidation number of manganese in permanganate, $\text{KMnO}_4$. Potassium is $+1$ and each oxygen is $-2$ (four of them, $-8$). The whole formula is neutral, so $(+1)+\text{Mn}+(-8)=0$, giving $\text{Mn}=+7$ – its maximum, which is why permanganate is a powerful oxidizing agent (it can only gain electrons).

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Watch electrons transfer in a redox reaction

In a redox reaction one species is oxidised (loses electrons) and another is reduced (gains them). Follow the electrons move from the metal to the non-metal.

Vocabulary Train

English	Chinese	Pinyin
redox	氧化还原	yǎng huà huán yuán
Oxidation	氧化	yǎng huà
reduction	还原	huán yuán

Exercise sheet

4.9

Exam tips

Balance every equation and work in moles — convert grams→moles, cross the mole ratio, then convert back.
Find the limiting reactant by comparing mole ratios; it sets the maximum product (theoretical yield).
In redox, remember OIL RIG: oxidation is loss of electrons, reduction is gain; every oxidation is paired with a reduction.
For titrations use the balanced ratio to link the known and unknown at the equivalence point.
A net ionic equation shows only the species that change; leave out spectator ions.

Recognizing a Chemical Reaction

Net Ionic Equations

Three Ways to Represent a Reaction

Physical Changes versus Chemical Changes

Stoichiometry

Scale reactants and products by the mole ratio

Introduction to Titration

Titrate an acid and find the equivalence point

Types of Chemical Reactions

Acid-Base Reactions

Move along the pH scale

Oxidation-Reduction (Redox) Reactions

Watch electrons transfer in a redox reaction

Exam tips

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