Study sheet: Types of Chemical Reactions

📋 Course Outline

1. Chemical Reactions and Classification
2. Combination and Decomposition Reactions
3. Single Replacement and Reactivity
4. Chemical Reactivity Series
5. Double Replacement Reactions
6. Neutralization and Precipitation
7. Oxidation and Reduction
8. Reaction Analysis and Applications

📖 1. Chemical Reactions and Classification

🔑 Key Concepts & Definitions

• Chemical reaction : a process in which bonds are broken in reactant molecules and new bonds are formed in product molecules, producing new substances with properties different from those of the reactants.

★ Must-know

• A chemical reaction may be indicated by:
  • gas evolution
  • a colour change
  • a temperature change
  • emission of light
  • formation of a precipitate

Further detail

• Chemical reactions are classified according to how atoms or ions rearrange and bind together, which helps explain how reactions occur and predict products.

📖 2. Combination and Decomposition Reactions

🔑 Key Concepts & Definitions

• Direct combination reactions : reactions in which two or more substances combine to form a new compound.
• Decomposition reactions : the reverse of direct combination reactions and involve a compound breaking into simpler components using energy such as heat or light.

★ Must-know

• When heated in oxygen, the metals show these observations and products: magnesium burns with a bright white flame and forms white magnesium oxide, steel wool burns with bright sparks and mainly forms magnetic iron oxide Fe₃O₄, copper forms a black layer of copper oxide CuO without a visible flame

• Some metal oxides decompose on heating into the metal, while oxygen gas is evolved.

Further detail

• The reaction $2\mathrm{NO}_{(g)}+\mathrm{O}_{2(g)}\longrightarrow2\mathrm{NO}_{2(g)}$ is a direct combination reaction because two substances combine to form a new compound.

• Red mercury oxide decomposes on heating into silver-coloured liquid mercury and colourless oxygen gas, which increases the glow of a lighted matchstick.

• Calcium carbonate decomposes on heating into calcium oxide and colourless carbon dioxide gas, which turns clear limewater turbid.

💡 Memory Hook

Combination builds a compound, whereas decomposition breaks one apart.

📖 3. Single Replacement and Reactivity

🔑 Key Concepts & Definitions

• Single replacement reactions : reactions in which an element replaces another element in one of its compounds.

★ Must-know

• Single replacement reactions include: replacement of hydrogen in an acid by a metal, replacement of hydrogen in water by a metal, replacement of a metal by another metal in its salt solution

• Magnesium, zinc, and iron react with dilute hydrochloric acid to form their metal chlorides and hydrogen gas, whereas copper produces no gas and does not react under these conditions.

📌 A highly reactive metal such as sodium replaces hydrogen in cold water to form a metal hydroxide and hydrogen gas.

• Zinc replaces copper in copper sulphate solution, producing colourless zinc sulphate solution and depositing red copper.

Further detail

• The rate of reaction with dilute hydrochloric acid decreases in the order magnesium, zinc, iron.

• Sodium reacts instantly and vigorously with cold water to form alkaline sodium hydroxide, hydrogen gas, and heat; the hydrogen may ignite with a yellowish-orange flame.

• Magnesium reacts with hot water but not visibly with cold water, showing that magnesium is less reactive than sodium and requires thermal energy to begin reacting.

💡 Memory Hook

Acid → water → salt solution: the three replacement contexts.

📖 4. Chemical Reactivity Series

🔑 Key Concepts & Definitions

• Chemical reactivity series : the arrangement of metal elements in descending order according to their chemical reactivity.

★ Must-know

• The chemical reactivity series from most to least reactive is potassium, sodium, lithium, barium, calcium, magnesium, aluminum, zinc, iron, tin, lead, hydrogen, copper, mercury, silver, platinum, and gold.

📌 Metals above hydrogen in the chemical reactivity series replace hydrogen in dilute acids, whereas metals below hydrogen cannot replace it.

📌 A metal earlier in the chemical reactivity series can replace a later metal from its salt solution.

Further detail

• Although aluminum precedes zinc in the series, aluminum may not react instantly with dilute acids because an aluminum oxide layer initially prevents acid from reaching the metal.

📖 5. Double Replacement Reactions

🔑 Key Concepts & Definitions

• Double replacement reactions : occur when the cations and anions of two ionic compounds, usually in aqueous solution, exchange positions to form two new compounds.
• Double replacement reactions : chemical reactions in which the cations and anions of two ionic compounds exchange positions to form two new compounds
• Neutralization reaction : occurs when an acid reacts with an alkali to form a salt and water
• Precipitation reaction : occurs when two salt solutions react to form a new substance that does not dissolve in water

★ Must-know

• The three types of double replacement reactions are:
  • acid with alkali
  • acid with salt
  • one salt solution with another salt solution

📐 Formula — Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water: $\mathrm{HCl}_{(aq)}+\mathrm{NaOH}_{(aq)}\rightarrow\mathrm{NaCl}_{(aq)}+\mathrm{H_2O}_{(l)}$.

📐 Formula — Sodium chloride solution reacts with silver nitrate solution to form a white precipitate of silver chloride: $\mathrm{NaCl}_{(aq)}+\mathrm{AgNO_3}_{(aq)}\rightarrow\mathrm{AgCl}_{(s)}+\mathrm{NaNO_3}_{(aq)}$.

Further detail

• An ionic compound consists of a positive cation and a negative anion, each of which may be a single ion or a charged atomic group.

• Universal indicator turns violet in an alkaline medium, green in a neutral medium, and red in an acidic medium.

• Hydrochloric acid reacts with sodium carbonate to form sodium chloride, water, and carbon dioxide: $\mathrm{Na_2CO_3}_{(s)}+2\mathrm{HCl}_{(aq)}\rightarrow2\mathrm{NaCl}_{(aq)}+\mathrm{H_2O}_{(l)}+\mathrm{CO_2}_{(g)}$.

💡 Memory Hook

Exchange ions → form water, gas, or precipitate

📖 6. Neutralization and Precipitation

🔑 Key Concepts & Definitions

• Precipitation reaction : a reaction between salt solutions that produces an insoluble solid precipitate.

★ Must-know

• Universal indicator is: violet in a strong alkali such as sodium hydroxide, green in a neutral solution such as sodium chloride, red in a strong acid such as hydrochloric acid

• Hydrochloric acid reacts with sodium carbonate to form sodium chloride and carbonic acid, which decomposes into water and carbon dioxide; the carbon dioxide turns clear limewater turbid.

Further detail

• Hydrochloric acid reacts with sodium hydroxide to form neutral sodium chloride solution and water.

• Silver nitrate solution reacts with sodium chloride solution to form sodium nitrate solution and a white insoluble silver chloride precipitate that turns violet after exposure to light.

💡 Memory Hook

Neutralization forms salt and water, whereas precipitation forms an insoluble solid.

📖 7. Oxidation and Reduction

🔑 Key Concepts & Definitions

• Oxidation : a chemical process involving loss of one or more electrons; traditionally, it involves an increase in oxygen or a decrease in hydrogen.
• Reduction : a chemical process involving gain of one or more electrons; traditionally, it involves a decrease in oxygen or an increase in hydrogen.
• Reducing agent : the substance that loses one or more electrons during a chemical reaction.
• Oxidizing agent : the substance that gains one or more electrons during a chemical reaction.
• Oxidation process : a chemical process in which a substance gains oxygen, loses hydrogen, or loses one or more electrons

★ Must-know

📌 Oxidation and reduction occur concurrently because the electrons lost in oxidation are gained in reduction.

• In every redox reaction, oxidation and reduction occur concurrently, and the number of electrons lost during oxidation equals the number gained during reduction.

📐 Formula — In the reaction $\mathrm{Zn}+\mathrm{CuSO_4}\rightarrow\mathrm{ZnSO_4}+\mathrm{Cu}$, zinc is oxidized and acts as the reducing agent, while copper ions are reduced and act as the oxidizing agent.

Further detail

• In the reaction between zinc and copper ions, zinc loses two electrons and is oxidized into Zn²⁺, while Cu²⁺ gains those electrons and is reduced into copper.

• Most metals usually act as reducing agents because they tend to lose electrons, while most non-metals usually act as oxidizing agents because they tend to gain electrons.

📐 Formula — The oxidation half-reaction of magnesium is $\mathrm{Mg}\rightarrow\mathrm{Mg^{2+}}+2\mathrm{e^-}$, while the reduction half-reaction of copper ions is $\mathrm{Cu^{2+}}+2\mathrm{e^-}\rightarrow\mathrm{Cu}$.

💡 Memory Hook

Oxidation loses electrons, whereas reduction gains electrons.

📖 8. Reaction Analysis and Applications

★ Must-know

📌 A metal that is higher in the chemical reactivity series can replace a less reactive metal from its salt solution, whereas a less reactive metal cannot replace a more reactive metal.

• When sodium reacts with water, sodium hydroxide and hydrogen are formed: $2\mathrm{Na}_{(s)}+2\mathrm{H_2O}_{(l)}\rightarrow2\mathrm{NaOH}_{(aq)}+\mathrm{H_2}_{(g)}$.

• Hard water contains high percentages of calcium and magnesium salts and can form hard deposits inside water pipes.

📐 Formula — In the reaction $\mathrm{Mg}_{(s)}+2\mathrm{HCl}_{(aq)}\rightarrow\mathrm{MgCl_2}_{(aq)}+\mathrm{H_2}_{(g)}$, magnesium loses electrons and hydrogen ions gain electrons.

Further detail

📐 Formula — When magnesium reacts with dilute hydrochloric acid, magnesium chloride and hydrogen are formed: $\mathrm{Mg}_{(s)}+2\mathrm{HCl}_{(aq)}\rightarrow\mathrm{MgCl_2}_{(aq)}+\mathrm{H_2}_{(g)}$.

• Precipitation reactions can treat hard water by removing dissolved calcium and magnesium salts as insoluble precipitates.

• When red mercury oxide is heated, silver-coloured mercury and oxygen gas are produced: $2\mathrm{HgO}_{(s)}\xrightarrow{\Delta}2\mathrm{Hg}_{(l)}+\mathrm{O_2}_{(g)}$.

💡 Memory Hook

Greater reactivity → displacement; electron transfer → redox behavior

📊 Synthesis Tables

Main Reaction Types

Reaction type	Defining change	Typical result
Direct combination	Two or more substances combine	One new compound
Decomposition	One compound breaks apart	Simpler substances
Single replacement	One element replaces another	A new element and compound
Double replacement	Cations and anions exchange	Two new compounds

Oxidation and reduction agents

Notion	Electron change	Role
Oxidation	Loss of electrons	Occurs in the reducing agent
Reduction	Gain of electrons	Occurs in the oxidizing agent
Oxidizing agent	Gains electrons	Is reduced
Reducing agent	Loses electrons	Is oxidized