Study sheet: Types of Chemical Reactions

📋 Course Outline

1. Chemical Reactions and Their Evidence
2. Direct Combination Reactions
3. Thermal Decomposition Reactions
4. Single Substitution and Metal Activity
5. Metal Reactions with Water
6. Metal Displacement from Salt Solutions

📖 1. Chemical Reactions and Their Evidence

🔑 Key Concepts & Definitions

• Chemical Reaction : Breaks bonds between reactant molecules and forms new bonds between product molecules, producing substances with different properties.

★ Must-know

• A chemical reaction can be recognized by:
  • color change
  • gas evolution
  • precipitate formation
  • temperature change
  • emission of heat and light
  • odor change

Further detail

• Burning magnesium in air is an example of a chemical reaction because magnesium forms magnesium oxide with different properties.

💡 Memory Hook

CGPTHO: Color, Gas, Precipitate, Temperature, Heat/light, Odor

📖 2. Direct Combination Reactions

🔑 Key Concepts & Definitions

• Synthesis Reaction : Occurs when two or more substances combine to form one new compound.

★ Must-know

📌 The reactivity of the metals in the oxygen examples decreases in the order magnesium > iron > copper.

Further detail

• Copper combines with oxygen when heated according to $2\text{Cu} + \text{O}_2 \xrightarrow{\Delta} 2\text{CuO}$, forming black copper oxide on its surface.

💡 Memory Hook

Mg, Fe, Cu combine with oxygen

📖 3. Thermal Decomposition Reactions

🔑 Key Concepts & Definitions

• Thermal Decomposition Reaction : Occurs when one compound breaks down into two or more simpler substances by heat or light.

★ Must-know

• Heating calcium carbonate decomposes it into calcium oxide and carbon dioxide gas.

Further detail

• Heating red mercury oxide decomposes it into liquid mercury and oxygen gas.

💡 Memory Hook

Heat or light causes one compound to split into simpler substances

📖 4. Single Substitution and Metal Activity

🔑 Key Concepts & Definitions

• Single Substitution Reaction : Occurs when one element replaces another element in one of its compounds.
• Chemical Reactivity Series : An arrangement of metals in descending order of their chemical reactivity.

★ Must-know

📐 Formula — Magnesium replaces hydrogen in hydrochloric acid according to $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$, producing a popping sound when hydrogen is tested with a flame.

• The reactivity of the tested metals with dilute hydrochloric acid decreases in the order zinc > iron > copper.

📌 Metals above hydrogen in the chemical reactivity series can replace hydrogen from dilute acids and release hydrogen gas, whereas metals below hydrogen cannot replace it.

Further detail

• The series given in descending reactivity is potassium, sodium, lithium, barium, calcium, magnesium, aluminum, zinc, iron, tin, lead, hydrogen, copper, mercury, silver, platinum, and gold.

📌 Aluminum does not react immediately with dilute hydrochloric acid because a thin protective aluminum oxide layer acts as a barrier until it gradually dissolves.

💡 Memory Hook

Above hydrogen replaces it; below hydrogen does not

📖 5. Metal Reactions with Water

★ Must-know

📌 Highly reactive metals at the top of the chemical reactivity series react with water and replace hydrogen from water molecules.

📌 Sodium reacts strongly with water, releasing hydrogen gas and a large amount of heat that can ignite the hydrogen and cause fire or explosion.

• 🔄 With water, magnesium: reacts very slowly with cold water, reacts readily with hot water, produces hydrogen gas, forms magnesium hydroxide as a white suspension

• Metal behavior with water depends on reactivity: highly reactive metals include: sodium, potassium, and calcium, magnesium, zinc, and iron as moderately reactive metals, copper, silver, and gold as less reactive metals

Further detail

📌 Sand is used to extinguish a sodium fire because water can intensify the reaction.

💡 Memory Hook

Cold water for highly reactive metals, hot water or steam for moderately reactive metals

📖 6. Metal Displacement from Salt Solutions

★ Must-know

📌 A more reactive metal can replace a less reactive metal from its salt solution, whereas a less reactive metal cannot replace a more reactive metal.

📐 Formula — Zinc displaces copper from copper sulfate solution according to $\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$.

Further detail

• 🔄 The zinc–copper sulfate reaction produces these observations: the blue copper sulfate color gradually disappears, colorless zinc sulfate solution forms, reddish-brown copper deposits on the zinc plate

• The zinc and copper sulfate reaction demonstrates that zinc is more reactive than copper and can replace copper from its salt.

💡 Memory Hook

Greater reactivity causes one metal to displace a less reactive metal

📊 Synthesis Tables

Metal Reactivity and Water

Metal group	Examples	Reaction with water
Highly reactive	Na, K, Ca	React rapidly with cold water
Moderately reactive	Mg, Zn, Fe	React with hot water or steam
Less reactive	Cu, Ag, Au	Do not react under normal conditions