Chapter Summary: Chemical Reactions and Equations

💡 Key Concepts We've Learned

Chemical Reactions

• A chemical reaction is a process where substances (reactants) transform into new substances (products) with different properties
• Chemical reactions involve the breaking and making of chemical bonds
• Signs of chemical reactions: color change, gas formation, temperature change, precipitate formation, odor change

Chemical Equations

• A chemical equation is a symbolic representation of a chemical reaction
• It shows reactants, products, and their physical states
• Word equations use names of substances: Magnesium + Oxygen → Magnesium oxide
• Chemical equations use formulas: 2Mg + O₂ → 2MgO
• State symbols: (s) - solid, (l) - liquid, (g) - gas, (aq) - aqueous solution

Balancing Chemical Equations

• Based on the Law of Conservation of Mass: matter cannot be created or destroyed
• We add coefficients (numbers in front of formulas) to balance atoms
• We never change the subscripts in chemical formulas
• The hit-and-trial method is a systematic approach to balancing equations

Types of Chemical Reactions

1. Combination Reactions (A + B → C)

• Two or more substances combine to form a single product
• Usually exothermic (release energy)
• Examples: burning fuels, formation of slaked lime

2. Decomposition Reactions (C → A + B)

• A single compound breaks down into two or more simpler substances
• Usually endothermic (absorb energy)
• Require energy input (heat, light, or electricity)
• Examples: electrolysis of water, thermal decomposition of limestone

3. Displacement Reactions (A + BC → AC + B)

• A more reactive element displaces a less reactive element from its compound
• Reactivity series determines if displacement will occur
• Examples: iron displacing copper from copper sulphate

4. Double Displacement Reactions (AB + CD → AD + CB)

• Ions of two compounds exchange partners
• Often result in precipitation or neutralization
• Examples: formation of barium sulphate precipitate

5. Oxidation-Reduction (Redox) Reactions

• Involve transfer of electrons between substances
• Oxidation: loss of electrons or gain of oxygen
• Reduction: gain of electrons or loss of oxygen
• Examples: combustion, respiration, corrosion

Effects of Oxidation

• Corrosion: deterioration of metals through oxidation

  • Rusting of iron, tarnishing of silver
  • Prevention: painting, galvanization, alloying, sacrificial protection

• Rancidity: oxidation of fats and oils in food

  • Causes bad taste and smell
  • Prevention: antioxidants, airtight packaging, refrigeration, nitrogen flushing

⚖️ Quick Comparison: Types of Reactions

Type	Pattern	Energy Change	Examples
Combination	A + B → C	Usually exothermic	Burning, formation of water
Decomposition	C → A + B	Usually endothermic	Electrolysis, thermal decomposition
Displacement	A + BC → AC + B	Can be either	Metal displacement reactions
Double Displacement	AB + CD → AD + CB	Can be either	Precipitation, neutralization
Redox	Involves electron transfer	Can be either	Combustion, corrosion, batteries

🧠 Important Memory Tricks

1. COPS for identifying chemical reactions:

   • Color change
   • Odor production
   • Precipitate formation
   • Sound/gas production/temperature change

2. LEO the lion says GER for redox reactions:

   • Loss of Electrons is Oxidation
   • Gain of Electrons is Reduction

3. BWSC for chemical equations:

   • Balance the equation
   • Write symbols correctly
   • State symbols for clarity
   • Coefficients in front, subscripts below

🌍 Real-World Connections

• Kitchen Chemistry: Cooking involves many chemical reactions (browning of food, rising of dough)
• Body Chemistry: Digestion and respiration are series of chemical reactions
• Industrial Processes: Manufacturing of cement, ammonia, metals all rely on chemical reactions
• Environmental Chemistry: Acid rain, carbon cycle, photosynthesis are all examples of chemical reactions

📝 Practice Questions

1. Identify the type of reaction:

   CaO + H₂O → Ca(OH)₂

2. Balance the following equation:

   Fe + H₂O → Fe₃O₄ + H₂

3. Identify which substance is oxidized and which is reduced:

   ZnO + C → Zn + CO

4. Predict whether a displacement reaction will occur:

   Cu + FeSO₄ → ?

5. Explain why food packages are often flushed with nitrogen gas.

🔜 Looking Ahead

Now that you understand chemical reactions and equations, you're ready to explore acids, bases, and salts in the next chapter. The concepts you've learned here will be essential for understanding those chemical interactions!

Remember, chemical reactions aren't just abstract concepts in textbooks—they're happening all around you, every moment of every day, from the air you breathe to the food you eat. The more you understand them, the better you can understand our wonderful world!