Chemistry Energy Changes Or Chemical Energetics

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🌡️ CHEMISTRY: ENERGY CHANGES

1. Exothermic & Endothermic Reactions

Exothermic: Reaction that releases heat to surroundings → Temperature RISES → ΔH is NEGATIVE (ΔH < 0)

Endothermic: Reaction that absorbs heat from surroundings → Temperature FALLS → ΔH is POSITIVE (ΔH > 0)

Type	Heat Flow	Temperature	ΔH Sign	Examples
Exothermic	OUT ↗	RISES ↑	NEGATIVE (−)	Combustion, respiration, neutralization
Endothermic	IN ↘	FALLS ↓	POSITIVE (+)	Photosynthesis, thermal decomposition

2. Enthalpy Change (ΔH)

Enthalpy Change (ΔH): Transfer of thermal energy during a reaction, measured in kilojoules (kJ)

Main Formula:
ΔH = Total energy of products − Total energy of reactants

Example 1 (Exothermic):
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
ΔH = −150 kJ (negative = heat released)

Example 2 (Endothermic):
NH₄Cl(s) → NH₃(g) + HCl(g)
ΔH = +136 kJ (positive = heat absorbed)

3. Reaction Pathway Diagrams

Exothermic Diagram:

Energy ↑ | | Reactants ●──────┐ | │ | │ ΔH = (−) | │ | └────→ ● Products | └─────────────────────────→ Progress

Endothermic Diagram:

Energy ↑ | | ┌────→ ● Products | │ | │ ΔH = (+) | │ | Reactants ●──────┘ | └─────────────────────────→ Progress

Key: Products LOWER = Exothermic | Products HIGHER = Endothermic

4. Activation Energy (Eₐ)

Activation Energy (Eₐ): Minimum energy that colliding particles must have to react

Energy ↑ | ╱●╲ ← Peak (transition state) | ╱ Eₐ ╲ | ●───╱ ╲ | ↑ ╲───→ ● Products | Reactants ΔH (−) └────────────────────────────→ Progress

Important: BOTH exothermic & endothermic reactions need activation energy to start!

5. Bond Breaking & Bond Making

Bond Breaking: REQUIRES energy (ENDOTHERMIC process) ☹
Bond Making: RELEASES energy (EXOTHERMIC process)

Key Formula:
ΔH = Energy absorbed (breaking bonds) − Energy released (making bonds)

Bond Energies (kJ/mol):

Bond	Energy	Bond	Energy
H−H	436	O=O	496
C−H	435	O−H	463
C−C	346	C=O	803
N≡N	945	N−H	391
Cl−Cl	242	H−Cl	431

6. Worked Example: H₂ + O₂ → H₂O

Reaction: 2H₂(g) + O₂(g) → 2H₂O(l)

Step 1: Bonds Broken (Energy IN)

2 H−H bonds: 2 × 436 = 872 kJ
1 O=O bond: 1 × 496 = 496 kJ
Total absorbed = 1368 kJ

Step 2: Bonds Made (Energy OUT)

4 O−H bonds: 4 × 463 = 1852 kJ
Total released = 1852 kJ

Step 3: Calculate ΔH

ΔH = 1368 − 1852 = −484 kJ
Conclusion: EXOTHERMIC (negative ΔH)

7. Calculation Steps

1. Write balanced equation
2. Draw all molecules showing all bonds
3. Count bonds broken (reactants)
4. Count bonds made (products)
5. Calculate total energy for breaking
6. Calculate total energy for making
7. Subtract: ΔH = Breaking − Making
8. Check sign: (−) = exothermic, (+) = endothermic

8. Important Formulas Summary

1. Enthalpy: ΔH = E(products) − E(reactants)

2. Bond Energy: ΔH = Σ(bonds broken) − Σ(bonds made)

3. Thermal Energy: Q = m × c × ΔT
(m=mass, c=4.2 J/g/°C, ΔT=temp change)

4. Combustion of Methane:
CH₄ + 2O₂ → CO₂ + 2H₂O; ΔH = −728 kJ/mol

5. Ammonia Formation:
N₂ + 3H₂ → 2NH₃; ΔH = −93 kJ/mol

6. Neutralization:
H⁺(aq) + OH⁻(aq) → H₂O(l); ΔH = −57 kJ/mol

9. Decision Rules

Condition	Result
Energy absorbed < Energy released	EXOTHERMIC (ΔH negative)
Energy absorbed > Energy released	ENDOTHERMIC (ΔH positive)
Products lower than reactants	EXOTHERMIC
Products higher than reactants	ENDOTHERMIC

10. Key Points

• Exothermic: Heat OUT, temp UP, ΔH = (−)
• Endothermic: Heat IN, temp DOWN, ΔH = (+)
• Breaking bonds: Needs energy (endothermic)
• Making bonds: Releases energy (exothermic)
• Activation energy: Needed for ALL reactions
• Catalyst: Lowers activation energy

11. Common Mistakes

❌ "Endothermic reactions are cold"
✅ Endothermic reactions BECOME cold by absorbing heat

❌ "Only exothermic reactions need Eₐ"
✅ ALL reactions need activation energy

❌ "Bond making absorbs energy"
✅ Bond BREAKING absorbs, bond MAKING releases

❌ "Negative ΔH = absorbed energy"
✅ Negative ΔH = RELEASED energy (exothermic)

12. Glossary

Activation Energy (Eₐ): Minimum energy for particles to react
Bond Energy: Energy to break 1 mole of a bond
Endothermic: Takes heat IN (ΔH positive)
Enthalpy (ΔH): Heat transfer in reaction (kJ)
Exothermic: Gives heat OUT (ΔH negative)
Fossil Fuels: Coal, petroleum, natural gas
Hydrocarbons: Compounds with H and C only

13. Exam Tips

1. Always check ΔH sign: (−) = exo, (+) = endo
2. Show ALL working in calculations
3. Count ALL bonds carefully
4. Include units (kJ or kJ/mol)
5. Draw clear energy diagrams
6. Label Eₐ and ΔH on diagrams

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