Energy Changes in Reactions and Combustion

Understanding Energy Changes in Reactions

Key Definitions

• Exothermic Reactions:

  • Release energy to the surrounding environment, causing increases in temperature.
  • Example: Burning of fuels.

• Endothermic Reactions:

  • Absorb energy from the surroundings, resulting in lower temperatures.
  • Example: Photosynthesis, instant cold packs.

Energy Changes in Reactions

• Energy change in a reaction can be calculated in terms of:
  • Q (heat energy change)
  • ΔH (molar enthalpy change)

Units of Measurement

• Energy is measured in kilojoules (kJ).
• Enthalpy change (ΔH) is expressed in kJ/mol.

Examples of Energy Changes

• Exothermic Process:

  • Reaction: Fe(s) + S(s) → FeS(s)
  • Energy change: $-100 ext{ kJ/mol}$ (indicates energy release).

• Endothermic Process:

  • Reaction: $CaCO<em>3(s) → CaO(s) + CO</em>2(g)$
  • Energy change: $+178 ext{ kJ/mol}$ (indicates energy absorption).

Calculating Heat Energy Change (Q)

Formula for Heat Energy Change

• $Q = m imes c imes \Delta T$
  • m: mass (g)
  • c: specific heat capacity (J/g°C)
  • ΔT: temperature change (°C)

Example Calculation

• Given:
  • Mass of water = 100 g,
  • Specific heat capacity of water = 4.2 J/g°C,
  • Temperature rise = 20°C.
• Calculation:
  • $Q = 100 imes 4.2 imes 20 = 8400 ext{ J}$
  • Convert to kJ: $8.4 ext{ kJ}$
  • Energy transferred = 8.4 kJ.

Molar Enthalpy Change Calculation

1. Calculate moles of propane burned:
   • Molar mass of propane (C3H8): 44 g/mol.
   • $ext{Moles} = ext{mass} / ext{molar mass} = 0.5 ext{ g} / 44 ext{ g/mol} = 0.01136 ext{ mol}$
2. Calculate molar enthalpy change:
   • $ext{ΔH} = Q / ext{moles} = 8.4 ext{ kJ} / 0.01136 ext{ mol} = 739 ext{ kJ/mol}$
   • This reaction is exothermic, therefore:
   • $ext{Molar Enthalpy change} = -739 ext{ kJ/mol}$

Sample Problem

• Determining Enthalpy of Combustion of Hexane:

1. Data collection:

   • Mass of water: 100 g
   • Temperature change from 18 °C to 44 °C = 26 °C.
   • Mass of hexane burned = 0.43 g.

2. Calculations:

   • $Q = 100 imes 4.18 imes 26 = 10920 ext{ J} = 10.92 ext{ kJ}$
   • Molar mass of hexane (C6H14) = 86 g/mol.
   • Moles burned = $0.43 ext{ g} / 86 ext{ g/mol} = 0.005 ext{ mol}$
   • $ext{Enthalpy change per mol} = -10.92 ext{ kJ} / 0.005 ext{ mol} = -2184 ext{ kJ/mol}$

Incomplete Combustion

• Definition: Occurs when there is insufficient oxygen, leading to the production of carbon monoxide (CO) and carbon (soot), along with water (H2O).
  • Example reaction with propane:
  • $C<em>3H</em>8 + 3O<em>2 ightarrow 2CO + C + 4H</em>2O$
• Consequences:
  • Health risks from carbon monoxide exposure, including hypoxia (reduced oxygen capacity of blood), and respiratory issues from soot.

Pollution from Sulfur Impurities

• Formation of Sulfur Dioxide (SO2):
  • Reaction: $S(s) + O<em>2(g) ightarrow SO</em>2(g)$
• SO2 can react with water to form sulfuric acid (H2SO4), causing acid rain which leads to:
  • Acidification of water bodies.
  • Damage to vegetation and buildings.

Reactivity of Hydrocarbons

• In combustion, energy released when bonds form is greater than the energy needed to break bonds in reactants, making it exothermic.
• Alcohols as clean fuels: Methanol and ethanol can be produced from renewable resources, presenting a more environmentally friendly option than traditional hydrocarbons.

Experimental Techniques

• Simple vs. Fractional Distillation:
  • Simple distillation is effective for liquids with significant boiling point differences.
  • Fractional distillation separates mixtures into fractions with varying boiling points (e.g., crude oil).

Hazards Associated with Fuels

• Flammability and risk of explosions are significant hazards when using or transporting fuels. Proper storage and handling are critical.
• The Buncefield oil depot explosion serves as a case study for understanding risks linked to fuel storage and transport.