Enthalpy Changes | OCR A Level Chemistry A Revision Notes 2015

Overview of Enthalpy Changes

• Enthalpy refers to the total chemical energy within a substance, also known as heat content.
• Enthalpy change (_H) is the change in chemical energy during reactions, represented by the symbol _H.
• Enthalpy changes can be either positive or negative.

Exothermic Reactions

• In exothermic reactions, the products have less energy than the reactants.
• Heat energy is released into the surroundings, increasing the temperature of the environment.
• The system's energy decreases and the enthalpy change (_H) is negative.
• Exothermic reactions are thermodynamically possible but may be kinetically controlled if the reaction rate is too slow._

Enthalpy Profile

• For exothermic reactions, the enthalpy profile shows a decrease in energy from reactants to products.

Endothermic Reactions

• Endothermic reactions have products with more energy than the reactants.
• Heat energy is absorbed from the surroundings, decreasing the temperature of the environment.
• The system's energy increases and the enthalpy change (H) is positive.

Enthalpy Profile

• For endothermic reactions, the enthalpy profile shows an increase in energy from reactants to products.

Enthalpy Profile Diagrams

• Energy level diagrams display:
  • Energy levels of reactants and products.
  • Transition states, which are higher in energy and cannot be isolated.
  • Activation energy (E_a), the minimum energy needed for reactant molecules to collide successfully and start the reaction.
  • The overall enthalpy change (H) of the reaction.

Examiner Tips

• Transition state does not need to be labeled in diagrams.

Worked Examples

Drawing Energy Level Diagrams

• Example: Combustion of methane
  • Chemical equation: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
  • Exothermic reaction (H negative), draw reactants higher in energy than products.

Calculating Activation Energy

• Example: Given _H for a reaction is +70 kJ mol^-1 and E_a for the reverse reaction is +20 kJ mol^-1.
  • E_a (forward reaction) is +90 kJ mol^-1._

Standard Enthalpy Definitions

• Thermodynamic measurements are under standard conditions:
  • Pressure: 100 kPa
  • Temperature: 298 K (25°C)
  • Substances in standard physical state.

Key Enthalpy Definitions

• Standard enthalpy change (H°) under standard conditions.

Worked Examples

Calculating Enthalpy Change of Reaction

• Example: 2H2(g) + O2(g) → 2H2O(l) with H°r = -572 kJ mol^-1.

Calculating Enthalpy Formation

• Example: _H°f[Fe2O3(s)] = -824.2 kJ mol^-1, 4Fe (s) + 3 O2 (g) → 2 Fe2O3 (s), _H°f = -1648 kJ mol^-1.

Important Tips

• _H°f of an element in its standard state is zero.
• Example: _H°f of O2(g) is 0 kJ mol^-1.

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