Enthalpy - Energy Processes

Introduction to Enthalpy

Enthalpy (H): The total heat content of a system, is a fundamental property in thermochemistry.

• Enthalpy assesses the total heat in a system, reflecting its current state. It is crucial for identifying energy changes during chemical reactions.

• Determines the heat exchange during reactions, helping classify them as either exothermic or endothermic.

Exothermic vs. Endothermic Reactions

• Exothermic Reactions:

  • Release heat (negative ΔH).
  • Example: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$

• Endothermic Reactions:

  • Absorb heat (positive ΔH).
  • Example: $H_2O_{(s)} \rightarrow H_2O_{(l)}$

Enthalpy Change Formula

• This is determined by subtracting the enthalpy of reactants from that of the products.

The Application of Hess's Law

Hess's Law: The total change in enthalpy of a chemical reaction is independent of its pathway.

• Utilises known enthalpy changes to estimate energy changes that are challenging to measure directly.

Photosynthesis

• Definition: Photosynthesis is an endothermic process converting carbon dioxide and water into glucose and oxygen.
• It absorbs energy, illustrating its endothermic nature.

Chemical Equation

• Balanced Reaction:

  $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

  • ΔH = +2814 kJ/mol.

Stages Involved

Light-dependent Reactions

• Occur in thylakoid membranes, capturing light energy via chlorophyll.

Calvin Cycle

• Takes place in the stroma.
• ATP and NADPH facilitate glucose synthesis.

Factors Influencing ΔH

• The wavelength of light, temperature, and environmental conditions affect the efficiency of photosynthesis.

Respiration

• Definition: Cellular respiration is an exothermic process that releases energy by breaking down glucose.

Chemical Equation

• Balanced Reaction:

  $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

  • ΔH = -2814 kJ/mol.

Biochemical Pathways

Glycolysis

• Converts glucose into pyruvate, releasing small amounts of energy.

Krebs Cycle

• Produces NADH and FADH₂.

Oxidative Phosphorylation

• Occurs in mitochondria, generating ATP through the electron transport chain.

Influence of Conditions

• Temperature affects enzyme activity.
• Oxygen levels impact the efficiency of respiration.

Integrating Hess's Law with Photosynthesis and Respiration

• Photosynthesis stores energy in glucose.
• Respiration releases this stored energy through the breakdown of glucose.
• The Enthalpy Cycle demonstrates the flow and conservation of energy between these processes.

Worked Examples

Example 1: Calculating Enthalpy Change in Photosynthesis

Problem: Calculate the total enthalpy change when 3 moles of glucose are produced during photosynthesis.

Solution:

1. The enthalpy change for producing 1 mole of glucose is +2814 kJ/mol
2. For 3 moles: ΔH = 3 × (+2814 kJ/mol) = +8442 kJ

Example 2: Using Hess's Law in Respiration

Problem: Calculate the energy released when 2 moles of glucose undergo complete cellular respiration.

Solution:

1. The enthalpy change for respiration of 1 mole of glucose is -2814 kJ/mol
2. For 2 moles: ΔH = 2 × (-2814 kJ/mol) = -5628 kJ
3. Therefore, 5628 kJ of energy is released

Identifying and Mitigating Errors

• Ensure accurate calorimetry measurements and control over environmental conditions.