Energy Changes in Chemistry

Introduction

Grasping energy changes in chemical reactions is essential for both predicting outcomes in chemistry and applying this knowledge to industry practices. Chemical equations depict these energy transfers, making comprehension of exothermic and endothermic reactions fundamental.

• Energy Transfer: Occurs between the system and its surroundings.
• Reaction Predictions: Energy changes categorise reactions as exothermic (energy is released) or endothermic (energy is absorbed).

Exothermic Reactions

Exothermic reactions release energy to the surroundings, often as heat, resulting in a rise in temperature.

• Examples:
  • Combustion: Supplies warmth and powers engines.
  • Respiration: Releases energy biologically.

Example: Combustion Process

• Reactants react with oxygen to produce carbon dioxide and water.
• Energy is released, which provides heat and light.

Chemical Equation:

$\mathrm{CH}_4 + 2\mathrm{O}_2 \rightarrow \mathrm{CO}_2 + 2\mathrm{H}_2\mathrm{O} + \mathrm{Energy}$

Endothermic Reactions

Endothermic reactions absorb energy, resulting in a drop in temperature. These reactions are vital for processes like photosynthesis.

Photosynthesis Example

• Plants transform carbon dioxide and water into glucose and oxygen using sunlight.
• Equation: $6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \mathrm{Energy~(sunlight)} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2$

Comparison with Exothermic Reactions

• Energy Transfer:
  • Exothermic: Energy released.
  • Endothermic: Energy absorbed.

Energy Terms in Chemical Reactions

• Exothermic ($\Delta H$ is negative): Energy is released.
• Endothermic ($\Delta H$ is positive): Energy is absorbed.

Symbols and Notations

• State symbols: Indicate energy changes due to phase transitions.
• Balancing equations: Includes $\Delta H$ notation for clarity.

Energy Profile Diagrams

• Exothermic Reactions:

• Endothermic Reactions:

Practical Lab Demonstrations

• Exothermic: Mix vinegar and baking soda.
• Endothermic: Dissolve ammonium nitrate in water.
• Use thermometers to measure energy changes.
• Safety Tips: Always wear goggles and gloves.

Calorimetry

• Simple Calorimeter: Suitable for basic measurements.
• Bomb Calorimeter: Provides precise results, especially for combustion reactions.

Worked Examples

Calculating Heat Change

1. Formula: $q = mc\Delta T$
2. Example: Determine the heat change for 100g of water heated from 25°C to 50°C.

Solution:

• Given: $m = 100\text{ g}$, $c = 4.18\text{ J/(g·°C)}$, $\Delta T = 50°C - 25°C = 25°C$
• Calculate $q = m \times c \times \Delta T = 100 \times 4.18 \times 25 = 10450 \text{ J}$
• Therefore, 10450 J of heat energy is transferred to the water.