2.3 Chemical Reactions Study Notes

2.3 Chemical Reactions

Learning Objectives

• By the end of this section, you will be able to:

  • Distinguish between kinetic and potential energy, including exergonic and endergonic chemical reactions.

  • Identify four forms of energy important in human functioning.

  • Describe the three basic types of chemical reactions.

  • Identify several factors influencing the rate of chemical reactions.

Metabolism

• Metabolism is the total sum of all chemical reactions that sustain an organism’s health and life.

• Types of metabolic processes:

  • Anabolic reactions: build larger molecules from smaller molecules or atoms (e.g., constructing proteins from amino acids).

  • Catabolic reactions: break down larger molecules into smaller molecules, releasing smaller atoms or molecules (e.g., digestion).

• Both types of reactions involve exchanges of matter and energy.

The Role of Energy in Chemical Reactions

• Chemical reactions require sufficient energy for reactants to collide with enough force to break old bonds and form new ones.

Kinetic and Potential Energy

• Kinetic energy: energy of matter in motion.

  • Example: Lifting and placing a brick on a wall utilizes kinetic energy.

• Potential energy: energy of position, or energy stored due to the arrangement of molecular structures.

  • Example: A brick wall stores potential energy that can be converted back to kinetic energy if it collapses.

• In the human body, potential energy is stored in bonds between atoms and molecules, referred to as chemical energy.

  • When chemical bonds form, chemical energy is invested; when they break, chemical energy is released.

Exergonic and Endergonic Reactions

• Exergonic reactions: release more energy than they absorb.

  • Example: Metabolic breakdown of food releases energy used by the body, some as heat.

• Endergonic reactions: absorb more energy than they release, requiring an energy input.

  • The resulting molecules store energy not only from their original components but also from the energy input.

  • Often linked to exergonic reactions providing the energy needed for these reactions.

Forms of Energy Important in Human Functioning

1. Chemical Energy: Energy stored in chemical bonds, crucial for metabolism.

2. Mechanical Energy: Energy stored in physical systems that powers movement.

   • Example: Muscles produce mechanical energy when lifting weights.

3. Radiant Energy: Energy transmitted as waves, varying in wavelength (e.g. electromagnetic spectrum).

   • Example: Ultraviolet energy from sunlight helps synthesize vitamin D in the skin.

4. Electrical Energy: Energy from electrolytes affecting voltage changes in cells.

   • Crucial in transmitting nerve impulses and muscle contractions.

Characteristics of Chemical Reactions

• Chemical reactions start with reactants: the substances entering the reaction (e.g., sodium and chloride forming table salt).

• The outcome of a chemical reaction is products: the resulting substances from the chemical reaction.

• Law of conservation of mass: matter cannot be created or destroyed. Therefore, all atoms present in the products must be accounted for in the reactants.

Chemical Equations

• Chemical equations display how reactants transform into products, read from left to right using arrows instead of an equal sign.

• Example:

  • Formation of ammonia: $N + 3H <br>ightarrow NH_3$

  • Decomposition of ammonia: $NH_3 <br>ightarrow N + 3H$

• Synthesis Reaction: a reaction forming larger compounds from separate components. General equation: $A + B <br>ightarrow AB$

• Decomposition Reaction: breaks down larger compounds into their constituent parts. General equation: $AB <br>ightarrow A + B$

• Exchange Reaction: involves both synthesis and decomposition, forming and breaking bonds. Possible general equations:

  • $A + BC <br>ightarrow AB + C$

  • $AB + CD <br>ightarrow AC + BD$

• Reversible reactions: indicated with double arrows. Example: $A + BC <br>ightleftharpoons AB + C$

Factors Influencing the Rate of Chemical Reactions

1. Properties of the Reactants:

   • Greater surface area facilitates easier collisions among reactants (e.g., chewing food increases surface area for digestion).

   • Gases react faster than liquids or solids.

   • Smaller molecules tend to react faster due to having fewer bonds.

   • Reactive elements (e.g., hydrogen) react more quickly than less reactive elements (e.g., helium).

2. Temperature:

   • Higher temperatures increase the kinetic energy of particles, leading to faster reactions.

   • Increased thermal energy equals faster moving particles, raising collision likelihood.

3. Concentration and Pressure:

   • Higher concentrations of reactants lead to an increased number of collisions.

   • Reducing the volume increases pressure, raising the concentration of particles in a reaction environment (e.g., a crowded dance floor).

4. Enzymes and Catalysts:

   • Catalysts: substances increasing the reaction rate without undergoing change themselves.

   • Enzymes: biological catalysts primarily made of proteins or RNA.

   • They reduce the activation energy required for reactions, making biochemical processes more efficient.

   • Most chemical reactions in the human body are facilitated by enzymes, vital for food breakdown and energy conversion.