Grade 12 Session 3

Week 6
Grade 12 - Al Ma’arifa School

Spontaneous Vs
Non Spontaneous Reactions

​A spontaneous reaction is one that proceeds without needing continuous external energy input. Once it starts, it continues by itself, such as when paper burns in oxygen to form carbon dioxide and water. An example is:

On the other hand, a non-spontaneous reaction requires a continuous supply of energy to proceed, like photosynthesis:

1. Enthalpy change (ΔH): Reactions can release energy (exothermic) or absorb energy (endothermic).

2. Entropy change (ΔS): The degree of disorder in the system.

3. Temperature (T): Affects how entropy and enthalpy influence spontaneity.

​Chemical reactions can either be spontaneous or non-spontaneous. Three main factors determine whether a reaction is spontaneous:

Multiple Select

Which of the following is an example of a spontaneous reaction?

• Iron rusting in the presence of water and oxygen.

• Photosynthesis in plants.

• A battery being charged by electricity.

🌡️ Enthalpy Change (ΔH)

Enthalpy change represents the heat energy change in a reaction. It helps us understand whether a reaction releases or absorbs energy.

🔥 Exothermic Reactions (ΔH = Negative)

• Exothermic reactions release energy, typically as heat. Think of a campfire 🔥 — it releases heat into the surroundings!

• These reactions go from higher energy to lower energy, making the system more stable (just like how nature prefers stability 🌍).

  ➡️ Examples: Combustion, rusting, and freezing.

💡 Why are Exothermic Reactions Spontaneous?

• Nature loves reactions that release energy because they make things more stable. Lower energy = higher stability.

• Since exothermic reactions release energy, they are often spontaneous ⚡.

❄️ Endothermic Reactions (ΔH = Positive)

• Endothermic reactions absorb energy, usually heat, from the surroundings. Picture ice melting ❄️ — it needs heat to melt.

• These reactions move from lower energy to higher energy, which is unfavorable because higher energy means less stability.

  ➡️ Examples: Photosynthesis, melting ice, and cooking food.

💡 Why are Endothermic Reactions Non-Spontaneous?

• Since these reactions absorb energy, they are less likely to happen on their own. Nature doesn't like reactions that require extra energy! These tend to be non-spontaneous 🚫.

​🌀 Entropy (S): The Measure of Disorder 🌀 Entropy (S) is a measure of the degree of randomness or disorder in a system. It tells us how spread out or disorganized the energy in a system is. 🔹 Gases have the highest entropy because gas particles are free to move around, creating the most disorder. 🔹 Liquids have medium entropy, with more organization than gases but still a good amount of freedom for particles. 🔹 Solids have the lowest entropy because their particles are tightly packed and highly organized.

⚖️ The Second Law of Thermodynamics

Everything in nature tends to move towards a state of higher entropy (more disorder). This is one of the basic principles of the Second Law of Thermodynamics.
Essentially, the universe loves chaos! 🌌

This is how entropy works: over time, things naturally tend to go from order to disorder unless energy is used to keep them organized! 

🔺 Entropy Change (ΔS)

When a reaction takes place:

• ΔS = S_products - S_reactants

If the entropy of the products is greater than that of the reactants (S_p > S_r), the entropy change (ΔS) is positive (ΔS > 0), and the reaction tends to be spontaneous 🌟.

This means the system becomes more disordered, and nature favors this type of change.

Reactions that lead to higher entropy (more disorder) are typically favored. This is why gases (which have more disorder) are more stable than solids in terms of entropy.

Multiple Choice

Which of the following best represents the concept of increasing entropy?

• a) Organizing books neatly on a shelf.
  

• b) Cleaning up clothes scattered on the floor.
  

• c) Clothes, books, and toys gradually spreading across the room over time.
  

• d) Folding clothes and putting them in a drawer.

Multiple Choice

In terms of entropy, what does a messy room represent? 

• a) A system with low entropy (high order).
  

• b) A system with high entropy (high disorder).
  

• c) A system where energy is being added to create order.
  

• d) A system where energy is being removed to maintain order.

Multiple Choice

Which of the following processes would decrease the entropy of a system? 

• a) Leaving the bed unmade and scattering clothes on the floor.
  

• b) Cleaning the room and organizing all the items neatly.
  

• c) Allowing books and papers to pile up on the desk over time.
  

• d) Letting toys and items spread out across the floor.

🌡️ Entropy Change (ΔS)

• Entropy (S) is a measure of disorder in a system.

• When substances go from a state of higher entropy (more disorder) to a state of lower entropy (less disorder), the entropy change (ΔS) is negative.

  • Formula: ΔS = S_products - S_reactants = negative

🔻 Non-Spontaneous Reactions:

• When a reaction results in lower entropy (less disorder), the entropy change (ΔS) is negative, which is unfavorable for spontaneity.

• Reactions that lead to less disorder in a system tend to be non-spontaneous because they go against the natural tendency towards higher entropy (disorder)

In summary:

• If ΔS is negative, the reaction is likely non-spontaneous ⚠️.

🌍 Example of Lower Entropy:

A situation where a system becomes more ordered (such as freezing or condensation) is an example where entropy decreases and the reaction is less likely to occur on its own. This goes against nature’s preference for chaos!

Multiple Choice

Which of the following indicates a negative entropy change (ΔS < 0) in a reaction? 

• a) A gas turning into a liquid.
  

• b) Ice melting into water.
  

• c) Water evaporating into steam.
  

• d) Solid salt dissolving in water.

Multiple Choice

When a reaction leads to lower entropy (less disorder), the reaction is likely to be: 

• a) Spontaneous
  

• b) Non-spontaneous
  

• c) Exothermic
  

• d) Endothermic