Video 4 Chemical Reactions: Kinetics, Equilibria, Catalysis, and Cellular Implications

Water (H2O) is a unique substance, different from its building blocks, hydrogen (H2) and oxygen (O2). When H2 and O2 combine, they create water, which has its own distinct characteristics.
If you break water down, you get hydrogen and oxygen back. But when hydrogen and oxygen combine to form water, the water is a completely new substance with different properties than the gases it came from.
Consider table salt (sodium chloride, NaCl). Sodium metal (Na) is very reactive, and chlorine gas (Cl2) is toxic. Yet, when they combine, they form safe table salt, which we use for food. In our bodies, sodium ions (Na+) and chloride ions (Cl−) also play crucial roles.
Here's a simple example showing how atoms rearrange during a reaction, rather than creating new elements:
- Left: methane and molecular oxygen; Right: carbon dioxide and water
- Let's check the balance — you'll find the same number of each type of atom on both sides:
- $ext{C}: 1 \, , \, ext{H}: 4 \, , \, ext{O}: 4$
- Chemical equation (illustrative):
- $ext{CH}4 + ext{O}2 \rightarrow ext{CO}2 + ext{H}2 ext{O}$
The main idea is that in reactions, atoms simply rearrange. The original substances turn into new ones with different properties, but the atoms themselves remain unchanged.

Atoms and elements are always preserved. A reaction simply shows them moving from their starting forms (reactants) to new forms (products), which have different traits.
Left side: methane (CH4) and molecular oxygen (O2); Right side: carbon dioxide (CO2) and water (H2O).
Understanding this rearrangement is how chemists write reactions and make sure all atoms are accounted for on both sides of the equation.

Kinetics, or reaction rate, tells us how fast products are made. This speed depends on how often and how effectively molecules bump into each other.
Reaction arrows can sometimes point both ways, showing that reactions can go forward (making products) and backward (making original reactants). The thickness of these arrows shows how likely a collision is to cause a reaction, not just how often collisions happen.
If making products is easier, then more products will form. But as more products build up, they will also start colliding and reacting to turn back into the original reactants.
Equilibrium is the point where the forward and reverse reaction rates are equal. This balance doesn't always mean a 50/50 mix of reactants and products; it depends on how favorable each reaction direction is.
For example, if a reaction starts with a lot of 'red' and 'green' reactants and the forward reaction is favored, you might end up with two-thirds 'purple/blue' product and one-third 'red/green' reactant. The exact balance at equilibrium relies on the energy barriers (activation energies) and how likely successful collisions are.

Activation energy is like a hurdle that molecules need to jump over for a collision to actually cause a chemical reaction.
A lower hurdle means it's easier for reactions to happen. A higher hurdle means reactions are less likely.
Reactions that are 'energetically favorable' (exothermic) release energy. These reactions tend to move forward more easily. For instance, burning methane (like in cooking gas): $ext{CH}4 + ext{O}2 \rightarrow ext{CO}2 + ext{H}2 ext{O}$ releases energy.
Remember: energetics tells you if a reaction will likely happen (energy-wise), while kinetics tells you how fast it will happen.

Catalysts are like special tools that help reactants meet up in exactly the right way so they can react. In living things, these catalysts are called enzymes.
Catalysts don't change the final balance (equilibrium) of a reaction. Instead, they speed up both the forward and reverse reactions, helping the system reach that balance much faster.
Imagine a demonstration: with a catalyst (like an enzyme), a reaction can finish much quicker (reaching its maximum speed, or Vmax) without changing the final mix of substances at equilibrium. For instance, converting 'red + green' to 'purple + blue':
- With an enzyme, it might take only about $40\,\text{seconds}$ to almost finish.
- Without an enzyme, it could take roughly $9\,\text{minutes}\,25\,\text{seconds}$ to get to the same point.

Cells need to create and copy these special shape-based catalysts (enzymes) so they can control specific chemical reactions over and over again.
The cell's internal machinery, like DNA and the process of making proteins, allows it to build and replicate these important catalysts.
Besides making reactions happen with catalysts, cells also need ways to move things in and out through their membranes, such as channels, gates, and pumps. This helps them control their internal chemistry.
In short, life depends on carefully organizing the making of catalysts and systems for transport to guide its metabolic processes and keep things balanced inside (homeostasis).