SCNC1112 Chap.5 Acid-Base Reactions and Oxygen Reduction
Acid-Base Reactions and Oxygen Reduction
The reaction half of Module 1 begins with water acting as a reactant rather than only as a solvent. An acid donates a hydrogen ion and a base accepts one, and water does both. Because hydrogen-ion concentrations in ordinary solutions span many orders of magnitude, they are reported on a logarithmic scale, so pH is the negative base-ten logarithm of that concentration.
The single most common error in this area is treating one pH unit as a small change: it is a factor of ten. That scale immediately connects to the environmental material later in the course. Clean rainwater is already mildly acidic near pH 5.6, because carbon dioxide from the air dissolves and forms carbonic acid, so acid rain names rainfall well below that baseline.
The same dissolving reaction in seawater is the link between rising atmospheric carbon dioxide and falling ocean pH. Buffers, which mop up or release hydrogen ions as needed, are why human blood holds near pH 7.4 despite constant metabolic additions. The second and more powerful idea is that two separate questions must be asked of any proposed reaction.
The first is whether it is energetically favourable, which depends on the energy difference between products and reactants together with the change in entropy. The second is whether there is a route over the energy barrier fast enough to observe, which is a question about kinetics.
A reaction can pass the first and fail the second, and the course states this directly: a feasible reaction will not be observed on a reasonable timescale if the kinetics are slow. Catalysts, including the enzymes of Module 3, act only on the second question; they lower the barrier and cannot make an unfavourable change happen.
Three of the ten lectures are given to the reduction of oxygen, which is a strong signal about what is examinable. Oxygen readily accepts electrons and the bonds formed when it does are strong, so passing electrons to oxygen releases a large amount of energy. The same overall change, glucose and oxygen becoming carbon dioxide and water, happens in a flame and in a living cell, and releases the same total energy either way.
What differs is the schedule: a cell breaks the change into many enzyme-catalysed steps and captures part of the energy at several of them, while a flame releases all of it at once. Energy is only useful when its release is controlled, which is why a cell is not simply a small fire. The module closes by turning this into a design brief.
A reaction that is favourable but slow is a stored opportunity, and finding a catalyst for it releases energy or material that was locked behind a barrier. Biology has solved a great many such problems with enzymes built from abundant elements, at ordinary temperatures and in water, which is exactly the operating envelope industrial chemistry finds hardest.
What this chapter covers
- 01
Water as Both Acid and Base
- 02
The Logarithmic pH Scale
- 03
Buffers and Resistance to Change
- 04
Energetic Feasibility Against Kinetics
- 05
Catalysts and Activation Barriers
- 06
Controlled Against Uncontrolled Oxygen Reduction
Separating feasibility from rate for a proposed process
- 2Answer the first question. The change is energetically favourable, so nothing about the energy balance forbids it. The proposal passes the feasibility gate.
- 2Answer the second question. The nitrogen molecule is held by a triple bond, so the barrier to getting started is very high and almost no molecular collisions at room temperature carry enough energy to clear it.
- 2Name the class of solution. A catalyst provides an alternative route with a lower barrier, and raising the temperature increases the fraction of collisions able to clear whatever barrier remains.
- 2State the limit precisely. A catalyst changes the rate and leaves the energy balance untouched, so it can make a favourable reaction practical and can never make an unfavourable one happen. Certain micro-organisms solve the same problem with an enzyme at ordinary temperature.
Key terms
- pH
- The negative base-ten logarithm of the hydrogen-ion concentration, so a fall of one unit means ten times more hydrogen ions.
- Buffer
- A combination of substances that absorbs or releases hydrogen ions, reducing how far the pH moves when acid or base is added.
- Activation Barrier
- The energy that reacting particles must acquire before a reaction can proceed, which sets the rate independently of the energy released.
- Catalyst
- A substance that provides a lower-barrier route for a reaction and is not consumed, changing the rate without changing the energy balance.
- Enzyme
- A biological catalyst, usually a protein, whose three-dimensional shape makes it specific to one reactant or a narrow family.
- Reduction
- A gain of electrons by a substance, which for oxygen releases a large amount of energy because the bonds subsequently formed are strong.
Acid-Base Reactions and Oxygen Reduction FAQ
Why can a reaction be favourable and still never happen?
Because favourability and rate are decided by different quantities. Favourability compares the energy and entropy of products with reactants, while rate depends on how high the barrier is between them. Diamond converting to graphite is the standard illustration: the change is favourable at ordinary conditions and the barrier makes it take geological time.
Is respiration chemically different from burning?
The overall change is the same and releases the same total energy, because energy released depends only on the start and end states. The difference is the route. A cell breaks the change into many small enzyme-catalysed steps and captures part of the energy at several of them, whereas a flame releases everything at once as heat and light.
How much difference does one pH unit really make?
A factor of ten in hydrogen-ion concentration. A move from pH 6.5 to pH 5.5 means ten times more hydrogen ions, not a small drift, which is why reports on lake or ocean acidification have to be converted before their size can be judged.
Exam move
Draw the two gates as a decision tree and force three examples through it, including one that is favourable and slow. Then practise converting pH differences into concentration ratios in both directions until the factor of ten is automatic, because that conversion turns up in environmental questions as often as in chemistry ones.
