Concept 2.4: Chemical Reactions – How Chemical Bonds Are Made and Broken

Every process that keeps you alive is a chemical reaction that breaks old bonds and builds new ones. If you can write the equation for water formation, balance photosynthesis, and explain why a reaction stops changing even though molecules are still colliding, you own the foundation for metabolism, respiration, and ecology questions on AP Biology, A-Level Biology, IB Biology HL, College Biology and USABO. In this definitive guide you will master what a chemical reaction really is, how matter is conserved while electrons are redistributed, why photosynthesis is the ultimate example of rearrangement with sunlight energy input, how reversibility and concentration control reaction rates, and what chemical equilibrium means for life.

Key Insights

• Chemical reactions break existing chemical bonds and form new bonds by redistributing electrons.
• Matter is conserved in reactions because atoms are rearranged, not created or destroyed.
• Photosynthesis rearranges CO₂ and H₂O into glucose and O₂ using sunlight energy.
• All reactions are theoretically reversible and reaction rate depends on reactant concentration.
• Chemical equilibrium is a dynamic state where forward and reverse rates are equal and concentrations stabilize.

Chemical reactions make and break chemical bonds

The making and breaking of chemical bonds, leading to changes in the composition of matter, are called chemical reactions. This is the central definition you must memorize. An atom does not disappear. Its valence electrons are simply placed into new positions around different nuclei.

A classic example is the reaction between hydrogen and oxygen molecules that forms water. This reaction is used in Campbell as the anchor example for the entire concept because it is clean, balanced, and biologically fundamental.

Chemical reaction 2 H₂ + O₂ forms 2 H₂O showing covalent bond breaking and forming and conservation of matter

In that reaction, the covalent bonds of H₂ and O₂ are broken, and the new O-H covalent bonds of H₂O are formed. No new electrons appear and none are lost. Each single H-H bond is a pair of shared electrons. Each O=O double bond is two shared pairs. When the reaction occurs, those shared pairs are split and then redistributed so that each oxygen now shares one pair with each of two hydrogens.

When we write the equation for a chemical reaction, we use an arrow to indicate the conversion of the starting materials, called the reactants, to the resulting materials, or products. The arrow means "is converted to" or "yields." It is not an equals sign.

The coefficients indicate the number of molecules involved. For example, the coefficient 2 before H₂ means that the reaction starts with two molecules of hydrogen. You read 2 H₂ + O₂ → 2 H₂O as "two molecules of hydrogen plus one molecule of oxygen yield two molecules of water."

This accounting is not optional. Notice that all atoms of the reactants must be accounted for in the products. Two H₂ gives you 4 H atoms on the left. O₂ gives you 2 O atoms on the left. On the right, 2 H₂O also gives you 4 H and 2 O. If the count does not match, the equation is not correct.

Callout Box:Conservation of Matter Is Non-Negotiable: Matter is conserved in a chemical reaction. Reactions cannot create or destroy atoms but can only rearrange, or redistribute, the electrons among them. This is the law of conservation of mass applied to biology. If you start with 6 carbons, 12 hydrogens, and 18 oxygens, you must end with 6 carbons, 12 hydrogens, and 18 oxygens, even if they are in completely different molecules. This is why balancing equations is tested so heavily.

Term in Equation	Meaning for Exams	Example in Water Formation
Reactants	Starting materials that are consumed, left side of arrow	2 H2 + O2
Products	New materials that are formed, right side of arrow	2 H2O
Coefficient	Number of molecules, multiplies entire molecule	2 in front of H2 and H2O
Subscript	Number of atoms inside one molecule, never changed to balance	2 in H2, H2O, O2
Arrow	Direction of conversion, indicates redistribution of electrons	→

Exam Trap:Students change subscripts to balance an equation. You must never change H₂O to H₃O to make numbers work. Change only coefficients. Changing subscripts changes the molecule itself into a different substance. AP, A-Level, and IB examiners will give zero marks for that.

Photosynthesis, which takes place within the cells of green plant tissues, is an important biological example of how chemical reactions rearrange matter. Humans and other animals ultimately depend on photosynthesis for food and oxygen, and this process is at the foundation of life in almost all ecosystems.

Photosynthesis equation 6 CO₂ + 6 H₂O plus sunlight yields glucose C₆H₁₂O₆ + 6 O₂ showing atom rearrangement

The overall reaction is summarized as:

6 CO₂ + 6 H₂O + Sunlight → C₆H₁₂O₆ + 6 O₂

The raw materials of photosynthesis are carbon dioxide (CO₂) and water (H₂O), which land plants absorb from the air and soil, respectively. Within the plant cells, sunlight powers the conversion of these ingredients to a sugar called glucose (C₆H₁₂O₆) and oxygen molecules (O₂), a by-product that can be seen when released by a water plant such as Elodea as bubbles of O₂ gas escaping from leaves submerged in water.

Although photosynthesis is actually a sequence of many chemical reactions involving photosystems, electron transport chains, the Calvin cycle, and dozens of enzymes, we still end up with the same number and types of atoms that we had when we started. Matter has simply been rearranged, with an input of energy provided by sunlight. On the left you have 6 carbons, 12 hydrogens, and 18 oxygens. On the right you also have 6 carbons in glucose, 12 hydrogens in glucose, and 6 oxygens in glucose plus 12 oxygens in 6 O₂, total 18 oxygens.

Sunlight does not provide atoms. It provides energy that is captured to make the rearrangement energetically possible and to produce high-energy covalent bonds in glucose that store chemical energy.

Exam Tip:For AP Biology and A-Level, when asked to explain photosynthesis as a chemical reaction, always mention three things: 1) Matter is conserved but rearranged, 2) Energy input from sunlight is required, 3) The actual process is many sequential reactions, not one step. That third point earns the synthesis mark.

All chemical reactions are theoretically reversible, with the products of the forward reaction becoming the reactants for the reverse reaction. For example, hydrogen and nitrogen molecules can combine to form ammonia, but ammonia can also decompose to regenerate hydrogen and nitrogen:

3 H₂ + N₂ ⇌ 2 NH₃

The two opposite-headed arrows indicate that the reaction is reversible. One arrow pointing right is the forward reaction, one pointing left is the reverse. In a closed system, both occur at the same time.

Reversible reaction 3 H2 + N2 ⇌ 2 NH3 showing forward and reverse rates and dynamic chemical equilibrium graph

One of the factors affecting the rate of a reaction is the concentration of reactants. The greater the concentration of reactant molecules, the more frequently they collide with one another and have an opportunity to react and form products. Rate depends on collision frequency. This is why increasing CO₂ concentration in a greenhouse can increase photosynthetic rate, up to a point, and why oxygen production by Elodea speeds up when more dissolved CO₂ is available.

The same holds true for products. As products accumulate, collisions resulting in the reverse reaction become more frequent. Imagine starting with only H₂ and N₂ and no NH₃. At first, only forward collisions are possible. As NH₃ builds up, NH₃ molecules begin colliding and decomposing back to H₂ and N₂. The reverse rate climbs.

Eventually, the forward and reverse reactions occur at the same rate, and the relative concentrations of products and reactants stop changing. The point at which the reactions offset one another exactly is called chemical equilibrium.

This is a dynamic equilibrium. Reactions are still going on in both directions, but with no net effect on the concentrations of reactants and products. Equilibrium does not mean that the concentrations of reactants and products are equal in concentration, but only that their concentrations have stabilized at a particular ratio.

Callout Box:Dynamic vs Static - The Most Misunderstood Idea in Chapter 2: Chemical equilibrium is not when everything stops moving. It is when forward and reverse movement are balanced. Think of people entering and leaving a store at the same rate. The number inside the store stays constant, but individuals are constantly coming and going. At equilibrium, molecules are still breaking and forming bonds every second.

The reaction involving ammonia reaches equilibrium when ammonia decomposes as rapidly as it forms. The equilibrium concentration ratio for 3 H₂ + N₂ ⇌ 2 NH₃ at a given temperature is constant, but the actual amounts of H₂, N₂, and NH₃ present at equilibrium depend on starting conditions.

In some chemical reactions, the equilibrium point may lie so far to the right that these reactions go essentially to completion; that is, virtually all the reactants are converted to products. Combustion of glucose in cellular respiration is an example where forward drive is very strong, though even that reaction is technically reversible in different metabolic pathways like gluconeogenesis.

Concept	What It Means	What It Does NOT Mean
Chemical equilibrium reached	Forward rate = Reverse rate	Reaction has stopped
Concentrations stable	No net change in or over time	=
Dynamic	Bonds continue breaking and forming both ways	Static, no molecular activity
Reversible arrow ⇌	Reaction can proceed forward or backward	It is equally likely both ways

We will return to the subject of chemical reactions after more detailed study of the various types of molecules that are important to life. In the next chapter, we focus on water, the substance in which all the chemical processes of organisms occur. Water is the solvent where concentrations matter, where photosynthesis reactants dissolve, and where equilibrium is biologically relevant.

Callout Box:Why This Concept Connects to Everything After: Without Concept 2.4 you cannot understand metabolism. Catabolic reactions like hydrolysis break bonds and release energy. Anabolic reactions like dehydration synthesis make bonds and require energy. Both obey conservation of matter and both can reach equilibrium. Photosynthesis and cellular respiration are essentially reverse reactions of each other, which is why the WHAT IF question about using photosynthesis products as respiration reactants is a classic exam twist.

Exam Tip:When you see a reversible arrow in AP or IB, immediately think about Le Chatelier's principle even if not named in this chapter: adding more reactant pushes equilibrium to make more product, adding more product pushes it back to reactant. Concentration is the lever.

Exam Trap:Students write that at equilibrium no reactions occur, or that reactants and products are equal in amount. Both lose marks. Always write "rates are equal, concentrations are stable but not necessarily equal, dynamic process continues."

Practice Questions

MCQs (Multiple Choice Questions)

01. What happens to atoms during a chemical reaction?

1. They are created
2. They are destroyed
3. They are rearranged with electron redistribution
4. They change into other elements

Correct: C
Explanation: Law of conservation of matter. Atoms are conserved. Bonds break and new bonds form via electron rearrangement. Elements do not transmute in biological reactions.

02. In the equation 2 H₂ + O₂ → 2 H₂O, the coefficient 2 before H₂ means:

1. Two atoms of hydrogen
2. Two molecules of hydrogen are reactants
3. Two molecules of water
4. Hydrogen has a subscript of 2

Correct: B
Explanation: Coefficient multiplies the whole molecule. 2 H₂ = two diatomic hydrogen molecules = 4 H atoms total. Subscript is inside molecule.

03. Which statement correctly accounts for atoms in 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂?

1. Matter is not conserved because glucose is more complex
2. 6 C, 12 H, 18 O on both sides
3. Energy from sunlight adds extra atoms
4. Oxygen atoms are lost

Correct: B
Explanation: Left: 6 C from CO₂, 12 H from H₂O, 18 O (12 from CO₂ + 6 from H₂O). Right: 6 C in glucose, 12 H in glucose, 18 O (6 in glucose + 12 in 6 O₂). Sunlight provides energy, not atoms.

04. Photosynthesis is described as a sequence of many reactions but summarized as one equation. Why is this valid?

1. Atoms are still conserved in the net sum
2. Only one enzyme is involved
3. Light is a reactant atom
4. Oxygen is not a product

Correct: A
Explanation: Net equation cancels intermediates. Even though dozens of steps occur, initial atoms and final atoms balance. This is standard for summarizing metabolism.

05. What does the symbol ⇌ indicate in 3 H₂ + N₂ ⇌ 2 NH₃?

1. Reaction goes only forward
2. Reaction goes only backward
3. Reaction is reversible and can go both directions
4. Reaction has reached equilibrium

Correct: C
Explanation: Double opposite arrows denote reversibility. It does not alone mean equilibrium has been reached, only that reverse is possible. Equilibrium is a state where rates equal.

06. Which factor directly increases collision frequency and thus reaction rate?

1. Decreasing reactant concentration
2. Increasing reactant concentration
3. Removing all reactants
4. Reaching equilibrium

Correct: B
Explanation: Higher concentration means more molecules per volume, more collisions per second, higher opportunity to react. This is a core principle for enzyme kinetics later.

07. At chemical equilibrium, which is true?

1. Forward and reverse reactions have stopped
2. Forward rate equals reverse rate and concentrations stabilize
3. Concentrations of reactants and products are equal
4. Only products exist

Correct: B
Explanation: Dynamic equilibrium: rates equal, no net change. Concentrations are stable at a particular ratio, not necessarily equal. Reactions continue both ways.

08. Why does adding more H₂ to the ammonia system at equilibrium increase NH₃?

1. It destroys N₂
2. It increases forward collisions to form more NH₃ until new equilibrium
3. It stops reverse reaction completely
4. It violates conservation of matter

Correct: B
Explanation: Le Chatelier shift. More reactant raises forward rate temporarily, consuming added H₂ and some N₂ to make more NH₃ until rates equal again. Reverse still occurs.

09. Which is the reverse of photosynthesis that occurs in your cells?

1. 6 O₂ + C₆H₁₂O₆ → 6 CO₂ + 6 H₂O + Energy
2. C₆H₁₂O₆ → 6 CO₂ + 6 H₂O
3. 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
4. 2 H₂ + O₂ → 2 H₂O

Correct: A
Explanation: Cellular respiration uses glucose and oxygen as reactants to produce CO₂, water, and usable energy (ATP). It is essentially photosynthesis in reverse, linking to breathing.

10. Some reactions go essentially to completion. What does this mean about equilibrium?

1. Equilibrium lies far to the right, almost all reactants become products
2. No equilibrium exists
3. Reaction is not reversible even in theory
4. Matter is not conserved

Correct: A
Explanation: All reactions are theoretically reversible, but equilibrium point may be so product-favored that completion appears to happen. Virtually all reactant converted, but tiny reverse still theoretically possible.

FRQs (Free Response Questions)

FRQ 1: Explain in terms of bond breaking and formation how 2 H₂ + O₂ → 2 H₂O represents a chemical reaction and conservation of matter. (4 points)

Rubric:

• H-H and O=O covalent bonds broken, electrons redistributed (1 pt)
• New O-H bonds formed in water (1 pt)
• Atoms not created/destroyed, only rearranged (1 pt)
• Counting: 4 H and 2 O atoms both sides, balanced (1 pt)

FRQ 2: Describe photosynthesis as an example of chemical reactions rearranging matter. Include reactants, products, source of raw materials, and role of sunlight. (5 points)

Rubric:

• Equation 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂ (1 pt)
• CO₂ from air, H₂O from soil/roots absorbed by plant (1 pt)
• Light provides energy, not atoms, powers conversion (1 pt)
• Glucose stores chemical energy in new C-C, C-H, C-O bonds, O₂ is by-product visible as bubbles from Elodea (1 pt)
• Net conservation: same number/type atoms before and after despite being multi-step sequence (1 pt)

FRQ 3: A student says "At equilibrium the amount of reactants equals the amount of products and reactions stop." Correct this statement using ammonia example 3 H₂ + N₂ ⇌ 2 NH₃. (4 points)

Rubric:

• Equilibrium means forward rate equals reverse rate, not amounts equal (1 pt)
• Concentrations stable at particular ratio, not necessarily equal (1 pt)
• Dynamic: both forward (forming NH₃) and reverse (decomposing NH₃) continue (1 pt)
• Ammonia decomposes as fast as it forms at equilibrium; analogy of people entering/leaving store (1 pt)

FRQ 4: Using collision theory, explain how reactant concentration affects reaction rate and how product accumulation leads to equilibrium. (4 points)

Rubric:

• Higher reactant concentration increases collision frequency and opportunity to react (1 pt)
• More collisions → higher forward rate initially (1 pt)
• As products accumulate, reverse collisions become more frequent, reverse rate increases (1 pt)
• When forward rate = reverse rate, net concentrations stop changing, equilibrium reached (1 pt)

FRQ 5: WHAT IF? Write equation using photosynthesis products as reactants and photosynthesis reactants as products. Name the cellular process and relate to breathing. (4 points)

Rubric:

• Equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy (1 pt)
• Process is cellular respiration / aerobic respiration (1 pt)
• In words: glucose and oxygen converted to carbon dioxide and water with release of energy for ATP (1 pt)
• Breathing takes in O₂ as reactant and expels CO₂ as product, linking gas exchange to this chemical reaction (1 pt)

Frequently Asked Questions

What is a chemical reaction in biology?

A chemical reaction is the breaking of existing chemical bonds and forming of new bonds, rearranging matter. Reactants are converted to products by redistributing electrons. Atoms are conserved, not created or destroyed, which is essential for metabolism and photosynthesis questions.

What do reactants and products mean?

Reactants are starting materials consumed during a reaction, shown left of the arrow. Products are new materials formed, shown right of the arrow. Coefficients tell how many molecules are involved. Balancing ensures all atoms from reactants appear in products.

How is photosynthesis a chemical reaction?

Photosynthesis rearranges 6 carbon dioxide and 6 water molecules using sunlight energy into one glucose and 6 oxygen molecules. Same atoms exist before and after, just in new bonds. It is actually many sequential reactions summarized as one net equation.

Are all chemical reactions reversible?

Yes, theoretically all are reversible. Products of the forward reaction can become reactants for the reverse. The symbol ⇌ shows reversibility. In cells, forward and reverse often use different pathways and enzymes but the chemical principle holds.

What affects the rate of a chemical reaction?

Concentration of reactants is a major factor. Higher concentration increases collision frequency and reaction rate. Temperature, enzyme presence, and surface area also affect rate, but concentration dependence is emphasized in Campbell Concept 2.4.

What is chemical equilibrium?

Chemical equilibrium is the point where forward and reverse reaction rates are equal, so reactant and product concentrations stop changing. It is dynamic—reactions continue both ways—but with no net change. It does not mean concentrations are equal.

Does equilibrium mean reactants and products are equal?

No. This is a common exam trap. Equilibrium means concentrations have stabilized at a particular ratio, not that they are equal. The ratio depends on the reaction. Some equilibria lie far to the right, going almost to completion.

How does chemical equilibrium relate to cellular respiration?

Cellular respiration C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy is essentially the reverse of photosynthesis. Breathing brings in O₂ as a reactant and removes CO₂ as a product, connecting organism-level gas exchange to molecular equilibrium and energy release.

Exam Mapping

Concept	A-Level Biology (AQA/OCR/Edexcel)	AP Biology	IB Biology HL	College Bio 1 & 2	USABO / IBO
Reaction definition, bonds break/form	1.1 Biological molecules, bond breaking in metabolism	Unit 1 Chemistry of Life, SYI-1 chemical reactions rearrange matter	2.1 Chemical basis, reactants → products	Ch 2 Chemical Context, chemical reactions	Definition of chemical reaction, electron redistribution
Balancing equations, conservation	Balancing symbol equations, conservation of mass	Conservation of matter, balancing photosynthesis equation	Balancing chemical equations, atom accounting	Stoichiometry basics, conservation law	Balancing, law of conservation
Photosynthesis as rearrangement example	Photosynthesis equation, limiting factors, Elodea experiment	Photosynthesis equation, energy input, matter rearranged	2.9 Photosynthesis overall equation, experimental evidence with O2 bubbles	Photosynthesis summary, light as energy not matter	Overall equation, raw materials source, multi-step nature
Reversible reactions ⇌	Reversible reactions, enzyme reversibility	Reversible metabolic reactions, forward/reverse	Reversible arrows, ammonia example	Reversible, hydrogenation/dehydrogenation	Meaning of ⇌, example 3 H2 + N2 ⇌ 2 NH3
Concentration and rate	Effect of substrate concentration on rate, collision theory	Collision frequency, rate increases with concentration	Factors affecting rate, concentration	Kinetics introduction, concentration dependence	Rate vs concentration, Le Chatelier introduction
Chemical equilibrium, dynamic	Equilibrium, stable concentrations, does not mean equal	Dynamic equilibrium, forward rate = reverse rate, not equal concentrations	Chemical equilibrium definition, dynamic nature, completion	Equilibrium, ratio, dynamic, completion far right	Dynamic equilibrium, concentrations stable not equal, completion

Conclusion

Chemical reactions are the bookkeeping of life: bonds break, electrons are redistributed, new bonds form, but atoms themselves never disappear. The simple arrow in 2 H₂ + O₂ → 2 H₂O hides bond breaking of H-H and O=O and bond making of O-H while conserving 4 hydrogens and 2 oxygens. Photosynthesis scales this same principle up to global importance, taking 6 CO₂ from the air and 6 H₂O from the soil and, powered by sunlight, rearranging them into energy-rich glucose C₆H₁₂O₆ and oxygen O₂, a sequence of many reactions whose net still obeys conservation. Because every reaction is theoretically reversible, concentration governs how often molecules collide and react, and as products build up the reverse reaction speeds up until forward and reverse rates match. That balanced, dynamic state called chemical equilibrium is not a dead stop, it is a busy two-way street with stable concentrations, sometimes lying so far to the product side that reactions appear to go to completion. Mastering these ideas prepares you for every pathway from hydrolysis to respiration, where matter is always conserved and energy is what makes the rearrangement possible.

References

Campbell Biology by Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Jane B. Reece - Latest Edition, Concept 2.4 Chemical reactions make and break chemical bonds. Primary source for reaction definition, reactants and products, coefficient meaning, conservation of matter, photosynthesis as example, reversibility, concentration effect on rate, and chemical equilibrium including dynamic nature and completion.