Connecticut Fall Foliage: Pigment Bond Chemistry

Connecticut Phenomenon · Deciduous Forests, New England · NGSS HS-PS1-4

Every autumn in Connecticut, hillsides of green transform into blazing reds, oranges, and yellows. This stunning color change is driven by chemistry at the covalent bond level. As days shorten and temperatures drop, enzymes break the conjugated C=C bonds in chlorophyll — an endothermic reaction that absorbs energy from the surroundings. Simultaneously, cooler nights trigger new covalent bond formation in anthocyanin synthesis — an exothermic reaction that releases energy. Together, these bond-level changes determine which pigments are present, and therefore what colors we see.

NGSS HS-PS1-4: Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy. Observable Feature 1.a.ii–iii: model shows bonds broken (energy in) and bonds formed (energy out) as distinct events.

Controls

CT Season

Sept 1

Temperature

−5 °C 22 °C 25 °C

Day Length

9 h 13.0 hrs 14 h

Pigment Levels

🟢 Chlorophyll 100%

🟡 Carotenoid (visible) 0%

🔴 Anthocyanin 0%

Net Reaction Direction

—

🔬 Prediction Prompt

As temperature drops and days shorten, the leaf will turn:

💡 Key Concept (HS-PS1-4)

When bonds break, energy must be absorbed from the surroundings (endothermic). When bonds form, energy is released to the surroundings (exothermic). The net direction depends on which total bond energy change is larger. In fall foliage: chlorophyll C=C bond breaking absorbs energy (blue arrow ←); new anthocyanin C–C bonds form and release energy (orange arrow →).

Data & Analysis

Bond Energy Change

Relative units — qualitative model (Assessment Boundary: no kJ/mol totals).

Light Absorption Spectrum

Remaining pigments absorb light — what reflects determines leaf color.

📌 Did You Know?

Chlorophyll has 11 conjugated C=C double bonds in its porphyrin ring — each bond absorbs specific wavelengths of light. When those bonds break in autumn, the green color disappears and the always-present yellow carotenoids (with their own conjugated system) are revealed. Anthocyanins are newly synthesized, giving the most vivid red and purple hues.

📊 Analysis Questions

Move the temperature slider from 20 °C to 5 °C. Describe what happens to the chlorophyll level and the leaf color. Which bonds are breaking, and is this reaction endothermic or exothermic? Justify using the energy arrow direction on the canvas.
At a temperature of 10 °C and a day length of 10.5 h, which reaction dominates — chlorophyll breakdown or anthocyanin synthesis? How does the bond energy chart show this?
Why does the leaf not turn red if the temperature drops below about 2 °C? Use the anthocyanin bar and the molecular diagram to explain.
If chlorophyll bond breaking is endothermic (energy in from surroundings), where does that energy come from in a real leaf? What environmental changes in Connecticut autumn reduce that energy source?
Compare the light absorption spectra at September 1 vs. October 25 conditions. Which wavelengths are now less absorbed? How does this explain the change in reflected color?