L35. Chemistry and Conservation of Energy

Stoichiometry and Energy

R-report

L35. Chemistry and Conservation of Energy

How can a battery make a lamp glow while the total energy in the system stays the same?

Central Idea:

Chemical energy is stored in bonds and is not lost when a reaction happens; it is transformed into other forms such as thermal, light, electrical, or motion energy, so the total energy of an isolated system is conserved. For example, in a simple galvanic cell a zinc oxidation and copper reduction pair can be written as Zn → Zn2+ + 2e− and Cu2+ + 2e− → Cu, overall Zn + Cu2+ → Zn2+ + Cu; atoms and charge are conserved while chemical energy drives electron flow. This is why a reaction that warms its surroundings has released chemical energy into thermal energy, not destroyed it.

How To Observe Changes:

Gather materials and identify the chemical system (battery, lamp, wires, multimeter), record what you measure: voltage (V), current (A), temperature change (°C) and light output, separate raw observations from your explanations, sketch the circuit and note energy flows, run a control where the lamp is off or the battery is disconnected, test reversibility by recharging or attempting to reassemble reactants when safe, check for new substances (change in color, precipitate, gas) and measure mass before and after if possible, calculate energy with E = V·I·t (include units) to track how much chemical energy became electrical and then light or thermal energy.

Apply To New Situations:

If you measure a temperature rise and light when two chemicals mix, then chemical energy likely converted to thermal and radiant energy; else if temperature stays the same and only shape or phase changed, then the change was probably physical. To falsify a chemical-change hypothesis look for evidence that would not fit it: if you can reverse the change by simple physical means (freeze, evaporate, separate), or if the mass and identities of substances are unchanged, that suggests no new substance formed. Quick tests: try to reverse the change, look for a gas (bubbles, odor), for a precipitate (solid forms), and measure mass before and after; also test electrical output with a circuit if you expect electron flow. Record all measurements with units and compare energy calculated from electrical measurements to observed thermal or light energy to check conservation.

Synthesize Ideas:

Conclusions: energy is conserved during chemical processes even when forms change; use precise terms like 'chemical energy', 'electrical energy', 'thermal energy', and 'energy transfer' rather than saying energy is 'lost'. Check equations and balances: Zn → Zn2+ + 2e−, Cu2+ + 2e− → Cu, overall Zn + Cu2+ → Zn2+ + Cu shows atoms and charge conserved while electrons move. In energy accounting use units (joules): for a 3.0 V battery delivering 0.5 A for 10 s, E = V·I·t = 3.0 V·0.5 A·10 s = 15 J, which should appear as electrical output then light or heat.

Check units and include them in every calculation, tell the particle-level story (which bonds broke and formed, which electrons moved), confirm atoms and charge balance in chemical equations, look for independent evidence (temperature change, light, gas, mass change), and report whether energy was transferred or transformed. Also state uncertainty and show units for all reported numbers. Include simple diagrams that label energy flows and units used.