Lesson 2 of 6 · Conserve atoms, account for energy
Chemical bonds and reactions
9 minute lesson · 3 checks along the way
Your goal for this lessonBalance a reaction, identify a limiting reactant, and distinguish net energy change from reaction speed.
Words with a dotted underline open a short definition right here in the lesson.
Rearrange the connections
Chemical bonding concerns the interactions that hold or together. In a covalent bond, atoms share electrons; an ionic solid is held together by attractions among charged constituents. A reaction changes the chemical arrangement, producing different substances. A formula records composition, while a balanced equation records the proportions in which reactants are consumed and products formed. Neither is a guarantee that the reaction proceeds quickly under every condition.
For the simplified complete combustion of methane, write CH₄ + 2 O₂ → CO₂ + 2 H₂O. There is one carbon atom, four hydrogen atoms, and four oxygen atoms on each side. The conserve each separately. The equation describes one chosen reaction; a real environment can have competing pathways, incomplete conversion, or different products. We will stipulate the simple pathway before using it.
Find the ingredient that runs out
Start an invented closed collection with five methane and eight oxygen molecules. Assume this reaction goes as far as those supplies allow, with no competing reactions. Each methane requires two oxygen molecules, so eight oxygen can react with only four methane. Oxygen is the . The final collection contains four carbon dioxide molecules, eight water molecules, and one unreacted methane molecule. No oxygen molecules remain in this idealized result.
Check the mass with rounded atomic masses H = 1, C = 12, and O = 16. Initially, 5 × 16 + 8 × 32 = 336 mass units. Finally, 4 × 44 + 8 × 18 + 1 × 16 = 336. Counting just the would miss the leftover methane. Particle number need not be conserved: bonding can regroup atoms into different numbers of molecules while preserving this atom and mass budget.
Check 1 of 3
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Breaking a bond is not the energy payoff
Separating bonded atoms requires energy; forming bonds releases energy. The net change depends on the whole set of bonds broken and formed, together with the states and conditions being compared. A fuel does not release energy simply because its bonds are broken. The complete rearrangement can release more during product formation than was required to separate the . Chemical energy accounting must include both parts.
In a simplified bond-energy exercise, breaking the required reactant bonds costs 900 energy units and forming product bonds releases 1,100. The reacting system’s net energy change is 900 − 1,100 = −200 units; 200 can be transferred to its surroundings. Those invented values are not methane data. They illustrate the sign convention and cannot specify a temperature rise without quantities such as heat capacity and energy losses.
A reaction budget includes what remains
Original instructional accounting diagram. The reaction pathway and maximum conversion are stipulated; it does not model reaction speed or actual environmental yields.
Open full-size diagram in a new tab: A reaction budget includes what remainsCheck 2 of 3
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Work through a decision
Energy available is not a reaction rate
Even a reaction that can proceed favorably under the specified conditions may face an . Reactants pass through a higher-energy transition arrangement before products form. A offers a different pathway with a lower barrier and is regenerated overall. It does not create extra atoms, turn an insufficient oxygen supply into a sufficient one, or change the overall energy difference between the same initial and final states.
For a planetary chemistry claim, ask four separate questions: are the required substances present, can the proposed reaction proceed under local conditions, how fast might it occur, and what evidence distinguishes that pathway from alternatives? A measured chemical imbalance may motivate an energy-source hypothesis. It does not establish that organisms used that energy. Explicit reactants, products, environmental conditions, and competing explanations turn a suggestive ingredient list into a testable chemical argument.
Check 3 of 3
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Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
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