Scivra
Pro 🔒~14 min

Chemical Reactions

Reactants, products, conservation of mass, and energy changes

How it works

A chemical reaction transforms reactants (starting materials) into products (ending materials) by breaking and forming chemical bonds. The atoms themselves are never created or destroyed — they just rearrange. This is the Law of Conservation of Mass: mass of reactants = mass of products. Reactions are classified by type: synthesis (A+B→AB), decomposition (AB→A+B), single displacement (A+BC→AC+B), double displacement (AB+CD→AD+CB), and combustion (fuel+O₂→CO₂+H₂O+heat). Energy changes: exothermic reactions release heat (ΔH<0), endothermic reactions absorb heat (ΔH>0). Reaction rate increases with temperature, concentration, surface area, and catalysts. A catalyst speeds up a reaction without being consumed.

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Step-by-step

  1. Select a reaction type and press Play to watch atoms rearrange.
  2. The molecular model shows bonds breaking and forming.
  3. Check the mass balance — left side should equal right side.
  4. Increase temperature to speed up the reaction.
  5. Add a catalyst (Pro) to lower activation energy and dramatically increase rate.

Key formulas

  • Reactants→Products(atoms conserved)\text{Reactants} \to \text{Products} \quad (\text{atoms conserved})Law of Conservation of Mass: atoms are rearranged, never created/destroyed
  • CH4+2O2→CO2+2H2OΔH=−890 kJ\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O} \quad \Delta H = -890\,\text{kJ}Methane combustion: balanced equation with energy released

Frequently asked questions

Balance: H₂ + O₂ → H₂O. How many molecules of each substance are needed?
2H₂ + O₂ → 2H₂O. Left side: 4H + 2O. Right side: 4H + 2O. ✓ Balanced. You need 2 hydrogen molecules and 1 oxygen molecule to make 2 water molecules.
If 32 g of sulfur reacts completely with oxygen, how many grams of sulfur dioxide can form?
Conservation of mass: S + O₂ → SO₂. 32 g S + 32 g O₂ = 64 g SO₂. Total mass is conserved.
Is photosynthesis exothermic or endothermic? What about cellular respiration?
Photosynthesis: endothermic (absorbs light energy to build glucose — ΔH>0). Cellular respiration: exothermic (releases energy from glucose — ΔH<0). They are essentially reverse reactions!
How does a catalyst increase reaction rate without being consumed in the reaction?
A catalyst provides an alternative reaction pathway with lower activation energy. More reactant molecules have enough energy to react (from the Maxwell-Boltzmann distribution). The catalyst participates temporarily in the reaction but is regenerated at the end — it's not consumed.