Chapter 3 Notes: Writing Chemical Equations

Notation and Conventions for Writing Chemical Equations

Chemical equations are symbolic representations of chemical reactions, showing how one or more reactants transform into new substances (products).
Most chemical equations are read left to right, with reactants on the left side and products on the right side, separated by an arrow that represents the change during the reaction.
The statement of a generic chemical equation is that reactants go to products, i.e., reactants are consumed and products are formed.

Reactants: substances present at the outset of a reaction and consumed during the reaction; they appear on the left-hand side of the equation.
Products: substances formed by the reaction; they appear on the right-hand side of the equation.
Formulas identify the specific chemical species involved.
Physical states are often indicated after the chemical formula:
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous (dissolved in water)
Example (gas-phase):
- All substances involved are gases as indicated by (g):
- ext{H2(g) + O2(g)
  ightarrow H_2O(g)}
Note: In order for a written equation to describe the actual process fully, additional information about conditions (temperature, light, pressure, etc.) can be shown above or around the arrow.

Chemical formulas identify specific substances; states provide information about the physical phase under the conditions represented.
Examples of state designations on formulas:
- ext{H2(g)}, ext{O2(g)}, ext{H_2O(g)} for gas-phase species in the hydrogen–oxygen example.
The practical implication is that the same reaction can have different representations depending on whether products are gases, liquids, solids, or aqueous solutions.

To indicate conditions that influence the reaction beyond the identities of reactants and products, symbols are placed over or near the reaction arrow.
Two common symbols:
- Heat: the Greek letter \Delta (Delta) above the arrow indicates heat is required or supplied.
- Light: the symbol hv above the arrow indicates the reaction is initiated by light energy (photochemical).
In notation:
- Heat-driven example: \Delta placed over the arrow to denote heat input.
- Photochemical example: the symbol hv placed over the arrow denotes light-driven initiation.
Illustrated usage (as described): two examples showing the Δ and hv indicators over the reaction arrow.

Hydrogen–oxygen combustion (gas-phase, all gases):
- Balanced representation showing the proper stoichiometry:
- 2\,\mathrm{H2(g) + O2(g) \rightarrow 2\,H_2O(g)}
- This equation describes the reaction where the reactants are gases and the product is water vapor under high-temperature conditions.
Photochemical reaction: hydrogen + chlorine to form hydrogen chloride, driven by light:
- \mathrm{H2(g) + Cl2(g) \xrightarrow{hv} 2\,HCl(g)}
- The hv notation above the arrow indicates initiation by light energy.
Comparative note: the first equation is presented as a more complete description of the hydrogen–oxygen reaction; the second equation explicitly shows a photochemical pathway driven by light energy to form hydrogen chloride.

Balloon demonstration: a balloon filled with a mixture of hydrogen and oxygen explodes when ignited by a candle flame.
- The gas mixture is stable until ignition; upon ignition, a rapid reaction occurs producing a ball of flame.
- A microscopic-scale illustration accompanies the demonstration, showing the molecular species present before and after the reaction.
This demonstration provides a tangible example of the gas-phase reaction and the transformation of reactants to products as described by the chemical equation.

Key conventions:
- Reactants on the left, products on the right; arrow indicates the direction of the reaction.
- State symbols (s, l, g, aq) are used to clarify the physical form of each species.
- Conditions such as heat (Δ) or light (hv) can be indicated over the arrow to show energy inputs required for the reaction.
Practical implications:
- Understanding these conventions is essential for balancing equations, performing stoichiometric calculations, and predicting amounts of reactants/products.
- Energy considerations (heat vs. light) relate to reaction kinetics and mechanisms (e.g., thermally driven vs. photochemical processes).
- Real-world demonstrations (e.g., balloon explosion) illustrate the energetic nature of chemical reactions and the transformation of molecular species.

Balancing equations ensures conservation of matter and atoms across reactants and products.
State designations connect to physical chemistry concepts such as phase behavior and reaction conditions.
Energy terms (Δ, hv) tie into reaction energetics and kinetics, reinforcing why certain reactions require heat or light to proceed.
The use of visual demonstrations helps bridge theoretical notation with observable phenomena, reinforcing the practical relevance of chemical equations.

Heat indicator on arrow: \Delta
Light indicator on arrow: hv
General gas-phase reactions: \mathrm{A(g) + B(g) \rightarrow C(g)} (example shown with H, O, H_2O)
Photochemical hydrogen–chlorine reaction: \mathrm{H2(g) + Cl2(g) \xrightarrow{hv} 2\,HCl(g)}
Balanced hydrogen–oxygen reaction: 2\,\mathrm{H2(g) + O2(g) \rightarrow 2\,H_2O(g)}

Reactants on the left; products on the right; balanced coefficients indicate the relative amounts.
States: (s), (l), (g), (aq).
Over-arrow indicators: \Delta for heat, hv for light.
Examples in the chapter illustrate both thermal and photochemical pathways and connect to lab demonstrations and real-world chemical behavior.