Chemical Reactions and Particle Diagrams

Understanding Particle Diagrams

Particle diagrams are visual representations of atoms and molecules before and after a chemical reaction. They help illustrate how substances change at the molecular level.

• Particle Diagram: A drawing that shows individual atoms or molecules as circles or shapes, often color-coded for different elements.

• States of Matter: In diagrams, particles may be spaced differently to indicate solid (s), liquid (l), gas (g), or aqueous (aq) states.

• Matching Equations: Each particle diagram can be matched to a chemical equation by counting the number and type of particles before and after the reaction.

• Example: A diagram showing two H2 molecules and one O2 molecule on the left, and two H2O molecules on the right, represents the reaction: $2\ \mathrm{H}_2(g) + \mathrm{O}_2(g) \rightarrow 2\ \mathrm{H}_2\mathrm{O}(l)$

Identifying Physical vs. Chemical Changes

It is important to distinguish between physical and chemical changes by observing particle diagrams and properties.

• Physical Change: A change in state or appearance without altering the chemical composition (e.g., melting, boiling, dissolving).

• Chemical Change: A process where substances are transformed into new substances with different chemical properties (e.g., rusting, burning, reacting acids and bases).

• Evidence of Chemical Change: Color change, gas production, formation of a precipitate, temperature change, or new odor.

• Particle Model: In a chemical change, bonds are broken and new bonds are formed; in a physical change, the arrangement or movement of particles changes, but the particles themselves remain the same.

• Example: Ice melting is a physical change (solid to liquid, no new substance). Hydrogen and oxygen reacting to form water is a chemical change (new substance formed).

Chemical Equations and Conservation of Mass

Law of Conservation of Mass

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

• Definition: In a closed system, the mass of substances before a reaction equals the mass after the reaction.

• Application: When balancing equations, ensure the number of each type of atom is the same on both sides.

• Equation: $\text{Total mass of reactants} = \text{Total mass of products}$

• Example: If 10 g of hydrogen reacts with 80 g of oxygen to form water, the total mass of water produced is 90 g.

Balancing Chemical Equations

Balancing chemical equations ensures the law of conservation of mass is obeyed. Each side of the equation must have the same number of atoms of each element.

• Steps to Balance:

  1. Write the unbalanced equation.

  2. Count the number of atoms of each element on both sides.

  3. Add coefficients to balance the atoms (do not change subscripts).

  4. Check your work.

• Example: $\mathrm{N}_2 + 3\ \mathrm{H}_2 \rightarrow 2\ \mathrm{NH}_3$

States of Matter Symbols in Equations

Chemical equations often include symbols to indicate the physical state of each substance.

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous (dissolved in water)

• Example: $\mathrm{NaCl}(aq)$ means sodium chloride dissolved in water.

Reactants vs. Products

In a chemical equation, substances on the left are reactants, and those on the right are products.

• Reactants: Starting materials, present before the reaction.

• Products: New substances formed as a result of the reaction.

• Example: In $\mathrm{C} + \mathrm{O}_2 \rightarrow \mathrm{CO}_2$, C and O2 are reactants; CO2 is the product.

Comparing Chemical and Physical Changes

Key Differences

Chemical and physical changes can be compared in terms of properties, observations, and molecular models.

Example: Boiling water (physical change); burning wood (chemical change).