Chemical Reactions and Atoms Find New Partners

Distinguishing Physical and Chemical Changes

• Physical Change: A change that does not affect the identity of a substance because no chemical bonds are broken or formed. Examples include changing states of matter ($H_2O_{(s)}$ to $H_2O_{(l)}$).
• Chemical Change: A change in which the identity of the substances involved changes as chemical bonds are broken and/or formed (e.g., $2Na_{(s)} + Cl_{2(g)} \rightarrow 2NaCl_{(s)}$).

Indicators of Chemical Change

• Color Change: Indicates a chemical alteration, such as iron turning to rust or clear air becoming brownish smog.
• Smell Change: Observations like souring milk or the aroma of baking brownies indicate chemical reactions.
• Energy Change: Includes exothermic reactions (energy released, temperature increase) and endothermic reactions (energy absorbed, temperature decrease or light absorption, e.g., photosynthesis).
• Gas Release: Observed as fizzing, such as tablets disintegrating in water.
• Precipitate Formation: A solid that separates out of a solution due to a chemical change, such as blue gelatinous $Cu(OH)_{2(s)}$ forming from $NaOH_{(aq)}$ and $CuSO_{4(aq)}$.

The Carbon Cycle and Greenhouse Effect

• Greenhouse Gases: Substances like carbon dioxide ($CO_2$), methane ($CH_4$), water, and ozone trap heat from the Sun.
• Virgin Earth Challenge: Sir Richard Branson announced a award of $\$25,000,000$ for technology that removes human-activity greenhouse gases for at least 10 years.
• Atmospheric Statistics:
  • Historic $CO_2$ levels: $200$ to $300\,ppm$.
  • Current average $CO_2$ concentration: Approximately $356\,ppm$.
  • $CO_2$ increase rate: About $1.5\,ppm/year$.
  • Temperature Change: Global surface temperature increased by $0.5^\circ C$ over the past century; potential increases of $4-5^\circ C$ may lead to a $2\text{ to }3\text{-foot}$ sea level rise.

Structural Components of Chemical Equations

• Reactants: Original substances on the left side of the equation.
• Products: New substances on the right side of the equation.
• Directional Arrows: (\rightarrow) for forward, (\leftarrow) for reverse, and (\rightleftharpoons) for reversible reactions.
• Physical State Symbols: $(s)$ stands for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous solution (dissolved in water).
• Chemical Coefficient: A number in front of a compound representing the whole-number ratio of reactants and products.
• Catalyst: A substance (acid, metal, or ionic compound) that speeds up a reaction without being consumed; its symbol is written over the reaction arrow (e.g., $KI$ in the decomposition of $2H_2O_{2(aq)}$).

Balancing Equations and Conservation of Mass

• Law of Conservation of Mass: Matter is neither created nor destroyed; atoms rearrange but their count remains the same on both sides of a balanced equation.
• Diatomic Elements: Seven elements exist as pairs when alone: $H_2, N_2, O_2, F_2, Cl_2, Br_2, I_2$ (mnemonic: BrINClHOF).

Questions & Discussion

• Q: Is the sublimation of dry ice a physical or chemical change? A: Physical change; the identity of the $CO_{2(g)}$ remains the same.
• Q: What are example indicators of chemical change in the home? A: Vinegar and baking soda producing bubbles (gas), milk spoiling (smell/solid curds), and wood burning (heat/light/ash).
• Q: How is the combustion of butane balanced? A: $2 C_4H_{10(g)} + 13 O_{2(g)} \rightarrow 8 CO_{2(g)} + 10 H_2O_{(g)}$.
• Q: Balance the reaction of aqueous sodium hydroxide and aluminum chloride. A: $3 NaOH_{(aq)} + AlCl_{3(aq)} \rightarrow Al(OH)_{3(s)} + 3 NaCl_{(aq)}$.