LAJU REAKSI : KIMIA SMA KELAS 11

Reaction Rates

Reaction rate is the speed at which a chemical reaction proceeds. Reactions can be slow, such as iron rusting, or fast, such as acid–base reactions, explosions, and fireworks burning. Reaction rate is commonly expressed as a change in concentration per unit time, such as mol L⁻¹ s⁻¹.

How Concentration Changes During a Reaction

As a reaction proceeds, reactant concentrations decrease and product concentrations increase. The rate can be described as either the decrease in reactant concentration per unit time or the increase in product concentration per unit time.

Reactant disappearance is conventionally written with a negative sign because the concentration falls:

Rate of disappearance = −Δ[reactant]/Δt

Product formation is written with a positive sign:

Rate of formation = +Δ[product]/Δt

For a fixed concentration change, a shorter reaction time means a faster rate.

Relationship Between Rates and Equation Coefficients

The rates at which substances are consumed or formed are proportional to their coefficients in the balanced chemical equation.

For example:

N₂ + 3H₂ → 2NH₃

The rates of consumption of N₂ and H₂ and formation of NH₃ are in the ratio 1:3:2.

Example 1: Balance the Equation, Then Compare Rates

Consider the decomposition reaction:

N₂O₅ → NO₂ + O₂

Balancing it gives:

2N₂O₅ → 4NO₂ + O₂

The rate of N₂O₅ decomposition compared with the rate of O₂ formation follows their coefficient ratio:

2:1

Balance the equation before using its coefficients to compare reaction rates.

Example 2: Calculate Rates from a Concentration Change

For 2N₂O₅ → 4NO₂ + O₂, suppose N₂O₅ decreases from 2.0 M to 0.5 M in 10 seconds.

1. Rate of N₂O₅ disappearance: Its concentration falls by 1.5 M over 10 seconds, so the disappearance rate is 0.15 M/s. The signed concentration-change rate, Δ[N₂O₅]/Δt, is −0.15 M/s.
2. Rate of NO₂ formation: The coefficient ratio is 4:2, so NO₂ forms at twice the N₂O₅ disappearance rate: 0.30 M/s.
3. Rate of O₂ formation: The coefficient ratio is 1:2, so O₂ forms at half the N₂O₅ disappearance rate: 0.075 M/s.

Main Takeaway

To solve basic reaction-rate problems:

• Track how reactant or product concentration changes over time.
• Use a negative sign for a reactant’s signed concentration change and a positive sign for a product’s.
• Balance the reaction equation first.
• Use the balanced coefficients to relate the rates of different substances.

Speakers and Sources

One narrator/instructor explains the concepts and worked examples. No other speakers or external sources are featured in the subtitles.