How is the Speed of a Reaction Measured?
The correct answer and explanation is:
Correct Answer:
The speed of a reaction, also called the reaction rate, is measured by the change in concentration of a reactant or product per unit time.
Explanation:
The speed or rate of a chemical reaction tells us how fast reactants are converted into products. Measuring this rate is essential in understanding the kinetics of a reaction, which helps control industrial processes, optimize yields, and understand biological mechanisms.
How is the rate measured?
The reaction rate is quantified by observing how the concentration of a reactant decreases or how the concentration of a product increases over time. This change is typically expressed in units like moles per liter per second (mol/L·s).
For example, consider a simple reaction:
A→BA \rightarrow B
- If we measure the concentration of AA at various times and notice it decreases from 1.0 M to 0.8 M over 10 seconds, the reaction rate can be approximated as:
Rate=Δ[A]Δt=0.8−1.010=−0.02 M/s\text{Rate} = \frac{\Delta [A]}{\Delta t} = \frac{0.8 – 1.0}{10} = -0.02 \, \text{M/s}
(The negative sign indicates a decrease in reactant concentration.)
Alternatively, if we track product BB forming from 0 to 0.2 M over the same 10 seconds:
Rate=Δ[B]Δt=0.2−010=0.02 M/s\text{Rate} = \frac{\Delta [B]}{\Delta t} = \frac{0.2 – 0}{10} = 0.02 \, \text{M/s}
Methods to Measure Concentration Change:
- Spectroscopy: Many substances absorb light at specific wavelengths. By measuring absorbance changes, we can determine concentration changes over time.
- Titration: Reactants or products can be titrated at intervals to determine concentration.
- Gas Volume Measurement: For reactions producing or consuming gases, measuring gas volume or pressure changes can indicate rate.
- Conductivity: Changes in the electrical conductivity of a solution as ions are formed or consumed.
Instantaneous vs. Average Rate:
- The average rate is calculated over a time interval.
- The instantaneous rate is the rate at a specific moment, found by determining the slope of the concentration vs. time curve at that point.
Summary:
Measuring the speed of a reaction involves tracking how quickly concentrations of reactants or products change over time, using various physical or chemical methods. This rate provides valuable information for controlling and understanding chemical reactions in research and industry.