Continuous Monitoring Method

Gas Volume Measurement

• If a reaction produces a gas, it can be collected using a gas syringe or an inverted measuring cylinder over water. The volume of gas is recorded at regular time intervals (e.g., every 20 seconds).

Mass Loss Method

• For reactions producing a dense gas like $CO_2$, the reaction vessel is placed on a high-precision balance. As the gas escapes, the total mass decreases, which can be plotted against time.

Colorimetry

• This is used when a reactant or product is colored. A colorimeter measures the absorbance of light. A calibration curve (absorbance vs. known concentrations) must be created first to convert absorbance readings into actual concentration values during the reaction.

Continuous monitoring is superior for identifying the full reaction mechanism and order, whereas initial rates is often simpler for reactions that are very fast or where products interfere with measurements.

• Tangent Accuracy: When asked to find the rate at a specific time, always draw a long tangent line that touches the curve only at that point. Use a large triangle to calculate the gradient ($\frac{\Delta y}{\Delta x}$) to minimize percentage error.

• Units: Always check the units for the rate. If concentration is in $mol \, dm^{-3}$ and time is in seconds, the rate units are $mol \, dm^{-3} \, s^{-1}$.

• Standard Form: Kinetic data is often provided in standard form (e.g., $2.5 \times 10^{-3}$). Ensure you maintain this notation throughout your calculations to avoid decimal errors.

• Calibration: If a question involves colorimetry, check if you need to use a calibration graph first before calculating the rate.

• Gas Density: Students often assume the mass loss method works for all gases. It is only effective for heavy gases like $CO_2$ ($M_r = 44$). It is not sensitive enough for light gases like $H_2$ ($M_r = 2$).

• Filter Selection: In colorimetry, you must use a filter that is the complementary color to the solution. For example, a blue solution absorbs orange light most strongly, so an orange filter should be used.

• Initial vs. Instantaneous Rate: The initial rate is the gradient at $t=0$. An instantaneous rate is the gradient at any other specific time $t$. Do not confuse the two in exam prompts.