Reaction time and kinetic curves
Chemical transformations vary drastically in duration. Some occur instantaneously, like the color change of tea under the action of lemon acid. Others have an intermediate duration—for example, it takes several days for construction concrete to fully cure. There are also extremely slow processes: it takes at least a year for fallen leaves to turn into humus.
External conditions are of utmost importance. If hydrogen and oxygen are mixed at room temperature, no traces of water will be found even after five years. However, when this mixture is heated to 518 °C, the water formation process is completed in just a couple of hours.
For graphical analysis of the process rate, graphs are plotted showing the dependence of a value (e.g., amount of substance or concentration) on time.
- Decay curves reflect the consumption of initial reactants. The flatter the line on the graph, the slower the process proceeds.
- Growth curves show the accumulation of reaction products. If the line rises steeply, it indicates a high rate.
Determining the exact time of complete reaction completion is difficult because the rate of change gradually decreases as the starting materials are consumed. Due to this uncertainty, chemistry more often uses the half-life period—the time interval required for half of the initial amount of a substance to react. This is a more reliable and objective criterion. For certain phenomena, such as radioactive decay or specific decomposition reactions, this period is a strictly constant value.
Types of rates and stoichiometric calculations
In kinetics, two main approaches to estimating the rate of chemical reactions are distinguished:
- Average rate. Shows the change in component concentration over a selected time interval. It is calculated as the ratio of the concentration difference to the elapsed time. The traditional unit of measurement is mol/(L·s).
- Instantaneous (true) rate. Characterizes the rate of the process at any specific moment in time when the time interval approaches zero. It is calculated via the derivative of concentration with respect to time.
The rate of concentration change of different substances within the same reaction is interrelated through stoichiometric coefficients. Consider the reversible process of hydrogen iodide synthesis from molecular hydrogen and iodine. During the reaction, hydrogen and iodine are consumed in equal amounts, and the concentration of hydrogen iodide increases by a doubled amount. Accordingly, the rate of hydrogen consumption will equal the rate of iodine consumption, and both will constitute exactly half of the product formation rate.
If the volume of the reaction vessel, the change in the amount of reactant, and the time elapsed are known, the average rate can be easily found. For example, with a decrease in hydrogen by 0.0016 mol in a closed vessel with a volume of 2.5 liters over 40 seconds, the average rate is calculated by dividing the decrease in the substance by the product of volume and time. In this example, it will be 0.000016 mol/(L·s). Based on the reaction stoichiometry, the rate with respect to hydrogen iodide will be exactly twice as high—0.000032 mol/(L·s).
Influence of the medium's phase state
Chemical reactions are classified depending on the phase state of the system:
- Homophasic (homogeneous) reactions. Proceed uniformly throughout the entire volume of the reaction mixture. Examples include interactions in gas mixtures or liquid solutions. Their rate is directly influenced by the temperature of the medium, the concentration of substances, and the presence of specific catalysts.
- Heterophasic (heterogeneous) reactions. Occur exclusively at the interface of contacting phases, for example, upon contact of a liquid or gas with the surface of a solid.
For heterogeneous processes, the contact surface area and the intensity of mixing are of decisive importance. Fine grinding of a solid reactant significantly increases the contact area, which leads to reaction acceleration. Mixing, in turn, continuously renews the layer of reactants in the interaction zone.
This is clearly visible when salt crystals dissolve. If the process proceeds without stirring, a dense layer of a heavy concentrated solution forms around the crystal. So-called diffusion inhibition occurs, hindering the access of pure solvent, and dissolution slows down significantly. Vigorous stirring eliminates this barrier, causing even small crystals to disappear several times faster.
Briefly: answers to topical questions
How is the law of mass action formulated for reaction rate?
The rate of a simple chemical reaction is proportional to the product of the concentrations of the reacting substances, raised to powers equal to their stoichiometric coefficients. This rule applies to simple homophasic reactions that proceed in a single stage. The kinetic equation of the reaction is expressed by the formula: $v = k \cdot c_1^{v_1} \cdot c_2^{v_2}...$. The probability of particle collision for different substances is proportional to the product of their concentrations, and for a single substance, to the square of its concentration.
What is the van 't Hoff rule?
The van 't Hoff rule describes the effect of temperature on the rate of a chemical reaction via the temperature coefficient ($\gamma$). This indicator represents the ratio of the rates of a given reaction at two temperatures differing by 10 °C (all other conditions being equal). The formula for calculating the coefficient is: $\gamma = \frac{v_{T+10}}{v_T}$. This regularity allows predicting the change in reaction rate upon heating or cooling, as well as calculating the temperature coefficient when the change in rate and temperature interval are known.
What is activation energy?
Activation energy is the energy barrier that reacting molecules must overcome for a chemical reaction to take place. During an active collision, the energy of the particles is sufficient to overcome this barrier, resulting in the formation of a transition state (activated complex) and a chemical transformation with the release or absorption of energy. The activation energy value enters the exponent of the Arrhenius equation with a minus sign; therefore, the larger this barrier, the slower the reaction proceeds.
How do catalysts increase the rate of a chemical reaction?
Catalysts increase the rate of a chemical reaction by decreasing the activation energy of the process. A catalyst changes the reaction mechanism, directing it along a new pathway where the rate constants are significantly higher. The working principle includes the following stages:
- Formation of an intermediate compound — the catalyst interacts with one of the reactants.
- Change in mechanism — the reaction proceeds via a new pathway through new stages.
- Regeneration — at one of the subsequent stages, the catalyst is restored and leaves the reaction in its original form.
Class and exam questions
Why is it difficult to determine the time of complete completion of a chemical reaction?
As the reactants are consumed, their concentration drops, and the process naturally slows down. Because of this, the moment of absolute completion of the reaction becomes indefinite, making it more convenient in practice to use the half-life period.
Why is it necessary to stir the system in heterogeneous reactions?
Stirring eliminates diffusion inhibition. It sweeps away the dense layer of concentrated solution around the solid substance, ensuring a constant influx of fresh reactants to the phase boundary.
What is the difference between average speed and instantaneous speed?
The average rate is calculated for a specific, finite time interval and shows the overall picture of changes. The instantaneous rate is calculated for a specific moment when the time interval approaches zero.
What else to study on this topic
- Mathematical derivation of true rate formulas via derivatives
- Mechanisms of diffusion inhibition
- Dependence of homogeneous reaction rates on temperature
- Specifics of rate calculations in reversible processes
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