Question

Why must the rate law for a chemical reaction be based on experimental evidence rather than the balanced equation for the reaction?

Short answer

The rate law for a chemical reaction must be based on experimental evidence rather than the balanced equation for the reaction because the balanced equation only provides the stoichiometric relationship between reactants and products, but not the reaction mechanism, rate-determining step, or the relationship between reaction rate and reactant concentrations. Experimental data is necessary to determine the accurate rate law, reaction orders, and rate constant to understand the factors affecting the reaction rate.

Step-by-step solution

1. Difference Between Rate Law and Balanced Equation

The rate law is an expression that describes how the rate of a chemical reaction depends on the concentrations of the reactants. It is usually in the form: rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of reactants, and m and n are the reaction orders, which describe how the rate is affected by the concentrations of the reactants. On the other hand, a balanced chemical equation provides the stoichiometric relationship between reactants and products. It shows the ratio in which the reactants combine to form products, but it does not provide any information about the reaction rate or how the reaction occurs.

2. Reaction Mechanisms

A chemical reaction often occurs through a series of elementary steps, also known as the reaction mechanism. These steps involve different species called reaction intermediates that do not appear in the overall balanced equation. The rate of the overall reaction depends on the slowest elementary step, which is the rate-determining step. The balanced equation does not provide any information about these elementary steps, reaction intermediates, or the rate-determining step. Therefore, the balanced equation cannot be used to derive the rate law directly.

3. Importance of Experimental Evidence

Since the balanced equation does not account for the reaction mechanism or rate-determining step, the rate law must be determined experimentally. By performing experiments and observing the reaction rate under different conditions and reactant concentrations, scientists can obtain the rate law. Experimental evidence is essential to confirm the reaction orders and the rate constant, which are necessary to accurately describe the relationship between the reaction rate and the reactant concentrations. In conclusion, the rate law for a chemical reaction must be based on experimental evidence rather than the balanced equation for the reaction because the balanced equation does not provide information about the reaction mechanism, rate-determining step, or the relationship between reaction rate and reactant concentrations. Experimental data is required to determine the accurate rate law and understand the factors affecting the reaction rate.

Key concepts

Reaction Mechanism

Every chemical reaction takes a specific path from reactants to products, and this path is known as the reaction mechanism. It's not just a single step but a series of elementary steps that happen at the molecular level. These elementary steps can involve the formation of short-lived species called reaction intermediates.

The mechanism unfolds in a sequence where each step has its own rate. One of these steps is typically slower than the others and is called the rate-determining step. Understanding this sequence is crucial because it provides insights into how a reaction actually progresses, rather than just how it appears in a simple balanced chemical equation. Such a mechanism must be confirmed through detailed experiments to pinpoint the intermediates and the rate at which they form and disappear.

• Sequences of events in which chemical transformations occur
• Involves multiple, often simplified, theoretical steps
• Helps identify the rate-determining step
• Explains how intermediates are formed and used up

Balanced Chemical Equation

A balanced chemical equation is a fundamental representation of a chemical reaction. It shows the starting substances, called reactants, transforming into end substances, called products. Balancing the equation ensures that the number of atoms of each element is the same on both sides, adhering to the law of conservation of mass.

While this provides valuable stoichiometric information, such as the ratios in which reactants combine, it doesn't tell us anything about the speed of the reaction (rate) or the steps involved in getting from start to finish (mechanism).

• Represents the conservation of mass
• Does not give insight into the reaction rate or mechanism
• Essential for predicting the amounts of each substance involved

Reaction Intermediates

These are the hidden players in the grand scheme of a reaction mechanism. Reaction intermediates are formed during one elementary step and consumed in another. They do not appear in the overall balanced chemical equation for the reaction.

Intermediates can be tricky to study because they are typically very unstable and exist only for a short time. However, detecting and understanding them is key to piecing together the puzzle of the reaction mechanism. Knowing about intermediates can help chemists theorize about the ways reactants are converted to products, allowing better control over the reaction pathway.

• Exist in the multi-step reaction process
• Formed and consumed as the reaction proceeds
• Generally unstable and short-lasting

Elementary Steps

Each step within a reaction mechanism is termed an elementary step. An elementary step is a single, molecular-level step, often involving just two molecules colliding and reacting.

The sequence of these steps — and the slowest among them — dictates the overall rate of the reaction. By analyzing elementary steps, chemists can better understand the rate-determining step and thus the overall speed and feasibilities of reactions.

• Basic molecular interactions building up to the overall reaction
• Help identify the rate-determining step
• Critical to formulating the rate law

Reaction Rate

The reaction rate tells us how fast a chemical reaction occurs. It's a measure of the change in concentration of reactants or products over time.

The reaction rate depends on various factors, including reactant concentrations, temperature, and the presence of a catalyst. However, knowledge of the rate law, which stems from experimental data, is essential to fully understand why the reaction proceeds at a certain rate. The rate law differs from the balanced chemical equation as it gives the power of prediction about how changing conditions affect the speed of the reaction.

• Measures the speed of a reaction
• Influenced by experimental variables
• Determines how different conditions affect reaction speed