Reaction rates, reversible reactions and
What reaction rate measures
Reaction rate describes how quickly reactants are consumed or products formed. A rate can be measured as a change in concentration per time or by a suitable observable such as gas volume, mass loss or colour intensity. The observable must track the intended chemical change. On a product-amount versus time graph, gives the production rate; a steep indicates a faster rate. An average rate uses a finite interval, while an instantaneous rate uses a tangent at a point. Rate often decreases as reactants are consumed, but not every reaction follows a simple pattern. State the units and measurement conditions alongside a numerical rate.
Collision theory and activation
Reacting particles must encounter one another with suitable and orientation for reaction to occur. Activation is the barrier associated with the pathway. More collisions alone do not guarantee the same proportion of successful collisions if temperature or molecular arrangement also changes. Raising temperature usually increases reaction rate because a larger fraction of particles has enough to pass the barrier, in addition to changes in encounter . A provides an alternative pathway with a lower activation barrier. It is regenerated overall, though it can participate in intermediate steps. The word does not that it remains completely uninvolved at every stage.
Concentration, and surface area
Increasing reactant concentration often raises rate by increasing encounter , but the detailed dependence must be established experimentally. For reacting gases, higher partial can similarly increase number . Increasing the exposed surface area of a solid often speeds a reaction occurring at that surface; powder can react faster than a large lump of equal mass. A solid's total mass and reactive surface area are not the same variable. Compare experiments with the same temperature, reagent amounts and measurement method when testing one factor. In a gas-collection experiment, leaks or delayed timing can imitate a slower reaction without reflecting its true chemical kinetics.
Rate laws and reaction order
An experimental rate law might have the form rate = k[A]ᵐ[B]ⁿ. The exponents are reaction orders determined from suitable evidence, not generally read from the overall balanced equation. If doubling [A] while holding other conditions fixed doubles rate, the data support first-order dependence on A over that range. If rate quadruples, second-order dependence is supported. Zero-order dependence means rate is unchanged by that concentration over conditions studied, not that no reaction occurs. Units of k depend on total order. Separate an observed empirical rate relationship from a proposed mechanism that seeks to explain it.
Dynamic in a closed system
A reversible reaction can proceed in both directions. In a closed system at , forward and reverse rates are equal, so macroscopic concentrations remain constant. Reactant and product concentrations need not be equal, and molecular events do not stop. A system initially containing only reactants can form products until the reverse rate grows enough to match the forward rate. Starting from products can reach the same composition under the same overall constraints. A steady-looking system is not automatically chemical if material continually enters or leaves. The closed-system and constant-condition assumptions matter to an exam explanation.
Writing expressions
For, the introductory concentration expression is Kc = [C]ᶜ[D]ᵈ/([A]ᵃ[B]ᵇ), using concentrations. Pure solids and pure liquids are omitted in the corresponding simplified heterogeneous expressions because their activities are treated as constant. The rigorous constant uses activities; school concentration expressions may carry unit conventions according to the course. A large K generally indicates products favoured in the written reaction under stated conditions, not a fast reaction. Reversing the written equation changes K to its reciprocal, and multiplying all coefficients changes its . Always specify reaction direction and temperature.
Predicting responses to concentration and
Le Chatelier's principle predicts a tendency to oppose an imposed disturbance when a system can re-establish . Adding a reactant can favour its consumption, while removing a product can favour further product formation. Increasing by reducing volume favours the side with fewer gas moles when there is a gas- difference; count balanced gaseous coefficients only. If both sides have equal gas moles, that idealised change gives no direct shift by this rule. Adding an inert gas at constant volume does not change ideal reacting-gas partial pressures, so it does not shift such an merely because total rises.
Temperature and catalysts at
Temperature changes the constant. For an exothermic forward reaction, heating favours the endothermic reverse direction and can reduce the product proportion. Cooling may improve that proportion but slow the rate, creating an industrial compromise. A makes reached faster without changing its final position at the same temperature and composition constraints. , feed composition and continuous product removal can also be chosen for practical yield, while cost, equipment and safety constrain the choice. Distinguish yield from product obtained per hour; a condition that improves one may worsen the other.
Industrial and experimental applications
The Haber process uses the reversible nitrogen–hydrogen reaction to make ammonia, and its conditions balance , rate and engineering limits. The Contact process similarly balances sulfur dioxide and downstream production needs. Enzymes are biological catalysts with conditions affecting their active structures; extreme conditions can reduce their activity rather than simply following an unlimited temperature-increase rule. In rate experiments, plot enough points to see a trend, control relevant variables and explain anomalies. Repeating a flawed setup consistently does not remove a systematic leak. An -profile diagram and a concentration–time graph answer different questions and should not be labelled interchangeably.
Self-check with explained results
For, compression favours ammonia because four gas moles on the left become two on the right. A increases both directions' rates but does not alter K at fixed temperature. For with [A] = 0.20 mol/dm³ and, Kc = 3 in the simple ratio treatment. If a rate experiment shows that doubling [A] at fixed [B] quadruples the initial rate, the supported A order is two. These results concern gas- balance, composition and kinetic evidence respectively; do not substitute one kind of explanation for another.