Lecture 13.1 Equilibrium Introduction
Understanding Chemical Equilibrium
Introduction to Chemical Equilibrium
- This lecture focuses on the nature of chemical equilibrium and aims to explain its dynamic characteristics.
- Chemical equilibrium occurs in a closed system where no components are added or removed, leading to stable concentrations of reactants and products.
Dynamic Nature of Equilibrium
- During a reaction, the concentrations of reactants decrease while products increase until a point is reached where these changes stop.
- At equilibrium, although concentrations remain constant over time, reactions continue at equal rates in both directions (forward and backward).
- The concept of dynamic equilibrium emphasizes that reactions are ongoing even when macroscopic properties appear unchanged.
Misconceptions about Equilibrium
- It is crucial to understand that "equilibrium" does not imply equal amounts of reactants and products; it signifies stability in concentration ratios.
Reversible Reactions
- An example reaction involves sodium carbonate reacting with calcium chloride to form calcium carbonate and sodium chloride.
- Both forward (reactants to products) and backward (products to reactants) reactions occur simultaneously, indicating a reversible process.
Representation of Equilibrium
- The double-headed arrow notation represents reversible reactions in chemical equations, indicating dynamic equilibrium between reactants and products.
Activation Energy Considerations
- Most chemical reactions are reversible under certain conditions; however, many learned in general chemistry are irreversible due to high activation energy barriers for reverse reactions.
- For instance, strong acid-base reactions often yield water as a product which does not easily revert back into its constituent acids and bases due to significant energy requirements.
Understanding Chemical Reactions and Equilibrium
Water Dissociation and Reaction Dynamics
- The decomposition constant of water is extremely low at 1 x 10^-4, indicating that under normal atmospheric conditions, water does not dissociate easily.
- Reactions involving gaseous products are often driven by entropy; such reactions tend to be spontaneous with a negative Gibbs free energy (ΔG).
- Entropically driven reactions are uni-directional and irreversible, contrasting with many other reversible reactions.
Definitions of Key Terms
- A reversible reaction proceeds in both directions simultaneously, where products can decompose back into reactants.
- An example includes the formation of calcium sulfate from calcium ions (Ca²⁺) and sulfate ions (SO₄²⁻), which can revert back to its reactants.
Concept of Equilibrium
- Equilibrium occurs when two opposing changes happen at the same rate; for instance, the rates of forward and backward reactions are equal.
- In chemical terms, equilibrium can be illustrated through the reaction between hydrogen gas (H₂) and iodine gas (I₂), forming hydrogen iodide (HI).
Characteristics of Dynamic Equilibrium
- Dynamic equilibrium is reached when the concentration of reactants and products remains constant over time despite ongoing reactions.
- At this state, both product formation and decomposition occur at equal rates, leading to no net change in concentrations.
Graphing Reaction Rates
- Reaction rates can be graphed by plotting concentration against time; this helps visualize how concentrations change during a reaction.
- For example, as H₂ and I₂ form HI initially, their concentrations decrease while HI's concentration increases until reaching equilibrium.
Observations Over Time
- Initially, as HI decomposes back into H₂ and I₂, its concentration increases due to lower initial amounts present in the system.
- Eventually, after some time has passed in a reversible reaction like this one, the rate of formation stabilizes as it reaches dynamic equilibrium.
Understanding Reaction Rates and Equilibrium Dynamics
Reaction Rate Changes Over Time
- The concentration of products increases over time, leading to a rise in reactant concentration. This results in an increase in the number of collision events, thereby enhancing the reaction rate.
- When plotting forward and backward reaction rates, it is observed that the forward reaction rate decreases while the backward reaction rate increases as time progresses.
- As reactant concentrations decrease over time, the frequency of collision events diminishes, causing a decline in the forward reaction rate.
- Conversely, product concentrations rise over time, increasing collision events and thus elevating the backward reaction rate until both rates stabilize.
Reaching Dynamic Equilibrium
- At a specific point in time, both forward and backward reactions reach a state known as dynamic equilibrium. In this state, the rates of formation for products and reactants become equal.