2.1
Chemical Reactions
A balanced chemical equation provides the information of chemical formulas of the reactants and products involved in the chemical change. A reaction’s…
Consider a balanced chemical reaction, such as the combustion of hydrogen gas. Here, the quantitative relationship between the reactants and products — H2, O2, and H2O — is that two molecules of H2 react with one molecule of O2 to produce two molecules of H2O. This quantitative relationship is known as stoichiometry.
In a chemical reaction, reactants interact with each other to form products. The reactant that is completely consumed is the limiting reactant, and the reactant present in a quantity greater than necessary to react completely with the limiting reactant is the excess reactant.
A chemical reaction involves the chemical transformation of reactants into products. As the reaction progresses, the concentration of reactants decreases while the concentration of products increases. This variance in concentrations of reactants and products can be plotted in a graph as a function of time.
The speed at which a reaction progresses is called the reaction rate. It measures the rate of reactants’ disappearance or the rate of products’ appearance and is expressed in units of molarity-per-second.
A reversible chemical reaction is one in which the conversion of reactants into products and the conversion of products back into reactants occur simultaneously. A double arrow between the reactants and products signifies its reversible nature.
When the rate of formation of products, or the rate of the forward reaction, equals the rate of formation of reactants, or the rate of the reverse reaction, chemical equilibrium is achieved.
The equilibrium constant expression is written as the molar concentrations of the products, C and D, over the reactants, A and B, at equilibrium, each raised to their respective stoichiometric coefficients. When solved, the expression is equal to the equilibrium constant, Kc.
An expression in the same form can also be written for the reactants and products at any concentration, and the calculated quantity is known as the reaction quotient, Qc.
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Q1: What is stoichiometry and why does it matter in chemical reactions?
Stoichiometry describes the quantitative relationship between reactants and products in a balanced chemical equation. For example, in hydrogen combustion, two molecules of H2 react with one molecule of O2 to produce two molecules of H2O. Stoichiometry helps predict how much reactant is needed to produce a desired amount of product or how much product forms from a specific reactant amount.
Q2: How do you identify the limiting reactant in a chemical reaction?
The limiting reactant is the substance completely consumed first, restricting the amount of product formed. To identify it, compute the molar amounts of each reactant and compare them to stoichiometric amounts in the balanced equation. The reactant present in less-than-stoichiometric quantity is limiting, while excess reactant remains unconsumed after the reaction completes.
Q3: What does reaction rate measure and how is it expressed?
Reaction rate measures the speed at which a chemical reaction progresses, quantifying either the disappearance of reactants or the appearance of products per unit time. It is expressed in units of molarity per second. As a reaction proceeds, reactant concentrations decrease while product concentrations increase, changes that can be plotted on a graph as a function of time.
Q4: What is a reversible chemical reaction and how does it reach equilibrium?
A reversible chemical reaction proceeds simultaneously in both forward and reverse directions, indicated by a double arrow in the equation. Chemical equilibrium is achieved when the rate of product formation equals the rate of reactant formation, causing reactant and product concentrations to remain constant over time. At this point, the system shows no net change in composition.
Q5: How does the reaction quotient differ from the equilibrium constant?
The equilibrium constant (Kc) is calculated from molar concentrations of products and reactants at equilibrium, each raised to their stoichiometric coefficients. The reaction quotient (Qc) uses the same expression form but applies to any concentration, not just equilibrium. Comparing Qc to Kc indicates whether a reaction will shift forward or reverse to reach equilibrium.
Q6: Why is excess reactant often used in chemical reactions?
Excess reactant is deliberately added to ensure complete conversion of the limiting reactant into product. By providing more than the stoichiometric amount of one reactant, chemists maximize product yield from the other reactant. This strategy is especially useful when one reactant is expensive or when complete consumption of a specific reactant is critical to the reaction's success.
Q7: How can you visualize changes in reactant and product concentrations during a reaction?
Concentration changes over time are plotted on a graph with time on the x-axis and molarity on the y-axis. As the reaction progresses, the reactant concentration curve decreases while the product concentration curve increases. This graphical representation clearly shows reaction rate, the point where equilibrium is reached, and the relative amounts of reactants and products throughout the reaction.