Amounts of Reactants and Products

Strategy Remember that the formula of an element or compound cannot be changed when balancing a chemical equation. The equation is balanced by placing the appropriate coefficients in front of the formulas. FoUow the procedure described on p. 74. 

In a balanced equation, the number and types of atoms on each side of the equation must be the same. We see that there is one A1 atom on the reactants side and there are two A1 atoms on the product side. We can balance the A1 atoms by placing a coefficient of 2 in front of A1 on the reactants side. 

This is a balanced equation. However, equations are normally balanced with the smallest set of whole number coefficients. Multiplying both sides of the equation by 2 gives whole number coefficients. 

Check For an equation to be balanced, the number and types of atoms on each side of the equation must be the same. The final tally is 

The equation is balanced. Also, the coefficients are reduced to the simplest set of whole numbers. 

Practice Exercise Balance the equation representing the reaction between iron(III) oxide, Fe203, and carbon monoxide (CO) to yield iron (Fe) and carbon dioxide (CO2). 

Which parts shown here are essential for a balanced equation and which parts are helpful if we want to carry ont the reaction in the laboratory  

A basic qnestion raised in the chemical laboratory is How much product will be formed from specific amounts of starting materials (reactants) Or in some cases, we might ask the reverse question How much starting material must be used to obtain a specific amount of product To interpret a reaction quantitatively, we need to apply our knowledge of molar masses and the mole concept. Stoichiometry is the quantitative study of reactants and products in a chemical reaction. 

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Using this insight Berthollet reasoned that the reaction is reversible, and that the relative amounts of reactants and products determine the direction in which the reaction occurs, and the final composition of the reaction mixture. We recognize a reaction s ability to move in both directions by using a double arrow when writing the reaction. 

Coulometric methods are based on Earaday s law that the total charge or current passed during an electrolysis is proportional to the amount of reactants and products in the redox reaction, ff the electrolysis is f00% efficient, in that only the analyte is oxidized or reduced, then the total charge or current can be used to determine... 

The following data were obtained during a kinetic study of the hydration of p-methoxyphenylacetylene by measuring the relative amounts of reactants and products by nuclear magnetic resonance (NMR). ... 

The thermochemical equation allows us to relate the enthalpy change to amounts of reactants and products, leading to conversion factors such as... 

Charles s and Gay-Lussac s law Relation stating that at constant P and n, the volume of a gas is directly proportional to its absolute temperature, 106-107, 111 Chelating agent Complexing ligand that forms more than one bond with a central metal atom the complex formed is called a chelate, 411-412 natural, 424-425 synthetic, 424-425 Chemical equation Expression that describes the nature and relative amounts of reactants and products in a reaction, 60-61. See also Equation, net ionic. 

Some of the most important information about a mechanism comes from experiments that determine how fast a chemical reaction occurs under various conditions. In chemical reactions, amounts of reactants and products change, so reaction rates are given in units of amount per unit time for example, molecules per second. Amounts also can be expressed as concentrations, so rates can be measured in units of concentration per unit time for example, molar per minute. 

In this chapter, you will study factors that affect reversible changes, notably those in chemical reactions. You will learn how to determine the amounts of reactants and products that are present when their proportions no longer change. You will also learn how to make qualitative predictions about the ways that chemists can change these proportions. Finally, you will see how industrial chemists apply their knowledge of reversible changes to increase the yield of chemicals that are important to society. 

Energy is an important component of most equilibrium systems. The input or output of energy in a system causes the temperature to change. Thus, the requirement that an equilibrium system be closed means that the temperature of the system must remain constant. In the next section, you will examine more closely the effects of thermodynamics on equilibrium systems. In particular, you will examine the factors that affect the amount of reactant and product in a reaction and the factors that determine whether or not a reaction is spontaneous. 

A balanced half-reaction shows relationships between the amounts of reactants and products and the amount of electrons transferred. 

A redox reaction equation requires well-defined amounts of reactants and products. The number (n) of electrons in such a reaction equation is related to the amount of charge transferred when the reaction is completed. Because each mole... 

We used small amounts of reactants and products to assure that we reached equilibrium, and we now cancel the subscripts in each term to arrive at... 

The precipitation method, however, does not form high-purity product as it contains small amounts of reactant and product ions that stick to the Hg2Cl2... 

Beyond chemical reactions lies the strange and wonderful concept of stoichiometry. Stoichiometry allows scientists to calculate quantitative (numerical) amounts of reactants and products based on balanced chemical reactions. In short (and in plain English), this part tells you how much of something a reaction will make. 

At equilibrium, amounts of reactants and products remain constant. Therefore, the color of the mixture does not change. 

The equilibrium constant is fixed and characteristic for any given chemical reaction at a specified temperature. It defines the composition of the final equilibrium mixture, regardless of the starting amounts of reactants and products. Conversely, we can calculate the equilibrium constant for a given reaction at a given temperature if the equilibrium concentrations of all its reactants and products are known. As we will show in Chapter 13, the standard free-energy change (A(3°) is directly related to Ke[Pg.61]

At equilibrium, the relationship among the amounts of reactants and products is given by the equilibrium constant expression... 

The subscript c in Kc means V I that the amounts of reactants and products are expressed as molar concentrations. 

To maintain consistent units in this calculation, we have expressed AG and AG° in units of kj/mol because R has units of J/ (K mol). The "per mole" in this context means per molar amounts of reactants and products indicated by the coefficients in the balanced equation. Thus, the free-energy change is —60.5 kj when 1 mol of N2 and 3 mol of H2 are converted to 2 mol of NH3 under the specified conditions. 

At equilibrium, depending on the temperature of the reaction, almost any concentration of the substances present can exist. If, at equilibrium in the reaction between sodium thiosulfate and silver ions, mostly silver ions and thiosulfate ions are present, we would not be successful in removing the silver ions to preserve a photo image. We need a system that shows us which substances are in excess at equilibrium, the reactants or products. It is possible to have equal concentrations of reactants and products at equilibrium, but this is usually not the case. At equilibrium, forward and reverse reaction rates are equal, not the amounts of reactants and products. At equilibrium, the rate of reactants making products equals the rate of products making reactants. This results in constant product and reactant concentrations. 

Equation (49) provides a means of determining the heat of a reaction without performing any calorimetric measurements. If an equilibrium can be established for the reaction with measurable amounts of reactants and products over a range of temperature, then AnnH° can be obtained from the slope of a graph of the logarithm of the equilibrium constant versus T x. Equation (49) can also be written as... 

Occasionally, not all equilibrium concentrations are known. When this occurs you must use equilibrium concepts and stoichiometry concepts to determine K. What you are trying to do in these problems is determine the amounts of materials at equilibrium. In Chapter 12, you learned that the balanced chemical equation shows you the relative amounts of reactants and products during the chemical reaction. For a reaction at equilibrium, the logic is the same. The mole ratios still apply. There is one major difference, however, between the stoichiometry... 

Writing the overall equation, with the reactants on the left and the products on the right, is only the first step in our study of a reaction. If we truly want to understand a reaction, we must also know the mechanism, the step-by-step pathway from reactants to products. To know how well the reaction goes to products, we study its thermodynamics, the energetics of the reaction at equilibrium. The amounts of reactants and products present at equilibrium depend on their relative stabilities. 

You have learned how to do stoichiometric calculations, using balanced chemical equations to find amounts of reactants and products. In these calculations, you assumed that the reactants and products occurred in the exact molar ratios shown by the chemical equation. In real life, however, reactants are often not present in these exact ratios. Similarly, the amount of product that is predicted by stoichiometry is not always produced. 

Recall that stoichiometry involves calculating the amounts of reactants and products in chemical reactions. If you know the atoms or ions in a formula or a reaction, you can use stoichiometry to determine the amounts of these atoms or ions that react. Solving stoichiometry problems in solution chemistry involves the same strategies you learned in Unit 2. Calculations involving solutions sometimes require a few additional steps, however. For example, if a precipitate forms, the net ionic equation may be easier to use than the chemical equation. Also, some problems may require you to calculate the amount of a reactant, given the volume and concentration of the solution. 

The relative amounts of reactants and products involved in a reaction maybe the ratio of moles, millimoles, or masses. 

For each reaction some quantities of the reactants were placed in separate containers and allowed to come to equilibrium. Describe the relative amounts of reactants and products that are present at equilibrium. At equilibrium, which is faster, the forward or reverse reaction in each case ... 

When K has a value much greater than 1, the product concentrations are relatively large and the reactant concentrations are relatively small. In both cases, however, the rate of the forward reaction equals the rate of the reverse reaction at equilibrium (this is a definition of equilibrium). 13. No, it doesn t matter in which direction the equilibrium position is reached. Both experiments will give the same equilibrium position since both experiments started with stoichiometric amounts of reactants or products. 15. When equilibrium is reached, there is no net change in the amount of reactants and products present since the rates of the forward and reverse reactions are equal. The first diagram has 4 A2B molecules,... 

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