The hydrogen peroxide produced subsequently oxidizes colorless iodide ion to yield brown iodine, which may be visually detected. Some strips include an additional substance that reacts with iodine to produce a more distinct color change.
The two test reactions shown above are inherently very slow, but their rates are increased by special enzymes embedded in the test strip pad. This is an example of catalysis , a topic discussed later in this chapter.
A typical glucose test strip for use with urine requires approximately 30 seconds for completion of the color-forming reactions. Reading the result too soon might lead one to conclude that the glucose concentration of the urine sample is lower than it actually is a false-negative result. Waiting too long to assess the color change can lead to a false positive due to the slower not catalyzed oxidation of iodide ion by other substances found in urine.
The rate of a reaction may be expressed in terms of the change in the amount of any reactant or product, and may be simply derived from the stoichiometry of the reaction. Consider the reaction represented by the following equation:. The stoichiometric factors derived from this equation may be used to relate reaction rates in the same manner that they are used to related reactant and product amounts.
The relation between the reaction rates expressed in terms of nitrogen production and ammonia consumption, for example, is:. Note that a negative sign has been added to account for the opposite signs of the two amount changes the reactant amount is decreasing while the product amount is increasing.
If the reactants and products are present in the same solution, the molar amounts may be replaced by concentrations:. Similarly, the rate of formation of H 2 is three times the rate of formation of N 2 because three moles of H 2 form during the time required for the formation of one mole of N 2 :. The rate of hydrogen production, for example, is observed to be three times greater than that for nitrogen production:. Expressions for Relative Reaction Rates The first step in the production of nitric acid is the combustion of ammonia:.
Write the equations that relate the rates of consumption of the reactants and the rates of formation of the products. Solution Considering the stoichiometry of this homogeneous reaction, the rates for the consumption of reactants and formation of products are:. Check Your Learning The rate of formation of Br 2 is 6. What is the instantaneous rate of production of H 2 O and O 2?
Solution Using the stoichiometry of the reaction, we may determine that:. The rate of a reaction can be expressed either in terms of the decrease in the amount of a reactant or the increase in the amount of a product per unit time. Relations between different rate expressions for a given reaction are derived directly from the stoichiometric coefficients of the equation representing the reaction.
What are the units of this rate? In chemistry, it is often important to know how efficient a reaction is. This is because when a reaction is carried out, the reactants may not always be present in the proportions written in the balanced equation.
As a result, some of the reactants will be used, and some will be left over when the reaction is completed. A reaction should theoretically produce as much of the product as the stoichiometric ratio of product to the limiting reagent suggests. This number can be calculated and is called the theoretical yield. However, the amount of product actually produced by the reaction will usually be less than the theoretical yield and is referred to as the actual yield. To evaluate the efficiency of the reaction, chemists compare the theoretical and actual yields by calculating the percent yield of a reaction:.
The theoretical yield of a reaction is percent, but this value becomes nearly impossible to achieve due to limitations. To accurately calculate the yield, the equation needs to be balanced. Next, identify the limiting reagent. Then the theoretical yield of the product can be determined and, finally, compared to the actual yield. Then, percent yield can be calculated. For example, consider the preparation of nitrobenzene C 6 H 5 NO 2 , starting with In theory, therefore, if all C 6 H 6 were converted to product and isolated, If Limiting Reactants and Percent Yield — YouTube : This video explains the concept of a limiting reactant or a limiting reagent in a chemical reaction.
It also shows how to calculate the limiting reactant and the percent yield in a chemical reaction. Privacy Policy. Skip to main content. Mass Relationships and Chemical Equations. Search for:. Reaction Stoichiometry Amount of Reactants and Products Stoichiometry is the study of the relative quantities of reactants and products in chemical reactions and how to calculate those quantities.
Learning Objectives Construct a balanced chemical equation. Key Takeaways Key Points To fully understand a chemical reaction, a balanced chemical equation must be written. Chemical reactions are balanced by adding coefficients so that the number of atoms of each element is the same on both sides.
Stoichiometry describes the relationship between the amounts of reactants and products in a reaction. Key Terms stoichiometry : The field of chemistry that is concerned with the relative quantities of reactants and products in chemical reactions and how to calculate those quantities. Molar Ratios Molar ratios, or conversion factors, identify the number of moles of each reactant needed to form a certain number of moles of each product.
Learning Objectives Calculate the molar ratio between two substances given their balanced reaction. Key Takeaways Key Points Molar ratios state the proportions of reactants and products that are used and formed in a chemical reaction.
Molar ratios can be derived from the coefficients of a balanced chemical equation. Stoichiometric coefficients of a balanced equation and molar ratios do not tell the actual amounts of reactants consumed and products formed. Key Terms stoichiometric ratio : The ratio of the coefficients of the products and reactants in a balanced reaction. This ratio can be used to calculate the amount of products or reactants produced or used in a reaction. Mole-to-Mole Conversions Mole-to-mole conversions can be facilitated by using conversion factors found in the balanced equation for the reaction of interest.
Learning Objectives Calculate how many moles of a product are produced given quantitative information about the reactants. Key Takeaways Key Points The law of conservation of mass dictates that the quantity of an element does not change over the course of a reaction. Therefore, a chemical equation is balanced when all elements have equal values on both the left and right sides. The balanced equation for the reaction of interest contains the stoichiometric ratios of the reactants and products; these ratios can be used as conversion factors for mole -to-mole conversions.
Stoichiometric ratios are unique for each chemical reaction. Key Terms conversion factor : A ratio of coefficients found in a balanced reaction, which can be used to inter-convert the amount of products and reactants. Example 2 If 4. Therefore, to calculate the number of moles of water produced: [latex]4. Mass-to-Mass Conversions Mass-to-mass conversions cannot be done directly; instead, mole values must serve as intermediaries in these conversions. Learning Objectives Calculate the mass of reactants and products from a balanced chemical equation and information about the amount of reactant s present.
Key Takeaways Key Points The law of conservation of mass dictates that the quantity of an element does not change over the course of the reaction. Therefore, a chemical equation is balanced when each element has equal numbers on both the left and right sides of the equation.
Stoichiometric ratios, the ratios of the amounts of each substance used, are unique for each chemical reaction. The balanced equation of a reaction contains the stoichiometric ratios of the reactants and products; these ratios can be used for mole -to-mole conversions.
There is no direct way to convert from the mass of one substance to the mass of another. Key Terms stoichiometric ratio : The quantitative ratio between the reactants and products of a specific reaction or chemical equation.
The ratio is made up of their coefficients from the balanced equation. Example This can be illustrated by the following example, which calculates the mass of oxygen needed to burn The molar amount of O 2 can now be easily converted back to grams of oxygen: [latex]6. Mass-to-Mole Conversions Mass-to-mole conversions can be facilitated by employing the molar mass as a conversion ratio.
Key Takeaways Key Points The mole is the universal measurement of quantity in chemistry. Although it is not possible to directly measure how many moles a substance contains, it is possible to first measure its mass and then convert that amount to moles. The molar mass constant can be used to convert mass to moles. By multiplying a given mass by the molar mass, the amount of moles of the substance can be calculated.
Key Terms molar mass : The mass of a given substance chemical element or chemical compound divided by its amount mol of substance. Example 1 For example, convert 18 grams of water to moles of water. Example 2 If you have Limiting Reagents The reagent that limits how much product is produced the reactant that runs out first is known as the limiting reagent.
The equation shows you that for every two moles of aluminium oxide oxidised, four moles of aluminium metal are obtained along with three moles of oxygen gas. The formula mass of each reactant or product can be used to calculate the reacting masses. Calculate the mass of aluminium formed when 51 g of aluminium oxide are electrolysed. Firstly, the formula mass of aluminium oxide can be calculated using the formula and masses found in the data book.
This formula triangle can be used to explain the relationship between the formula mass the mass of one mole with any mass and the number of moles that it represents.
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