Passage
Living organisms that require oxygen to respire can build up hydrogen peroxide (H₂O₂) as a byproduct of respiration. Because hydrogen peroxide can cause oxidative damage to cells, the amount of H₂O₂ in cells must be closely regulated. Although H₂O₂ decomposes spontaneously, a catalyst is needed for the reaction to occur at a sufficiently rapid rate suitable for biological purposes. In biological systems, an enzyme called a catalase accelerates the catalytic decomposition of H₂O₂ into water and oxygen (Reaction 1) .
Reaction 1In a laboratory setting, several different inorganic compounds can also serve as catalysts for H₂O₂ decomposition, allowing researchers to compare how different catalysts affect the efficiency of the reaction. To compare Fe(NO₃) ₃ (a metal homogeneous catalyst) and NaI (a halogen homogeneous catalyst) , researchers measured the amount of heat evolved after placing each catalyst in a fresh solution of 3% hydrogen peroxide.To perform the measurements, the researchers placed 50 mL of 3% H₂O₂ in an insulated coffee cup to be used as a calorimeter, as shown in Figure 1. Before adding any catalyst, the initial, baseline temperature of the H₂O₂ solution was determined by recording the temperature every 30 seconds for 5 minutes. Then 10 mL of 0.10 M Fe(NO₃) ₃(aq) catalyst was added to the H₂O₂ solution. The temperature was recorded every 30 seconds for another 15 minutes. The experiment was then repeated using 10 mL of 0.50 M sodium iodide in 0.01 M NaOH as the catalyst. The results for each reaction are shown in Figure 2.
Figure 1 Coffee cup calorimeter
Figure 2 Temperature vs. time measurements for reactions catalyzed by Fe(NO₃) ₃ and NaI
Adapted from: C. Marzzacco, "The effect of a change in catalyst on the enthalpy of decomposition of hydrogen peroxide." Chem13 News Magazine. ©2008 University of Waterloo.
-Below is a reaction energy diagram for the decomposition of H₂O₂. As the reaction proceeds, the enthalpy of the reaction is:

Question

Passage
Living organisms that require oxygen to respire can build up hydrogen peroxide (H₂O₂) as a byproduct of respiration. Because hydrogen peroxide can cause oxidative damage to cells, the amount of H₂O₂ in cells must be closely regulated. Although H₂O₂ decomposes spontaneously, a catalyst is needed for the reaction to occur at a sufficiently rapid rate suitable for biological purposes. In biological systems, an enzyme called a catalase accelerates the catalytic decomposition of H₂O₂ into water and oxygen (Reaction 1) .

Passage Living organisms that require oxygen to respire can build up hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) as a byproduct of respiration. Because hydrogen peroxide can cause oxidative damage to cells, the amount of H<sub>2</sub>O<sub>2</sub> in cells must be closely regulated. Although H<sub>2</sub>O<sub>2</sub> decomposes spontaneously, a catalyst is needed for the reaction to occur at a sufficiently rapid rate suitable for biological purposes. In biological systems, an enzyme called a catalase accelerates the catalytic decomposition of H<sub>2</sub>O<sub>2</sub> into water and oxygen (Reaction 1) . <strong>Reaction 1</strong>In a laboratory setting, several different inorganic compounds can also serve as catalysts for H<sub>2</sub>O<sub>2</sub> decomposition, allowing researchers to compare how different catalysts affect the efficiency of the reaction. To compare Fe(NO<sub>3</sub>) <sub>3</sub> (a metal homogeneous catalyst) and NaI (a halogen homogeneous catalyst) , researchers measured the amount of heat evolved after placing each catalyst in a fresh solution of 3% hydrogen peroxide.To perform the measurements, the researchers placed 50 mL of 3% H<sub>2</sub>O<sub>2</sub> in an insulated coffee cup to be used as a calorimeter, as shown in Figure 1. Before adding any catalyst, the initial, baseline temperature of the H<sub>2</sub>O<sub>2</sub> solution was determined by recording the temperature every 30 seconds for 5 minutes. Then 10 mL of 0.10 M Fe(NO<sub>3</sub>) <sub>3</sub>(aq) catalyst was added to the H<sub>2</sub>O<sub>2</sub> solution. The temperature was recorded every 30 seconds for another 15 minutes. The experiment was then repeated using 10 mL of 0.50 M sodium iodide in 0.01 M NaOH as the catalyst. The results for each reaction are shown in Figure 2. <strong>Figure 1</strong> Coffee cup calorimeter <strong>Figure 2</strong> Temperature vs. time measurements for reactions catalyzed by Fe(NO<sub>3</sub>) <sub>3</sub> and NaI Adapted from: C. Marzzacco, The effect of a change in catalyst on the enthalpy of decomposition of hydrogen peroxide. Chem13 News Magazine. ©2008 University of Waterloo. -Below is a reaction energy diagram for the decomposition of H<sub>2</sub>O<sub>2</sub>. As the reaction proceeds, the enthalpy of the reaction is: A) negative, but the activation energy is positive. B) equal to the activation energy. C) positive, but the activation energy is negative. D) zero, but the activation energy is nonzero and positive.

Reaction 1In a laboratory setting, several different inorganic compounds can also serve as catalysts for H₂O₂ decomposition, allowing researchers to compare how different catalysts affect the efficiency of the reaction. To compare Fe(NO₃) ₃ (a metal homogeneous catalyst) and NaI (a halogen homogeneous catalyst) , researchers measured the amount of heat evolved after placing each catalyst in a fresh solution of 3% hydrogen peroxide.To perform the measurements, the researchers placed 50 mL of 3% H₂O₂ in an insulated coffee cup to be used as a calorimeter, as shown in Figure 1. Before adding any catalyst, the initial, baseline temperature of the H₂O₂ solution was determined by recording the temperature every 30 seconds for 5 minutes. Then 10 mL of 0.10 M Fe(NO₃) ₃(aq) catalyst was added to the H₂O₂ solution. The temperature was recorded every 30 seconds for another 15 minutes. The experiment was then repeated using 10 mL of 0.50 M sodium iodide in 0.01 M NaOH as the catalyst. The results for each reaction are shown in Figure 2.

Figure 1 Coffee cup calorimeter

Figure 2 Temperature vs. time measurements for reactions catalyzed by Fe(NO₃) ₃ and NaI
Adapted from: C. Marzzacco, "The effect of a change in catalyst on the enthalpy of decomposition of hydrogen peroxide." Chem13 News Magazine. ©2008 University of Waterloo.
-Below is a reaction energy diagram for the decomposition of H₂O₂.

As the reaction proceeds, the enthalpy of the reaction is:

Quizplus · Accepted Answer

11ecba2d_1219_5fdb_a3bf_2986cfaf84db_MD0008_00 A reaction energy diagram shows the relative energies E of the chemical species as a reaction proceeds. During a reaction, reactant bonds are broken and new bonds form to make the products. The energy difference between the reactants and the products is the heat of the reaction ΔH, or enthalpy: 11ecba2d_1219_5fdc_a3bf_efdab4193dbc_MD0008_00 If the reactants are higher in energy than the products, ΔH is negative, which indicates that the reaction is exothermic and releases energy. Conversely, if the reactants are lower in energy than the products, ΔH is positive, which indicates that the reaction is endothermic and absorbs additional energy from the surroundings. The activation energy E_a is the minimum amount of energy required to initiate a reaction. It is the difference between the energy of the transition state and the energy of the reactants: 11ecba2d_1219_5fdd_a3bf_594923190be3_MD0008_00 Because E_a is the energy required by the reactants to form a higher energy transition state, E_a will always be positive. As seen in the reaction energy diagram for the decomposition of H₂O₂ given in this question, the reactants are higher in energy than the products; therefore, ΔH is negative. This is also confirmed by Reaction 1 in the passage because "heat" is one of the reaction products, which indicates this reaction is exothermic. Therefore, the decomposition of H₂O₂ proceeds with a negative ΔH and a positive E_a. (Choices B, C, and D) According to the reaction energy diagram for the decomposition of H₂O₂, and according to the equations for ΔH and E_a, ΔH is a negative value because the products are lower in energy than the reactants, and E_a is a positive value because the transition state is higher in energy than the reactants. As a result, ΔH and E_a cannot be equal, and ΔH cannot be zero because the reactants and products have different energies. Educational objective: The difference in energy between the products and the reactants is the heat of the reaction (enthalpy). The enthalpy is negative for exothermic reactions and positive for endothermic reactions. The difference in energy between the transition state and the reactants is the activation energy, which is always a positive value.

Passage Living Organisms That Require Oxygen to Respire Can Build Up