Passage
Combustion occurs when an oxidation-reduction reaction takes place between a reduced fuel source and an oxidizer, most frequently oxygen. The reaction between hydrogen and oxygen forms water (Reaction 1) and is highly exothermic and thermodynamically favorable. At the completion of hydrogen combustion, essentially all H₂ molecules have been converted to water. Nevertheless, at room temperature and atmospheric pressure, molecular hydrogen and oxygen can coexist quite stably.2 H₂ + O₂ → 2 H₂OReaction 1A group of researchers designed two prototype internal combustion engines, each of which could inject a hydrogen/oxygen mixture into a 5-mL combustion chamber at a total pressure of 1 atm and could repeat this process for several cycles. In one engine design, a spark was generated to initiate combustion as hydrogen and oxygen were injected. In another method, the combustion chamber was coated with a small amount of platinum powder. Platinum dramatically increases the reaction rate at room temperature without the need for a spark. The platinum itself is not altered by the reaction and can be reused. Researchers measured the maximum power output of both engine designs under identical injection and temperature conditions (Figure 1) .
Figure 1 Relative power outputs of hydrogen engines with different ignition sources at 298 KThe power output of an engine is directly related to the rate of combustion. In the presence of platinum, researchers measured the engine's rate of water production at various temperatures and H₂/O₂ mixture compositions. The results are shown in Figure 2.
Figure 2 Rate of combustion in platinum-coated engines under various conditions
Adapted from Schultze M, Mantzaras J. Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum: Fuel-lean versus fuel-rich combustion modes doi: 10.1016/j.ijhydene.2013.06.069
-Assuming the combustion of hydrogen behaves like an elementary reaction with respect to its rate, which of the following explains why an increase in the H₂/O₂ ratio causes an increase in the reaction rate?

Question

Passage
Combustion occurs when an oxidation-reduction reaction takes place between a reduced fuel source and an oxidizer, most frequently oxygen. The reaction between hydrogen and oxygen forms water (Reaction 1) and is highly exothermic and thermodynamically favorable. At the completion of hydrogen combustion, essentially all H₂ molecules have been converted to water. Nevertheless, at room temperature and atmospheric pressure, molecular hydrogen and oxygen can coexist quite stably.2 H₂ + O₂ → 2 H₂OReaction 1A group of researchers designed two prototype internal combustion engines, each of which could inject a hydrogen/oxygen mixture into a 5-mL combustion chamber at a total pressure of 1 atm and could repeat this process for several cycles. In one engine design, a spark was generated to initiate combustion as hydrogen and oxygen were injected. In another method, the combustion chamber was coated with a small amount of platinum powder. Platinum dramatically increases the reaction rate at room temperature without the need for a spark. The platinum itself is not altered by the reaction and can be reused. Researchers measured the maximum power output of both engine designs under identical injection and temperature conditions (Figure 1) .

Figure 1 Relative power outputs of hydrogen engines with different ignition sources at 298 KThe power output of an engine is directly related to the rate of combustion. In the presence of platinum, researchers measured the engine's rate of water production at various temperatures and H₂/O₂ mixture compositions. The results are shown in Figure 2.

Figure 2 Rate of combustion in platinum-coated engines under various conditions
Adapted from Schultze M, Mantzaras J. Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum: Fuel-lean versus fuel-rich combustion modes doi: 10.1016/j.ijhydene.2013.06.069
-Assuming the combustion of hydrogen behaves like an elementary reaction with respect to its rate, which of the following explains why an increase in the H₂/O₂ ratio causes an increase in the reaction rate?

Quizplus · Accepted Answer

11ecba2d_11b3_7198_a3bf_ef1a703578db_MD0008_00 The law of mass action is usually expressed with respect to the equilibrium constant of a reaction. However, when applied to the dynamic equilibrium of an elementary reaction, the law of mass action also states that the rate of a reaction is proportional to the molar amount (concentration or partial pressure) of each reactant raised to the power of its reaction order. An elementary reaction is a reaction that proceeds in a single step, and in these reactions, the reaction order of each species is equal to its stoichiometric coefficient. For non-elementary reactions, the order must be determined empirically. Many non-elementary reactions behave as if they were elementary with respect to reaction rate. Assuming the combustion of hydrogen is one such non-elementary reaction, it is second order with respect to H₂ but only first order with respect to O₂. Therefore, the rate of the reaction depends on the partial pressure of oxygen and on the square of the partial pressure of hydrogen. As long as enough oxygen is provided for the reaction to proceed, removing some oxygen and replacing it with the same number of moles of hydrogen will result in a rate increase because hydrogen makes a greater contribution to the rate than oxygen does. (Choice A) The Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers. It is not related to the reaction rate. (Choice C) Boyle's law says that the pressure of a gas is inversely proportional to the volume it occupies. It does not directly predict the rate of any reaction. (Choice D) Henry's law states that the amount of a gas that dissolves in a liquid is proportional to the partial pressure of that gas. It is not predictive of the reaction rate. Educational objective: The law of mass action states that the rate of a reaction is proportional to the molar amount of each reaction component raised to the power of its reaction order. For elementary reactions, the reaction order of each species is equal to its stoichiometric coefficient.

Passage Combustion Occurs When an Oxidation-Reduction Reaction Takes Place Between a a Reduced