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
-In the absence of platinum, a spark initiates combustion by doing which of the following?

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
-In the absence of platinum, a spark initiates combustion by doing which of the following?

Quizplus · Accepted Answer

11ecba2d_11b2_6026_a3bf_7b07a65e4f5e_MD0008_00 Most chemical reactions can only occur when two or more molecules collide with enough energy to break bonds, allowing new bonds to form. This required energy is called the activation energy. Temperature is a measure of the average kinetic energy of the molecules in a system. At higher temperatures, the molecules move more quickly and collide more often and with more energy. Therefore, an increase in temperature will increase the rate of a reaction by providing more molecules with the required activation energy. The reaction between hydrogen and oxygen is thermodynamically favorable, meaning that it occurs spontaneously. However, the reaction has a high activation energy. At room temperature, most of the molecules lack the energy necessary to initiate the reaction, and the reaction proceeds so slowly that it effectively does not occur at all. A spark releases heat into the system, raising the temperature and therefore the kinetic energy of nearby molecules. These molecules are then able to react exothermically, releasing more heat into the system and providing more molecules with sufficient energy to react. (Choice A) The equilibrium position of a reaction is not appreciably altered by the presence of a spark. Instead, the rate at which equilibrium is achieved changes. (Choice B) The spark consumes a negligible amount of oxygen. Because oxygen is a reactant, its consumption would drive the reaction toward reactant formation based on Le Châtelier principle. (Choice D) Oxidizers such as oxygen gain electrons in combustion reactions. They do not lose electrons. Educational objective: Molecules must collide with sufficient energy, known as the activation energy, for a reaction to occur. Increased temperature increases the kinetic energy of the molecules in a system, and therefore increases the rate of a reaction.

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