Question

Hydrogen reacts explosively with oxygen. However, a mixture of \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) can exist indefinitely at room temperature. Explain why \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) do not react under these conditions.

Short answer

A mixture of \(H_{2}\) and \(O_{2}\) does not react spontaneously at room temperature because the energy barrier for the reaction is too high to be overcome by the energy present in the system. The reaction between Hydrogen and Oxygen requires a certain activation energy, which can be provided in the form of heat, sparks, or a flame. Without an external source of energy to provide the necessary activation energy, the molecules remain in a stable state and do not break their existing bonds to form new ones with each other.

Step-by-step solution

Understanding the reaction between Hydrogen and Oxygen

The reaction between Hydrogen (H2) and Oxygen (O2) is a well-known chemical reaction that produces water (H2O). The balanced chemical equation for this reaction is: \[2\,\mathrm{H}_{2} + \mathrm{O}_{2} \rightarrow 2\,\mathrm{H}_{2}\mathrm{O}\] This reaction is exothermic, which means it releases energy, and under certain conditions, it can be quite explosive.

Identifying the conditions necessary for a reaction

For a chemical reaction to occur, there needs to be a certain amount of energy provided to the reactants, known as the activation energy. This energy helps the reactants overcome the energy barrier and form new bonds, ultimately leading to the formation of products. In the case of the reaction between Hydrogen and Oxygen, this activation energy can be provided in the form of heat, sparks, or a flame.

Explaining the absence of reaction at room temperature

At room temperature, the energy present in the system is not sufficient to provide the required activation energy for the reaction between Hydrogen and Oxygen to occur. This means that even if H2 and O2 are mixed, they do not react because the energy barrier is too high for them to overcome spontaneously. In other words, the H2 and O2 molecules are in a stable state at room temperature, and they do not have enough energy to break their existing bond and form new bonds to create water molecules.

Factors affecting the reaction rate

Other factors may also affect the reaction rate between Hydrogen and Oxygen, such as the pressure and concentration of the reactants. Generally, a higher pressure or concentration of reactants can increase the reaction rate by further lowering the activation energy. However, these factors are also insufficient to initiate the reaction at room temperature without an additional source of energy, like heat or a flame. To summarize, a mixture of hydrogen and oxygen does not react spontaneously at room temperature because the energy barrier for the reaction is too high to be overcome by the energy present in the system. As a result, the reaction cannot occur without an external source of energy, such as heat or a spark, to provide the necessary activation energy.

Key concepts

Exothermic Reactions

An exothermic reaction is a type of chemical process that releases energy, usually in the form of heat or light, as products are formed from reactants. When hydrogen and oxygen combine to form water, the reaction is highly exothermic; that is, it releases a significant amount of energy.

In a typical exothermic reaction, the energy released as products form is greater than the energy required to break the bonds in the reactants. In the hydrogen-oxygen reaction, the formation of the strong O-H bonds in water releases more energy than is used to break the H-H and O=O bonds in the individual gas molecules.

However, despite being exothermic, such reactions often need an initial input of energy, known as activation energy, to get started. This is metaphorically similar to needing a spark to ignite a fuel; without the spark, the fuel won't burn, even though it could release a considerable amount of energy when it does.

Reaction Rate Factors

The rate of a chemical reaction can be affected by various factors. One key factor is temperature, which increases the kinetic energy of molecules, making collisions more frequent and more energetic, thus increasing the likelihood of overcoming the activation energy barrier.

Alternatively,

Concentration and Pressure

can also influence the reaction rate. An increase in the concentration of reactants leads to an increased number of collisions per unit time. Similarly, higher pressure, which is directly related to concentration in gases, brings particles closer together, thereby increasing the frequency of collisions.

Catalysts

are substances that speed up reactions without being consumed. They work by providing an alternative pathway for the reaction with a lower activation energy. Lastly, the presence of

Surface Area

in solid reactants can also affect the rate of reaction; a greater surface area allows more collisions to occur.

Chemical Equation Balancing

Balancing chemical equations is essential to accurately represent the conservation of mass in a chemical reaction. Each element must have the same number of atoms on both sides of the equation.

In the equation for the reaction between hydrogen and oxygen, the balanced form is crucial for correctly portraying the stoichiometry of the reaction, which is necessary for predicting the proportions of reactants and products. Through the equation \[2\,\mathrm{H}_{2} + \mathrm{O}_{2} \rightarrow 2\,\mathrm{H}_{2}\mathrm{O}\], we understand that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water. Unbalanced equations may lead to incorrect calculations and a faulty understanding of the reaction at hand.

Reactant Concentration Effects

The concentration of reactants can significantly affect the rate at which a chemical reaction proceeds. In an environment where the concentration of reactants is high, the probability of collisions between reactant molecules is greater.

This heightened probability means that reactants have more opportunities to collide with the energy necessary to overcome the activation energy barrier, leading to a higher reaction rate. Conversely, at low concentrations, such as mixtures of hydrogen and oxygen at room temperature, molecules collide less frequently, making the initiation of the reaction far less likely without external energy input.