Problem 109 - Chemistry The Science in Context

Question

The following reactions were carried out in sealed containers. Will the total pressure after each reaction is complete be greater than, less than, or equal to the total pressure before the reaction? Assume all reactants and products are gases at the same temperature. a. \(\mathrm{N}_{2} \mathrm{O}_{5}(g)+\mathrm{NO}_{2}(g) \rightarrow 3 \mathrm{NO}(g)+2 \mathrm{O}_{2}(g)\) b. \(2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{SO}_{3}(g)\) c. \(\mathrm{C}_{3} \mathrm{H}_{8}(g)+5 \mathrm{O}_{2}(g) \rightarrow 3 \mathrm{CO}_{2}(g)+4 \mathrm{H}_{2} \mathrm{O}(g)\) d. \(4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \rightarrow 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g)\)

Step-by-Step Solution

Answer

a. N2O5(g) + NO2(g) -> 3NO(g) + 2O2(g) b. 2SO2(g) + O2(g) -> 2SO3(g) c. C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(g) d. 4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O(g) Answer: a. Greater b. Less c. Greater d. Greater

Step 1: Count the number of moles of reactants and products

To do this, we simply add up the stoichiometric coefficients of the reactants and products: Reactants: 1 (N2O5) + 1 (NO2) = 2 moles Products: 3 (NO) + 2 (O2) = 5 moles

Step 2: Compare the number of moles of reactants and products

Since there are 2 moles of reactants and 5 moles of products, we can conclude that the total pressure after the reaction will be greater than the total pressure before the reaction. #b. 2SO2(g) + O2(g) -> 2SO3(g)#

Step 1: Count the number of moles of reactants and products

Reactants: 2 (SO2) + 1 (O2) = 3 moles Products: 2 (SO3) = 2 moles

Step 2: Compare the number of moles of reactants and products

Since there are 3 moles of reactants and 2 moles of products, we can conclude that the total pressure after the reaction will be less than the total pressure before the reaction. #c. C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(g)#

Step 1: Count the number of moles of reactants and products

Reactants: 1 (C3H8) + 5 (O2) = 6 moles Products: 3 (CO2) + 4 (H2O) = 7 moles

Step 2: Compare the number of moles of reactants and products

Since there are 6 moles of reactants and 7 moles of products, we can conclude that the total pressure after the reaction will be greater than the total pressure before the reaction. #d. 4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O(g)#

Step 1: Count the number of moles of reactants and products

Reactants: 4 (NH3) + 5 (O2) = 9 moles Products: 4 (NO) + 6 (H2O) = 10 moles

Step 2: Compare the number of moles of reactants and products

Since there are 9 moles of reactants and 10 moles of products, we can conclude that the total pressure after the reaction will be greater than the total pressure before the reaction.

Key Concepts

Stoichiometry

Stoichiometry is like the math of chemistry, helping us understand the quantities involved in chemical reactions. It's all about balancing equations to make sure both sides reflect the same number of each type of atom. To do this, we use stoichiometric coefficients which are the numbers in front of the molecules in a chemical equation. These numbers tell us how many moles, a unit of measurement in chemistry, are involved in the reaction.
In the exercises, stoichiometry helps determine the change in total pressure by counting moles of gases. Gases behave in a predictable way under the same conditions. If the number of moles of gas changes, the pressure changes. More moles generally mean more pressure! So, by comparing the moles of reactants and products, we can figure out if the pressure will increase, decrease, or stay the same after the reaction.
Remember, when balancing equations, the goal is to have the same number of each type of atom on both sides of the equation. Each reaction example in the exercise uses this principle to predict pressure changes.

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into new substances, called products. These reactions can be simple, involving the breaking and forming of bonds or more complex changes involving energy transfer.
In the exercises described, each reaction takes place in a sealed container with gases. This setup assumes that all changes will mainly affect pressure and anything else inside remains constant, like temperature. Understanding how these gases interact, break and make bonds, is key to predicting the pressure outcomes.

• Reactions that produce more moles of gas than they consume will have a pressure increase. The first example reaction has this type of outcome.
• Conversely, reactions that result in fewer moles of gas will lead to a pressure decrease, like in the second example.
• Sometimes, the total moles of gases does not change, keeping the pressure constant.

Knowing these outcomes lets us predict how a chemical reaction affects containers of gas, vital for practical applications in industries.

Pressure Changes

The pressure in a system of gases is closely related to the number of moles present and their kinetic energy. According to the ideal gas law, which is often summarized as \( PV = nRT \), pressure (\(P\)) is related to the number of moles of gas (\(n\)), the temperature (\(T\)), and the volume (\(V\)) of the container. Here, \(R\) is a constant that doesn't change.
When a chemical reaction occurs in a sealed container, if there is an increase in the total moles of gas, the pressure will typically rise because more gas particles are moving around in the same space. This is what happens in examples c and d in the exercise. If there are fewer moles after the reaction, as seen in example b, the pressure will go down.
Changes in pressure are important for understanding many phenomena:

• In industries using chemical processes, controlling pressure can improve safety and efficiency.
• In natural processes, changes in pressure can affect weather patterns and more.

By considering stoichiometry and chemical reactions collectively, predicting pressure changes becomes more intuitive and supports understanding of real-world applications.