Question

When \(6 \mathrm{~g}\) of granulated \(\mathrm{Zn}\) is added to a solution of \(2 \mathrm{M}\) \(\mathrm{HCl}\) in a beaker at room temperature, hydrogen gas is generated. For each of the following changes (at constant volume of the acid) state whether the rate of hydrogen gas evolution will be increased, decreased, or unchanged: (a) \(6 \mathrm{~g}\) of powdered \(\mathrm{Zn}\) is used, (b) \(4 \mathrm{~g}\) of granulated \(\mathrm{Zn}\) is used, (c) \(2 M\) acetic acid is used instead of \(2 M \mathrm{HCl}\), (d) temperature is raised to \(40^{\circ} \mathrm{C}\).

Short answer

a) Increased, b) Unchanged initially, c) Decreased, d) Increased.

Step-by-step solution

Understanding Zinc's Surface Area and Reaction Rates

The reaction between zinc ( Zn ) and hydrochloric acid ( HCl ) is influenced by the surface area of the zinc used. Powdered zinc has a larger surface area compared to granulated zinc, allowing more zinc particles to be in contact with the acid, thereby increasing the rate of hydrogen gas evolution.

Considering Mass and Reaction Rates

Reducing the initial amount of zinc from 6 g to 4 g decreases the availability of zinc for the reaction at any given time. However, assuming there is an excess of acid, the rate of reaction might initially be maintained because sufficient acid is available to react with the zinc. Nevertheless, with less zinc, the total hydrogen gas evolved will eventually be less after the reaction completes.

Comparing Acids: Strong vs Weak

Hydrochloric acid (HCl) is a strong acid, which dissociates completely in solution, leading to a faster reaction rate than acetic acid, a weak acid that does not fully dissociate. Therefore, replacing HCl with an equimolar acetic acid solution will decrease the rate of hydrogen gas evolution.

Effect of Temperature on Reaction Rates

Increasing the temperature from room temperature to 40°C accelerates the rate of reaction because (1) the kinetic energy of the reactant molecules increases, resulting in more frequent and energetic collisions, and (2) more molecules surpass the activation energy barrier, speeding up the overall reaction.

Key concepts

Surface Area in Reactions

When it comes to chemical reactions, surface area can significantly affect how quickly a reaction occurs. Imagine zinc ( Zn ) reacting with hydrochloric acid ( HCl ). If you use powdered zinc instead of granulated zinc, you increase the surface area available for the reaction. This means more zinc particles are exposed to the acid at once.

• More surface area means more frequent particle collisions.
• Higher collisions often lead to a higher reaction rate.

So, using powdered zinc rather than larger granules speeds up the production of hydrogen gas. This is because the tiny zinc particles react more quickly due to enhanced exposure to the acid.

Concentration of Reactants

The amount or concentration of reactants influences how fast a reaction takes place. In our example, the hydrochloric acid is kept at a constant concentration of 2 M . When there's enough acid to react with the zinc, maintaining this concentration means there's a steady rate of reaction initially.

• Higher concentrations lead to more collisions between reactant molecules.
• This increases the likelihood of a reaction occurring.

If you reduce the amount of zinc from 6 g to 4 g, the rate of reaction can initially remain unchanged, provided there is still excess acid. However, less zinc ultimately means less product formed in total.

Effect of Temperature on Reaction Rates

Temperature is a critical factor that significantly influences reaction rates. Consider raising the temperature of the reaction from room temperature to 40°C. Such an increase results in:

• Higher kinetic energy of molecules, leading to more frequent and effective collisions.
• The likelihood of more molecules having enough energy to overcome the activation energy barrier is higher.

Both of these factors mean that as temperature goes up, the rate of reaction usually goes up too. This is why reactions tend to be faster at higher temperatures, leading to quicker hydrogen gas evolution in our scenario.

Strong vs Weak Acids

The nature of the acid used in a reaction is pivotal to its rate. Strong acids like hydrochloric acid ( HCl ) dissociate completely in water, providing more hydrogen ions available for reaction instantly. This leads to faster reactions. Conversely, weak acids such as acetic acid dissociate partially:

• Less complete dissociation leads to fewer available hydrogen ions.
• This results in a slower reaction rate compared to strong acids.

So, swapping HCl for acetic acid would decrease the rate of hydrogen gas evolution due to the fewer reactive ions present.