Exercises: Factors affecting reactions

A banana ripening on the kitchen table happens much faster than inside the refrigerator. What factor is accelerating ripening in one case and slowing it down in the other? Briefly explain how this factor acts.

Olivia adds a sugar cube to her coffee with milk and Marco adds the same amount of granulated sugar to his, as shown in the figure. Both stir equally.

Which of the two will dissolve first? What factor affecting the rate of reaction explains the difference?

The fish we buy is kept in the freezer so that it takes much longer to spoil. What factor are we controlling by freezing it, and why does it slow down decomposition reactions?

To make an antacid take effect sooner, pharmacies sell it in effervescent tablets that dissolve in water, or it is recommended to crush the tablet. What factor is being exploited in both cases?

In soup broth, fine salt dissolves almost instantly, while coarse salt takes much longer, even if the amount is the same. Explain the difference using the particle model: consider which salt particles are in contact with water in each case.

An oak log can burn for hours in a fireplace, but sawdust from the same oak, if scattered, burns almost instantly.

1. a) What factor explains this difference?
2. b) Explain why smoking is banned in carpenter shops and why accumulated sawdust is cleared away frequently.

An effervescent vitamin C tablet takes $80\ \text{s}$ to dissolve completely in a glass of water at $10\ \celsius$ and only $20\ \text{s}$ in water at $30\ \celsius$. The graph summarizes the data:

1. a) How many times faster is the reaction at $30\ \celsius$ than at $10\ \celsius$?
2. b) Explain this result considering how particles move as temperature increases.

Almost all chemical reactions go faster when the temperature rises. The figure represents the particles of a gas at two different temperatures:

Explain it using the particle model: what happens to the reactant particles and what consequence does this have for their collisions?

Steel wool used for scrubbing burns with sparks when brought near a flame, whereas a nail made of the same material only rusts slowly over the years.

1. a) What factor explains why the same material reacts so differently?
2. b) What is the slow oxidation undergone by the nail called?

Ethan wants to check that temperature affects the rate at which cocoa powder dissolves in milk. Design the experiment for him: what materials he needs, which variable will change between the two glasses, what things he must keep exactly the same, and what he will measure to compare.

You want to test the effect of surface area using two identical effervescent tablets: one whole and the other crushed into powder.

1. a) Describe the procedure step by step.
2. b) Predict the result and justify it using the particle model.
3. c) Explain why you must use the same amount of water at the same temperature in both glasses: what would happen to your conclusions if you did not?

Aitana mixes two colorless solutions in the lab and measures the time it takes for a yellow solid (a precipitate) to appear. She repeats the mixture at different temperatures and obtains: $80\ \text{s}$ at $10\ \celsius$, $40\ \text{s}$ at $20\ \celsius$ and $20\ \text{s}$ at $30\ \celsius$. The data are shown on the graph:

1. a) Describe in your own words the trend shown by the data.
2. b) Looking at the trend, how much time would roughly be expected at $40\ \celsius$?
3. c) Justify the trend using the particle model.

Explosions have occurred in grain silos and old flour mills without any visible flame: a small spark in air laden with flour dust was enough.

1. a) Explain why suspended flour dust burns much faster than the same flour piled in a sack.
2. b) Which of the factors that speed up reactions is acting to an extreme degree?
3. c) Propose a safety measure for these places and justify it.

You prepare four glasses of water with sugar: glass 1 with cold water and a sugar cube, glass 2 with cold water and the same amount of granulated sugar, glass 3 with hot water and a sugar cube, and glass 4 with hot water and granulated sugar. The figure shows the four glasses:

1. a) Which two glasses would you compare to study only the effect of temperature? Justify your choice.
2. b) And which two glasses would you compare to study only the effect of surface area?
3. c) Which glass will take the least time to dissolve the sugar? Give reasons for your answer by combining both factors.