Exercises: Factors affecting reactions

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

Lucía adds a sugar cube to her white coffee 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?

When we blow on barbecue coals, they burn more fiercely. What are we supplying when we blow? What factor that speeds up reactions comes into play?

What is a catalyst? Explain what effect it has on the rate of a reaction and whether it is consumed in it. Give an example of a catalyst you know.

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?

A glowing wooden splint catches fire with a flame when inserted into a tube filled with pure oxygen. What factor explains why combustion in that tube is much more vigorous than in air?

Cars have a catalytic converter in their exhaust pipe. What role does it play regarding the pollutant gases leaving the engine? Why do you think it is called that?

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) Reason why smoking is banned in carpenter workshops and 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 summarises 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.

If you chew a piece of bread for a long time, it ends up tasting sweet: an enzyme in saliva begins to break down the starch in the bread into sugars.

1. a) What role does this enzyme play in the reaction?
2. b) Why are enzymes described as biological catalysts?
3. c) Name another process in the human body in which enzymes are involved.

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?

Iker 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.

In many gas burners there is an adjustable air inlet: with a small air inlet the flame is yellow and blackens pans, and with a larger inlet it is blue and heats better.

1. a) What factor are you modifying when adjusting the air inlet?
2. b) Explain why combustion is more complete and efficient with the inlet open.

Design a complete experiment to test how the concentration of a reactant affects the reaction rate, using vinegar and bicarbonate of soda: you would prepare several glasses with different amounts of vinegar, making them all up to the same volume with water. Indicate:

1. a) Which variables you keep constant and why.
2. b) Which variable you change from one glass to another.
3. c) How you would measure the rate of reaction (what you would observe or time).
4. d) What result you expect to obtain and how you would interpret it.

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 colourless solutions in the laboratory 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.

To make homemade yoghurt, milk with ferments is kept at around $40\ \celsius$ for several hours. If the mixture boils, yoghurt is no longer formed.

1. a) What are ferments, from the point of view of a chemical reaction?
2. b) Explain why they work well at $40\ \celsius$ but stop acting if the mixture boils.
3. c) Predict what would happen if you left the mixture at $10\ \celsius$ instead of $40\ \celsius$ and give reasons for your answer.

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.

A magnesium ribbon burns in air with an intense white flame, but inside a jar full of pure oxygen the combustion is much more violent. The figure represents the gas particles inside each jar:

1. a) Knowing that air contains only $21\ \%$ oxygen, explain the difference using the particle collision model.
2. b) Predict what would happen if the jar were filled with an oxygen-free gas, such as nitrogen, and justify your prediction.

A car's catalytic converter contains metals such as platinum, which speed up reactions that convert toxic exhaust gases into less harmful gases.

1. a) Explain what it means that a catalyst "is not consumed" in the reaction.
2. b) If it is not consumed, why do you think a catalytic converter might stop working properly after many years? Think about what could happen to its surfaces with use.