Chemistry Unit 7.1: Speed of Reaction

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Unit 7.1 Speed of Reaction

1. Describe the effect of concentration, particle size, catalysis and temperature on the speeds of reactions.

Here are some factors that speed up the rate of a reaction:

• surface area: the larger the surface area, the faster the rate; this is because this increases the chance of particles colliding with each other
• concentration: a higher concentration means there are more particles in each cm^3 of space, so there are more collisions
• catalysis: a catalyst speeds up the rate of a reaction; it lowers the amount of energy needed for the reaction to take place, so it occurs more easily
• temperature: a higher temperature provides energy for articles to move more quickly, so they collide more often

2. Describe a practical method for investigating the speed of a reaction involving gas evolution.

An example to investigate this is using magnesium and hydrochloric acid to test the rate of reaction. The equation goes as follows:

magnesium + hydrochloric acid --> magnesium chloride + hydrogen

Equipment:

• conical flask

• gas syringe

• stopwatch

• hydrochloric acid

• magnesium

Using this apparatus, we can design an experiment that, for example, measures how long it takes for 20cm^3 of gas, using the volume of magnesium as our independent variable (the one you change). 

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The diagram above uses magnesium ribbons instead of powder, but that's completely fine.

3. Devise a suitable method for investigating the effect of a given variable on the speed of a reaction.

For the experiment above, time was the variable that we changed. The experiment could also be designed around other variables:

• the concentration of acid

• temperature conditions

• surface area of magnesium (chips? ribbon? powder?)

• the reaction with a catalyst and without

4. Interpret data obtained from experiments concerned with speed of reaction.

Once you have carried out the experiment and obtained data, you'll be able to come to a conclusion on what you were investigating.

5. Describe the application of the above factors to the danger of explosive combustion with fine powders (e.g. flour mills) and gases (e.g. mines).

So you know that variables affecting the rate of a reaction are temperature, concentration, and surface area. These are applicable to real life situations.

In a flour mill, flour is produced. Flour particles are very small, so they have a large surface area. If there is a lot of flour in the air, a small spark can cause an explosion between flour and oxygen.

In a coal mine, the air is filled with flammable gases. If the gases reach a certain concentration they can form an explosion with air. 

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6. Describe and explain the effects of temperature and concentration in terms of collisions between reacting particles (concept of activation energy will not be examined).

As mentioned above, a higher temperature will give particles more energy to move and collide, and a higher concentration means there are more particles in the space that will collide. 

As a reaction occurs and the reactants get used up, the concentration of the substance decreases; this is why reactions slow down as they happen over a span of time.

7. Define catalyst as an agent which increases rate but which remains unchanged.

A catalyst is a substance thats speeds up a reaction, but the catalyst itself will not be used up.