An investigation into how surface area affects the rate of reaction

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To investigate the factors which affect the rate of reaction between marble chips and hydrochloric acid, with one in specific detail.

An investigation into how surface area affects the rate of reaction

Aim: To investigate the factors which affect the rate of reaction between marble chips and hydrochloric acid, with one in specific detail.

Scientific knowledge:

To speed up the rate of a reaction, you can:

  • Add a catalyst
  • Increase the temperature
  • Increase the surface area
  • Increase the concentration

These four methods all link in with the "Collision Theory"

The basics of Collision Theory:

Collision Theory relates to the number of molecules in the reaction, the chance of colliding correctly and the correct amount of energy.

For a reaction to take place between two substances, their particles must collide, so if more collisions occur, the rate of the reaction increases. However only some collisions are successful. This is because not all particles have enough energy to break bonds in other particles. We can alter the conditions under which the reaction takes place so that more collisions occur in a given period of time and so that the rate will increase.

Catalysts:

The introduction of a catalyst means that less energy needs to be used for a successful collision. If each collision requires less energy, then more collisions can take place in a shorter period of time. They also provide a surface for the molecule to attach to, increasing the probability of particles bumping into each other. However, each different reaction needs a unique catalyst.

Temperatures:

Increasing the temperature gives the particles more energy, resulting in faster moving particles and, consequently, more collisions and an increased rate of reaction.

When the reactants are heated, the rate generally increases. When the reactants are cooled, the reaction generally goes slower. According to the collision theory, particles must collide in order to react. Heat gives the particles more energy to move around and so they increase their speed. This increases the chances of a collision, and more importantly a successful one, as they are hitting each other with greater force with increased energy. On the other hand, cooling has the opposite effects and the chances of successful collisions are reduced.

Surface Area:

A smaller piece within a large surface area means more areas of contact, resulting in an increased number of collisions and, so, an increased reaction rate.

When the surface area of the solid reactant is increased, i.e. the reactant is powdered, the rate of the reaction will also increase. This is because there are more particles exposed for the particles of the other reactant, e.g. acid to collide with. The more particles exposed, the more collisions and so a faster rate. If there are less particles exposed, e.g. the reactant is in lumps, there will be less collisions and the rate will decrease.

Concentration of acid

If an acid is used in a reaction, its concentration can affect the rate. At a high concentration, there are more acid particles in a certain volume. This means there is a higher chance of a collision because there are more acid particles. As there are more collisions, the rate of the reaction will increase. At a low concentration, there are less acid particles in a certain volume. Therefore there is a lower chance of a collision because there are less acid particles. Because there are fewer collisions, the rate will decrease.

An increased concentration means more particles. This increases the chance of a collision and, hence, an increased rate of reaction.




Low pressure or concentration High pressure or concentration

Prediction: (activation energy and collision theory)

In Brief:

Using the scientific knowledge, stated above, I predict the rate of reaction increases when the surface area of a solid reactant is increased. This happens because by increasing the surface area we are also increasing the rate of collisions between the particles. When there is more of the reactant to react with the reaction speeds up. Therefore, my hypothesis is: the reaction will be faster when using small chips, so the gas will be produced faster (the same amount of gas will be produced in all experiments).

I predict that as the size of particles decreases, surface area will increase causing the average collision of particles to increase. An increase in rate of collision causes an equal increase in the rate of reaction. This is based on the collision theory that states that for a reaction to occur between two particles, an effective collision must take place to form product molecules. The reaction rate is the measure of how frequently effective collisions occur.

For every chemical reaction, there is a certain minimum energy needed in the collisions before a reaction can occur, because the bonds between the atoms must be broken before new molecules can be made. This minimum energy is called the activation energy (EA) of the reaction. If the colliding molecules have enough energy, the collision is successful and a reaction occurs

As a result, of increase in surface area, I predict that there is greater area for contact between the chips and the acid for reactions to take place therefore increasing rate of reaction. The rate simply depends on how fast and often particles collide with each other.

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The energy (gas) given off becomes more as surface area increases, therefore I predict the reaction using the powdered chips will produce the highest amount of gas in the time allowed.

Based on my own knowledge, if I were to predict the shape of the graph for a certain size of chips at this early stage in the investigation, given the amount of gas produced and the time taken, I would expect the graph to be like this:

Furthermore, I hypothesis that if the surface area is increased, the rate of reaction will increase at a proportional rate.

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