The Effects of Strong and Weak Acids on the Order of a Reaction.

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Aim

To investigate the effect strong and weak acids have on the rate of a reaction and its order.

Introduction

What is a chemical reaction? This is the first question that needs to be answered before tackling this investigation. A chemical reaction is the process which results in the chemical conversion of one substance into another. These reactions can be placed into two groups; exothermic and endothermic reactions. If the energy given out of the reaction is greater than the energy taken in, the reaction is known as an 'Exothermic Reaction'. Similarly, if the energy taken in is greater than the energy given out, the reaction is known as an 'Endothermic Reaction'. The easiest way to determine if a reaction is Exo/Endothermic is to observe any temperature changes within or after the reaction has taken place. If the surrounding temperature drop, the reaction is exothermic and if the temperature increases, the reaction is exothermic.

Although it is true that every substance has the potential to be converted into another, it is also true that before a reaction happens, certain criteria needs to be met. For a reaction to happen, the particles must collide with the correct amount of energy - also known as the 'Activation Energy'. The Activation energy is the lowest amount of energy a particle needs in order for it to successfully collide with another and for the reaction to take place. So, for a reaction to 'go', a certain number of particles have to have the correct amount of energy. Although the activation energy for a reaction can never be changed, outside factors can affect how many particles in the reaction have the minimum energy requirement to react successfully.

Figure 1: http://www.docbrown.info/page03/3_31rates.htm#3a

As you can see from the above diagrams, the Activation energy (Ea) is the difference between the transition state of the particles - the state where the old bonds aren't yet fully broken and the new bonds aren't yet fully formed.

You will also notice that Endothermic reactions have a much greater Activation energy than Exothermic reactions and this is because the energy taken to form new bonds is greater than the energy taken to break them.

The rate of a reaction is the time taken for the particles to reach the activation energy and for the reaction to 'go'. The fewer the number of particles with the activation energy, the slower the rate of reaction, and vice-versa. Not every particle in a substance can have the minimum energy requirement for the reaction and this is when outside factors can affect the rate of the reaction. Outside factors, such as the temperature, concentration, surface area and the use of a catalyst can affect the rate of the reaction by speeding it up or slowing it down.

Effect of Surface Area

The number of correct collisions in the reaction mixture affects the rate of the reaction. This is directly proportional to the surface area of the reactants. The greater the surface area, the higher the chance of correct collisions taking place. The smaller the surface area, the smaller the chance of correct collisions taking place. This is due to the number of particles exposed to the others - the greatest example of this is the use of Magnesium Powder in comparison to Magnesium Ribbon.

Figure 2: http://www.sciencepages.co.uk/keystage4/GCSEChemistry/m3ratesofreaction.php#rate7

As you can see, the smaller chips - or Mg powder - would be more exposed to the reactants and will therefore encounter more successful collisions. This will increase the rate of reaction. However, sometimes the particles can be too small and increase the rate of reaction too much for it to be observed. It is for this reason that Mg ribbon would be the easiest to use in my experiment - more on that later on.

Effect of Concentration

The increase in the concentration of a substance will increase the rate of its reaction. This is because there are more particles to be reacted with and therefore more collisions to be made - this will increase the number of successful collision which is what makes the reaction 'go'. The concentration of something is the number of moles of a product per given volume. SO the increase in concentration means that more particles are in closer contact with each other and the collision between the correct molecules is more likely.

Figure 3: http://www.docbrown.info/page03/3_31rates.htm#3a

Effect of temperature

As stated previously, every particle has some energy within them - this is called Kinetic energy. As the temperature is increased, the kinetic energy these particles have is also increased. This means that the particles are not only moving faster - which will help them surpass the activation energy - but there is also a greater chance of correct collisions. With more particles able to react now due to their increase in kinetic energy, the number of correct collisions will increase which means the overall rate of reaction will increase.

As you can see from the diagram above, T1 is at a lower temperature than T2 and has a significantly lower number of molecules with energy greater than the Activation energy.

Effect of a Catalyst

The use of a catalyst increases the rate if a reaction. It is wrong to say, however, that the catalyst lowers the activation energy of a reaction. The definition of a catalyst is a substance which provides an alternate route for the reaction with a lower activation energy and can be used in a reaction without getting used up, itself. By providing an alternate route for the particles with a lower activation energy, more particles have the minimum energy requirement and so can take part in the reaction - this leaves more scope for the number of correct collisions per second.

Figure 4: http://www.chemguide.co.uk/physical/basicrates/catalyst.html

As you can see from the figure above, the use of a catalyst shows that a significantly larger proportion of particles have the minimum needed amount of energy to take part in the reaction.

While an alternate activation energy is provided, the thermodynamics of the reaction remain unchanged. There are two types of catalysis - Homogenous and Heterogeneous.

Homogenous catalysis is when both the reactants and the catalyst are in the same phase. An example of this would be like the synthesis of an Ester; both the Carboxylic acid, the alcohol and the Hydrogen ion catalyst used are in an aqueous solution.

Heterogeneous catalysis is when both the reactants and the catalyst are in different phases. An example of this would be in the Haber process in the synthesis of Ammonia. The reactants (Hydrogen and Nitrogen) are gases and the catalyst (Iron) is a solid.

Working out the Activation Energy (Ea)
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To work out the activation energy of a reaction, the Arrhenious equation must be used. This is:

rate = Ae-(Ea / RT)

Where, A = constant,

e = exponential function,

R = gas constant and

T = temperature.

This equation can also be written as:

ln (rate) = ln (A) -(Ea / RT)

ln, being the natural log of the number. Using the natural log makes the question far easier to use.

To work out the activation energy, one can substitute the equation above and write ...

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