Rates of reactivity.

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Introduction

Rates of reactivity

The rate of reaction is simply how fast it takes for a complete reaction to occur. In this coursework the following factors will be investigated to discover whether or not they affect the rate of reaction:

  • Surface area of reactants.
  • The temperature of reactants.
  • The concentration of reactants.
  • The involvement of a catalyst in a reaction.

Collision Theory

This states that reactions only commence when reacting particles collide with each other with a sufficient amount of energy, the minimum amount of energy.

When the temperature of the reactants is increased particles absorb energy and move around a lot faster. This increases the chance of collisions.

Increased concentration leads to more reactive particles and therefore more chance of collisions, increasing the rate of reactivity.

Small pieces of reactants mean a large surface area. This allows more collision to take place resulting in a faster rate of reaction.

Catalysts allow particles to react with a lower amount of energy. They also provide a surface for the particles to attach to; therefore the chances of collisions are higher.

What is meant by rate of reaction?

Magnesium + Hydrochloric Acid                     Magnesium Chloride + Hydrogen

Mg (s) + 2HCl(aq)         MgCl2 (aq) + H2 (g)

In order for the magnesium and acid particles to react together, they must have the following bullet points present:

  • They must collide with each other.
  • The collision must have enough energy for the reaction to take place.

        Acid particles

        Water molecules

        Magnesium atoms

The particles in the liquid move around continually. Here an acid particle is about to collide with a magnesium atom.

        Here the reaction is taking place.

If enough energy is present then the reaction would take place and magnesium chloride and hydrogen will be formed.

                   Acid particles

        Water molecules

        Magnesium atoms

If not enough energy is present then the will be no reaction. The acid particles rebound away again. If there are more collisions in a given one minute, then the reaction is much more faster therefore it has a high rate. If not many collisions take place then the rate if low. The rate of reaction depends on how many successful collisions there are in a give unit of time.  

Changing the rate of reaction

Increasing the rate with concentrations

The increase in concentration of an acid results to an increase to the rate of reaction. This is explained on the nest page:

 

        Acid particles

                                                           Water molecules

        Magnesium atoms

In this diluted acid there are so many acid particles this results to having not much chance of an acid particle hitting a magnesium atom.

Here the acid id more concentrated therefore there are more chances of a successful collision occurring. The more successful collision there is, the faster the reaction.

Increasing the rate with temperature

At low temperatures the particles of reacting substances do not have much energy. However, when the substances are heated, the particles take in energy. This causes them to move faster and collide more often. The collisions have more energy, so more of them are successful. Therefore the rate of reaction increases.

Increasing the rate with surface area

The reaction that took place between magnesium and acid can be much faster when the metal is powered:

 

        Acid particles        

                                                               Water molecules

                                                                Magnesium atoms

The acid particles can collide only with those magnesium atoms in the outer most layer of the metal ribbon.

When the metal is powdered, many more atoms are exposed. So there is a greater chance of successful collisions.

Increasing the rate with catalyst

 Adding a catalyst can speed up reactions. In the presence of a catalyst, a collision needs less energy in order to be successful. The result is that more collisions become successful, so the reaction goes faster. Catalysts are very important in industry, because they speed up reactions even at low temperatures. This means that less fuel is needed so money is saved.  

The following diagrams also explain why the reaction between magnesium and hydrochloric acid slows down as time goes up:

At the start, there are plenty of magnesium atoms and acid particles, but they get used up during the process of successful collisions.

After some time, there were fewer magnesium atoms, and the acid is less concentrated. So the reaction is down.

Background Information

Heating it, adding water to it or mixing it with another substance can change a substance. The change that takes place will either be a chemical change or a physical change. In this investigation the experiments that will take place are all chemical reactions. Chemical changes means when two substances are mixed to make a new substance.

A chemical change is usually called a chemical reaction. You can tell if a chemical reaction has taken place by these signs:

  • One or more chemical substances are formed.
  • Energy is either given in or out.
  • The change that has taken place cannot be reversed.

The physical reaction can be reversed. Physical change is when no new chemical substance is formed. An example of physical reaction is ice melting to water. The rate of reaction means how fast the reaction is preceded in a certain amount of time. To find out how fast we observe either how quickly the reactants are used up or how quickly the product is being formed.

How to calculate a mole?

During the experiment we need to calculate the amount of moles we need of that substance. A mole is a simple name given to a number.

A mole is 6.023 x 1023.One mole of atoms or molecules of any substance will have a mass is grams equal to the relative formula of that substance.

Example:

Carbon has a relative mass of 12.

So moles of carbon weight exactly 12g.

This means that 12g of carbon weight one mole.

Example 2:

Hydrochloric acid has a relative molecular mass of 36.5.

So 1 mole of HCI would equal to 36.5g in 1000ml of water (H2O)

If we wanted to work out 0.1 molar of HCI in water we would divide it by 10.

0.1Mole of HCI = (36.5)

                         10

0.5 moles of HCI = 36.5

                          2

2 moles of HCI = 36.5 × 2

More information

Chemical rates and their Mechanism

Some reactions, such as explosions, occur rapidly. Other reaction, such as rusting, takes place slowly. Chemical kinetics is the study of reaction rates, shows that three conditions must be met at the molecular level if a reaction is to occur, the molecules must collide.

They must be positioned so that the reacting groups are together in a transition state between reactants and products.

And the collision must have enough energy to form the transition state and convert it into products.

Fast reactions occur when these three criteria are easy to meet. If even one is difficult, however, the reaction is typically slow, even though the change in free energy permits a spontaneous reaction.

Rates of reaction increase in the presence of catalysts, substance that provide a new, faster reaction mechanism but the are unchanged or regenerated so that they can continue the process. Mixtures of hydrogen and oxygen gases at room temperature do not explode. But the introduction of powered platinum leads to an explosion as the platinum surface becomes covered with absorbed oxygen. The platinum atoms stretch the bonds of the oxygen molecules, weakening then and lowering the activation energy. The oxygen atoms then react rapidly with hydrogen molecules, colliding with them, forming water, and regenerating the catalyst. The steps by which a reaction occurs are called the reaction mechanism.

Rates of reaction can be changed not only by catalyst but also by changes in temperature and by changes in concentration. Raising the temperature increases the rate by increasing the kinetic energy of the molecules of the reactants, thereby increasing the number of collisions per second and the likelihood of transition states being achieved. Increasing the concentration can also increase the reaction rate by increasing the rate of molecular collisions.

Chemical Equilibrium

As a reaction proceeds, the concentration of the reactants usually decreases as they are used up. The rate of reaction will, therefore, decrease as well. Simultaneously, the concentrations of the products increase, so it becomes more likely that they will collide with one another to reform the initial reactants. Eventually, the decreasing rate of the forward reaction becomes equal to the increasing rate of the reverse reaction, and net changes ceases. At this point the system is said to be at chemical equilibrium. Forward and reverse reactions occur at equal rates.

Changes in systems at chemical equilibrium are described by Le Chatelier’s principle, named after the French scientist Henry Louis Le Chatelaine. Any attempt to change a system at equilibrium causes endothermic reactions to occur, lowering the temperature causes exothermic reactions. Raising the pressure favours reactions using up the added material, decreasing any concentration favours forming that material.

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Preliminary test

 

Before this experiment was taken place a Preliminary test was taken, to see if the apparatus was set properly, and there was no difficulties in the Preliminary test.

Resources used

During this experiment work much searched was used to find out more information about rates of reaction. The resources used were the library, the Internet, my teacher and books. I got the information about ‘the collision theory’ from the GCSE Chemistry Revision Guide. For the Internet I got help from .

Safety precautions  

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