Rates of Reaction

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Rates of Reaction coursework

Aim: My aim is to see how the concentration of hydrochloric acid affects the rate of reaction, using marble chips.

Prediction:

Primarily I state my hypothesis to be; that the increase in concentration (hydrochloric acid) of reactants will increase the collision frequency between the reactants, this and calcium carbonate. Therefore the effective collision frequency also increases.  A chemical reaction is where the molecules of reactants either combine or separate, in this case we are looking at the displacement of the between calcium carbonate and hydrochloric acid, below is the formula for this reaction:

2HCl (aq) + CaCO3 (s) ----> CaCl2 (aq) + H2O (l) + CO2 (g)

Hydrochloric acid + Calcium carbonate → Calcium chloride + water + carbon dioxide

Reactants:


2HCl (aq): hydrochloric acid (aqueous)
CaCO
3 (s): calcium carbonate (solid)

Products:


CaCl
2 (aq): calcium chloride (aqueous)
H
2 O (l): water (liquid)
CO
2 (g): carbon dioxide (gas)

I will now state the principals of the collision theory in order to elucidate the reaction. Max Trautz and William Lewis first proposed this theory in 1916 that qualitatively explains how reactions occur and why rates differ depending on the reactants and reactions. It assumes that for a reaction to be successful the reactant particles must collide, but only a certain fraction of the total collisions, the fruitful (successful) collisions, cause the transformation of reactant molecules into products. Furthermore factors, which contribute to whether the particles successfully collide and react, vary from speed, the angle of collision and consequently the orientation of the molecules when they collide.

In relationship to this experiment, the higher concentration of hydrochloric acid added would result in a higher collision frequency, meaning that more particles will collide with each other, which would conclude in more successful reactions taking place.

The fraction of the molecules have sufficient energy and the right orientation at the moment of impact to break the existing bonds and form new bonds; The minimal amount of energy needed so that the molecule is transformed is called activation energy (Ea), this is also known as threshold energy.

The activation energy is the height of the barrier (sometimes called the energy barrier) separating two minima of potential energy (of the reactants and of the products of reaction).  This can be interpreted through the diagram to the left:

This graph also illustrates what happens to the reactants within the reaction, as you can see from the graph the reactants remain stationary, it then increases as you can see a dramatic rise of the line; though successful collisions only take place where the activation energy is shown, the deterioration of the line shows the reaction is thus coming to an end and there is a lowering of the collision frequency making successful collisions less frequent.  As you can see from the diagram above that this reaction is exothermic; in an exothermic reaction, reactants have a higher energy level than the products. The difference between these two energy levels is the energy released to the environs in the reaction, and an energy level diagram shows this as a vertical drop from a higher to a lower level. Generally further energy is required to ‘activate’ the reaction. This is called the activation energy and is drawn in energy level diagrams as a hump.

Fundamentally the collision theory is based on the Kinetic theory; this theory describes the physical properties of matter in terms of their behaviour, principally movement of its component atoms or molecules. In addition to this It states that all matter is formed by very small particles that are in constant motion, and can be used to explain the properties of solids, liquids, and gases, as well as changes of state from one to another.

A reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles, as explained by collision theory previously. However, the main reason why it increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions (when bonds are formed between reactants).

This can be exemplified further using the Maxwell-Boltzmann Distribution below.

The graph shows the range of energies possessed within the fluid. The majority of particles have a consistent amount of energy, with the exception to other particles, which either concern a small, or large amount of energy.

However, for the reaction to take place, the top section of the curve must exceed the activation point, so that most of the particles posses the minimal amount of energy in order to collide and react. This can be done via increasing the temperature, however my aim is to see the effects of concentration on the rates of reaction, therefore Temperature will be a control variable in the experiment, explained later.

During the experiment I will have to use equal sized marble chips because the surface area can affect the results attained. After having tried the powder, large and small chips I discovered that the medium would be most suited and preferred for my usage.  So thus we can see how the concentration affects the reaction rate not the surface area that will be a controlled variable. I will be using the following solutions: 0.5, 1.0, 1.5, 2.0, and 2.5 molars of hydrochloric acid to help me see the pattern in which each of the chemical reactions is being affected.

However I have produced some of the molar solutions myself in order to give more accurate results, for example to form 0.5M I used 10ml of water and 10 ml of hydrochloric acid with a molar of 1 which ultimately creates 20ml of 0.5M acid. I predict that the 0.5M will react at a much slower pace and less gas would be released though this is primly due to the fact that there are fewer molecules capable of reacting, as the acid due to the weakness of the acid.

In conjunction to this the 2.5M would have the quickest reaction rate as there gas is given off in larger amounts patent to the fact that there are more molecules, which would increase the collision frequency.

I will time the experiment for a period of 2 minutes; results will be recorded every 10 seconds in order to achieve more accurate results.

In the graph shown below conveys the relationship between the concentrations of the hydrochloric acid against the time.  

As you can see from the graph, the greater the concentration of the hydrochloric acid within the solution the quicker the rate of reaction occurs. It also shows how fast the reactants are used up in each solution. So evidently I would expect to see my graph to have similar correlations to this one.

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If I were going to take the concentration of the hydrochloric acid further approximately up to 3M I would expect to see again an increase in the curve of the graph.

As the brown line (3M) has steepened considerably, it shows that the greater the concentration used the faster the rate of the reaction. This is simply due to the fact that there are more particles able to collide with others in the solution, as there are a larger amount of particles which means there is a higher collision frequency and a fraction of these particles would ...

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