What factors effect the rate of a chemical reaction?

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What factors effect the rate of a chemical reaction

Introduction

During any chemical reaction the concentrations of the reactants decrease and the concentration of products increase with time. The rate of reaction (reaction velocity) may be defined as the rate of change of concentration of a stated reactant or product. The rate of a reaction is found by measuring the amount of a reactant used up per unit of time or the amount of a product produced per unit of time. A reaction can be made to go faster or slower by changing a number of factors. In order for a reaction to occur it is necessary that: particles must collide with each other and the collision must have enough energy. If this happens the original bonds are broken and new bonds are formed – so that new products are formed. Successful collisions (those with sufficient energy) can be increased (or decreased) by a number of factors.

 

Aim

I am trying to find out what factors effect the rate of a particular reaction – the reaction between Calcium Carbonate and Hydrochloric acid.

 

Key Variables

The key variables that I could change are:

 

Surface area of solid – The surface area has an effect on the rate of reaction. If the solid has a large surface area then there are more opportunities for collisions to occur between the solid and liquid. This is because there are more opportunities for collisions to occur. If the surface area is small, collision can only occur with the outer atoms and is therefore limited. The diagram below illustrates this:

 

 

The temperature of the reaction - When the temperature is low, the particles in the reaction do not have much energy so collision is unlikely to occur. However on heating, particles take in energy causing them to move faster and collide more often. Because the collisions have more energy they are more likely to be successful in breaking and reforming bonds. Therefore the rate of the reaction will increase. When the temperature increases by 10C, the rate of the reaction roughly doubles.

 

 

 

Concentration of the reactants

 

 

The more concentrated the solvent, the more particles are present and the greater the chance of a successful collision. In dilute acids, with fewer particles the chance of successful collisions occurring is reduced. Therefore if concentration is increased, the reaction rate also increases. This also applies in reactions between two gasses when increasing the pressure has the same effect as increasing the concentration.

 

Amount of the reactants

It is important to keep the amount of reactants constant in the experiment because, as the reactant is used up, so the reaction slows down. This means a small amount of solid will have a different reaction time to a large amount. A small amount of solid will have fewer atoms so will be less likely to successfully collide with acid particles. The limiting factor would be the amount of the smaller reactant.

 

Presence of a catalyst

A catalyst increases the rate of a chemical reaction without itself being used up. Catalysts are usually transition metals. In the presence of a catalyst, less energy is needed by a collision in order to be successful. Therefore there are relatively more successful collisions and so the reaction rate increases.

 

In order to ensure a fair test with reliable results, I will need to keep all variables constant apart from the one I am intending testing.

 

The investigation

I am going to investigate how a change in temperature will effect the rate of the reaction between calcium carbonate (marble chips) and hydrochloric acid). The word equation for the reaction is:

 

Calcium carbonate + hydrochloric acid À calcium chloride + water + carbon dioxide

 

The chemical formula for this reaction is as follows:

 

CaCO3(s) + 2HCl(aq)À CaCl2(aq) + H2O  + CO2(g) 

 

I will collect the carbon dioxide as a measure of the rate of reaction.

Prediction:

I can predict that there will be a positive correlation between the rate of the reaction and the temperature so that as the temperature is increased, the rate of reaction will increase

Graph to show relationship between temperature and rate of reaction:

The rate of reaction changes throughout, but is greatest at the start and decreases as the reaction proceeds as atoms and particles are used up during successful collisions. However, the higher the temperature, the faster the reaction will occur.

 

Scientific explanation:

For a reaction to occur, not only must particles collide with one and other but they must have enough energy so that original bonds are broken and new bonds formed. The energy needed to break a mole of bonds is known as bond energy. If the collision has insufficient energy no reaction occurs. The more successful collisions, the faster the reaction. Successful collisions depend on particles moving faster to increase the chance of colliding. Increasing the temperature means particles take in more energy causing faster movement and therefore more collisions. Heated particles both collide more often and also these collisions will have more energy so will be more likely to be successful.   Conversely, reducing the temperature will decrease the amount of energy, slow down movement and decrease the number of collisions. All reactions need energy to start them off. The activation energy is the minimum energy needed to break enough bonds to start a reaction. 

Diagram of activation energy:

From the diagram it is possible to see that an increase in temperature causes an increase in energy so bonds are broken more easily.

 

I have ensured that I referred to a variety of different sources to make my information more accurate and my prediction more reliable. These have included the Internet, a variety of text books (see bibliography) and talking to scientists.

 

Planning procedure:

In order to plan an appropriate strategy to justify my prediction, I will use my scientific knowledge together with careful planning and an appropriate range of temperatures to ensure both accurate results and a fair test.

 

Apparatus:

Hydrochloric acid – A reactant

Marble chips – A reactant

Bunsen burner – to produce heat for the reaction

Thermometer – To monitor the temperature

Conical flask – To house the reaction

Tripod – For the flask to stand on

Beaker – Containing a specified amount of water to act as a water bath

Gas Syringe – To collect the carbon dioxide gas

Stop watch – To time the reaction

 

Diagram of apparatus:

What I will measure

The following measurements may be used to find the reaction rate for this experiment.

Ø     How much calcium carbonate disappears per unit of time

Ø     The amount of acid used per unit of time

Ø     The amount of carbon dioxide formed per unit of time.

 

I am going to collect and measure the amount of carbon dioxide given off during the reaction at different temperatures. I have chosen this method as it gives the least error and is easy to collect accurately. I will record the time as each 10mls of carbon dioxide is given off up to 100mls of carbon dioxide. I will record the results cumulatively in minutes and seconds. The calcium carbonate would have to be dried and weighed if I was to use the first method and therefore cause inaccurate results. The amount of acid may not change significantly and it would therefore be hard to get an accurate reading.

 

Variables

In order to carry out a fair test the only variable I will change is the temperature of the reaction. I will ensure that the following variables are kept constant:

Ø     Surface area of solute (marble chip)

Ø     Concentration of solvent (dilute hydrochloric acid)

Ø     Amount of reactants (9-12mm size marble chip and 50cm3 of hydrochloric acid)

Ø     Catalyst (no catalyst will be involved)

A fair test will also require the experiment to be repeated several times at each temperature for reliable results

 

Plan

I plan to use a range of temperatures from 20°C to 70°C increasing by 10°C for each test. This should give me an appropriate range of results to confirm my prediction. I will repeat the test twice for each temperature and measure the time taken to produce a specified amount of carbon dioxide as a measure of reaction rate. By repeating each test twice I can take the average result for each temperature and ignore any anomalous results.

 

The apparatus was set up  as shown in the diagram:

 

900ml of water was placed in a trough and heated up to the first temperature (70°C) using the Bunsen burner. 50cm3 of hydrochloric acid was placed in a conical flask inside the trough and allowed to heat up for 1 minute. 1 marble chip was added which was 9-12mm in size. The stopwatch was started and the time recorded each time 10ml of carbon dioxide was produced up to 100ml. For each temperature tested, 3 trials were recorded so that any anomalous results could be discarded and the average of the three tests calculated to give more accurate and reliable results.

 

Trial Run

I will carry out a trial run to ensure that the way I have planned my experiment will actually work in practice and give me the range of results I require. I will do this by carrying out one test run for each extreme of temperature i.e. 20°C and 70°C and checking the time taken to collect 100mls of carbon dioxide decreases as temperature increases.

Obtaining evidence

Results of temperature trials showing times in minutes and seconds

 

*Trial 3 was ignored in the average as an anomalous result

 

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Analysing results

The average results for each temperature were considered in turn, plotted on a graph, and a line of best fit for each temperature was drawn.

 

Table of average results (in minutes and seconds) for each temperature

 

The results showed that the reaction between dilute hydrochloric acid and calcium carbonate occurred fastest at the highest temperature (70ºC). This was shown by the fact that 100ml of one of the products of the reaction (CO2 gas) was collected in the shortest time when the temperature was 70ºC. The longest time for this amount of ...

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