DIAGRAM
I did not use the factors below for many different reasons:-
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Temperature- was not used since it would be difficult to keep the temperature of HCl constant through out the whole experiment therefore an accurate set of results would not be obtained.
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Pressure- was not used since at least one of the reactants needed to be a gas so that the rate of the reaction will increase. As you can see from my word equation on the first page, none of the reactants is a gas (calcium carbonate is a solid and hydrochloric acid is a solution).
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Surface area- of CaCO3 was not used since the surface area of the small marble chips was large enough to obtain a good set of results.
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A catalyst- was not used since the rate of the reaction was fast enough already therefore a catalyst was not needed to be used for this experiment.
BACKROUND KNOWLEDGE
A chemical reaction takes place only when the particles of the reactants collide and react with each other and with sufficient energy. The collision theory suggests particles need to collide with enough energy to react sufficiently and the higher the level of a particlular factor (e.g. concentration) the higher the rate of the reaction. This minimum energy reacting particles must have if they are to react successfully is called the activation energy, Ea. If the reacting particles are given enough energy (activation energy), they will collide with other particles causing their bonds to stretch and break therefore causing a reaction to occur.
From the kinetic theory of gases in a reaction between 2 gases called ‘A’ and ‘B’, a molecule of ‘A’ must collide with ‘B’ for the reaction to proceed. However, in a concentrated solution there will be a higher percent of reactants which will have more energy. A reaction only takes place if the reacting particles collide with a certain minimum energy called the activation energy. This is what produces the products of the reaction.
Energy profiles describe energy changes over the course of a reaction. Energy (as H or G) is graphed on the y-axis; reaction progress (reactants to products) is graphed on the x-axis.
Reaction profiles are shown in several figures in the diagram below. Key features of reaction profiles are the hills. There is one hill for each step in the mechanism. The leftmost point represents the potential energy of the reactants. The top of the hill is the transition state. (If there is more than one hill, there is more than one transition state.) The energy difference between the reactants and the top of the hill is the activation energy (Ea).
The rightmost point is the potential energy of the products. The energy difference between reactants and products is the enthalpy (H) or the free energy (G) of the reaction. If the energy of the reactants is higher than the energy of the products, the reaction is exothermic (H) or exergonic (G). If the energy of the reactants is lower than the energy of the products, the reaction is endothermic (H) or endergonic (G).
If there is more than one hill, the slow step is the one with the highest hill. Any valley between the hills represents the energy of the intermediate. The transition state is the energy of the activated complex.
Raising the temperature increases the rate of reaction because the added kinetic energy allows a larger fraction of reactants to "go over" the hill. Adding a catalyst actually lowers the activation energy (size of the hill).
The H is the energy difference between products and reactants. Ea is the energy difference between reactants and transition state. The transition state is the highest energy between reactants and products.
This diagram shows the activation energy for an exothermic reaction:
This diagram shows the activation energy for an endothermic reaction:
For a reaction to happen the particles must overcome this energy barrier. Several factors can change the amount of energy each particle has, or the amount of energy needed to overcome this barrier.
PREDICTIONS
- Concentration of HCl
As the concentration of hydrochloric acid increases, the rate of the reaction increases. This is because there are more H+ ions per unit volume therefore the frequency of collisions increases and the rate of the reaction increases.
However, if the concentration of the hydrochloric acid is doubled, the rate of the reaction will also double. This is because there will be twice the number of H+ ions in the same volume of hydrochloric acid. Therefore, there will be twice the number of collisions per unit time.
If the concentration of hydrochloric acid decreases, i.e. the original solution is diluted with more water, the rate of the reaction will decrease. This is because there are less H+ ions per unit volume. Therefore the frequency of collisions decreases and the rate of the reaction decreases.
Diagram:
- Temperature of HCl
As the temperature increases, the rate of the reaction increases. This is because the particles gain kinetic energy and begin to move faster. As a result, the reacting particles collide more frequently and with greater energy. Therefore there are more successful collisions per second and the rate of the reaction increases.
Increasing the temperature by 10ºC will almost double the rate of the reaction.
Diagram
- Pressure
When one more of the reactants is a gas, an increase in pressure will increase the rate of the reaction. This is because when the pressure is increased, the particles are forced closer together. Therefore when the pressure is increased, there are more particles per unit volume. Thus the frequency of collisions increases and the rate of the reaction increases.
Diagram
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Surface area of CaCO3
As the surface area of calcium carbonate increases, the rate of the reaction increases. This is because when one of the reactants is a solid, the reaction takes place on the surface of the solid. By breaking up the calcium carbonate (marble chips) into smaller pieces, the surface area increases and there is a greater surface available for collisions to take place. Thus, there are more collisions per second and the rate of the reaction increases.
Diagram
- Catalyst
A catalyst is a substance which speeds up the rate of a reaction but remains chemically unchanged itself.
A catalyst provides a new route for the reaction, which has a lower activation energy than the normal (uncataysed) reaction. Therefore, there are more successful collisions per second and the rate of the reaction increases.
Diagram
PRELIMINARY EXPERIMENT
DIAGRAM
The preliminary experiment was carried out so that a reasonable mass of the marble chips and a reasonable volume of the hydrochloric acid could be found to be used in the actual experiment. Another reason was so that suitable results could be obtained from the actual experiment to plot a graph to show the volume of the gas produced which is carbon dioxide against time.
EXPERIMENT 1
Powdered CaCO3 0.48g
40cm³ of HCl (2.0 mol dm‾ ³)
In this experiment I found that the reaction was far too quick to gain enough results, therefore meaning that the results are not reliable. Therefore a graph cannot be drawn from these results. This means that powdered CaCO3 is not suitable to use for my actual experiment.
EXPERIMENT 2
Small marble chips CaCO3 0.77g
40cm³ of HCl (2.0 mol dm‾ ³)
In this experiment I found that the reaction was far too slow. I did not use enough of CaCO3 therefore did not produce enough of CO2 gas. In order to speed up to reaction I need to use more CaCO3 .
EXPERIMENT 3
Small marble chips CaCO3 0.88g
40cm³ of HCl (2.0 mol dm‾ ³)
In this experiment the reaction was at a good, steady speed therefore for my actual experiment I will use 0.88g of CaCO3 in the form of small marble chips and 40cm³ of HCl (2.0 mol dm‾ ³). This is because this reaction was not as fast as experiment 1 where I used powdered CaCO3 and not too slow like experiment 2 where I did not use enough of CaCO3 (0.77g).
ACTUAL EXPERIMENT
CaCO3 (s) + 2HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)
DIAGRAM
VARIABLE:- Concentration of dilute hydrochloric acid
THINGS KEPT CONSTANT:-
- The volume of the solution (40cm³)
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The surface area of CaCO3 (small chips)
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The size of the CaCO3 (2-3mm)
- The temperature of dilute HCl (room temperature)
- The temperature of distilled water (room temperature)
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The mass of CaCO3 (0.88g)
I will measure the volume of carbon dioxide gas every 10 seconds.
To make different concentrations of hydrochloric acid, I will dilute the original solution with distilled water.
RESULTS
Experiment 1
0.88g marble chips ( CaCO3 )
40cm³ HCl (2.0 mol dm‾ ³)
Experiment 2
0.88g marble chips ( CaCO3 )
35cm³ HCl (2.0 mol dm‾ ³) and 5cm³ of distilled water = 1.75 mol dm‾ ³
Experiment 3
0.88g marble chips ( CaCO3 )
30cm³ HCl (2.0 mol dm‾ ³) and 10cm³ of distilled water = 1.5 mol dm‾ ³
Experiment 4
0.88g marble chips ( CaCO3 )
25cm³ HCl (2.0 mol dm‾ ³) and 15cm³ of distilled water = 1.25 mol dm‾ ³
Experiment 5
0.88g marble chips ( CaCO3 )
20cm³ HCl (2.0 mol dm‾ ³) and 20cm³ of distilled water = 1.0 mol dm‾ ³
From the results, 5 graphs of time were plotted against the volume of carbon dioxide, for the five different concentrations of dilute HCl..
The gradient t = 0 must be found. This is because the exact concentration of hydrochloric acid is known at zero time. Therefore, the gradient at t = 0 gives the initial rate. Thus, the effect of concentration on the reaction rate can be measured.
The concentration of HCl decreases with time, therefore the rate of the reaction decreases with time.
SAFETY PRECAUTIONS
Overall and goggles were worn to protect the eyes from the acid.
BIBLIOGRAPHY
- Microsoft Excel
- Microsoft Word
- Key Science Chemistry- Eileen Ramsden
