Investigating the rate Of reaction between Acid and marble

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Variables

Independent variables

Dependant variables

Concentration of acid

Rate of reaction

Volume of acid

Yield

Temperature of acid

Surface area of marble (size)

Amount of marble

I have identified as important variables in this experiment. An independent variable is an input variable which means it is something that will affect the outcome of the experiment. A dependant variable is an output variable, an outcome of the experiment. The dependant variable I am going to monitor is the 'rate of reaction'.

Concentration of acid

I am going to change the concentration of acid whilst the other variables will be kept constant throughout the experiment. As the concentration of acid increases so does the concentration of hydrogen ions, which then results in an increase in the rate of reaction. This is because an acid is a donor of hydrogen ions and the strength of an acid is a measure of it ability to 'release' hydrogen ions

Temperature

I need to control the temperature of the reactants this is due to several reasons. As heat is transferred to the particles, they gain energy as heat and convert it to kinetic energy which means that they will move much faster thus resulting in more collisions between the particles and an increased rate of reaction. The particles will react more frequently and are then more likely to react. The cooler the reactant is the less movement there is and it is then less likely that a reaction will take place quickly.

Surface area of marbles

Another variable which I will have to control is the surface area of the marble. The larger the surface area of the marble the more collisions there will be between the hydrogen ions, this will result in an increase in the rate of reaction. I have a choice of various sizes of marble that I can use, I can use large chips, small chips or powdered marble. I have to control this by using the same one each time or it will affect the results of the experiment.

Preliminary work

Acid

As part of my planning I had to do some preliminary tests so that I could find out what the best conditions are to get the best results out of my experiment. I did an experiment to find out what the best acid to use for my experiment was. The acids that I used for my preliminary work are as follows:

Sulphuric acid - H2SO4

Nitric acid - HNO3

Carbonic acid - HCO3

The tables below show the results which I obtained from my preliminary experiments. To make my experiments fair, I made sure that I kept all of the other factors of the experiment the same throughout the experiment. I used 20cm3 of 2 molar acid and 2 grams of small sized marble chips. I monitored each one for 4 minutes.

Sulphuric acid Nitric acid

Time/minutes

Weight/grams

0.00

22.94

0.30

22.93

.00

22.93

.30

22.93

2.00

22.93

2.30

22.93

3.00

22.93

3.30

22.92

4.00

22.92

Time/minutes

Weight/grams

0.00

22.94

0.30

22.93

.00

22.93

.30

22.93

2.00

22.93

2.30

22.93

3.00

22.93

3.30

22.93

4.00

22.91

Carbonic acid

Time/minutes

Weight/grams

0.00

22.91

0.30

22.91

.00

22.91

.30

22.91

2.00

22.91

2.30

22.90

3.00

22.90

3.30

22.90

4.00

22.90

Sulphuric acid is the strongest acid used but it isn't suitable for my experiment as the range of change in mass was small (0.02), HCO3 was the weakest acid I used and gave the smallest range (0.01) of results and so is the least suitable for my experiment. Hno3 gave the largest range (0.03) and so is most suitable for this experiment:

2HNO3 + CaCO3 --> Ca(NO3)2 +CO2+H2O

Marble

As surface area has an effect on the rate of reaction I am going to do some preliminary tests on this to find out what the best type of marble is for my experiment. I am testing small marble chips, large marble chips and medium marble chips. Again I had to ensure that this was a fair test so I kept other factors constant throughout the experiment. I used 20cm3 of 2 molar nitric acid and 2 grams of marble chips and then I monitored the change in weight for 4 minutes. The results are shown below.

Small marble chips Medium marble chips

Time/minutes

Weight/grams

0.00

22.63

0.30

22.54

.00

22.48

.30

22.43

2.00

22.37

2.30

22.33

3.00

22.29

3.30

22.27

4.00

22.24

Time/minutes

Weight/grams

0.00

22.92

0.30

22.87

.00

22.82

.30

22.76

2.00

22.72

2.30

22.67

3.00

22.64

3.30

22.61

4.00

22.58

Large marble chips

Time/minutes

Weight/grams

0.00

23.27

0.30

23.23

.00

23.19

.30

23.16

2.00

23.11

2.30

23.09

3.00

23.04

3.30

23.02

4.00

22.98

I have decided to use small marble chips as they have given the largest range difference thus will be the best choice for my experiment. The small marble chips give a larger range difference for my results then the medium and large marble chips.

Prediction

I predict that if were to increase the concentration of acid the rate of reaction would also increase in proportion to the concentration of acid, I also predict that the rate of reaction will gradually decrease as there will be less marble chips for the acid to react with. The concentration of acid is the variable that I am going to be changing; all other variables will be controlled and kept the same. If I were to decrease the concentration of acid, the acid would be more dilute, there would be a higher ratio of water particles to acid particles, as there are less acid particles they would not react with the marble as quickly thus a slower rate of reaction.
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I believe this as when I double the concentration of the acid the numbers of hydrogen ions will also double this will also lead to an increase in the probability of collisions, the hydrogen ions will not always collide with the marble they may sometimes collide with each other or with water molecules, this is demonstrated in the diagram below. Notice how in the diagram with the higher concentration of acid there are far more hydrogen ions to collide with the marble.

KEY: - Water particle - Marble particle - Hydrogen ion

Plan

Equipment

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