PH in Descaling
PH is used to measure the acidity or alkalinity a substance is, the scale ranging from 1-14. If the substance is 7 on the scale, it is neutral. If it is less than 7 it is acidic, but if it higher than 7 it is an alkaline. The lower the number, the stronger the acid is. The higher the number the more alkaline it is. If a substance is acidic, you can use an alkaline to make it neutral, and vice-versa. This is called neutralisation. The experiment we conducted was a neutralisation reaction test. We had used citric acid, which has a pH of 2.2, to neutralise calcium carbonate, which has a pH of 9.4. This makes the solutions pH 7.2, making it a neutral substance. Citric acid is mainly used as it is not too strong, thus not damaging the kettle and/or the person using it.
Monobasic, Dibasic and Tribasic Acids
Substances become acidic due to their hydrogen ions. These hydrogen ions come in various different forms, each different form changing the type of acid. These are called monobasic, dibasic and tribasic acids. Monobasic acids have one free hydrogen to donate to a base, dibasic acids have two to donate, while tribasic have three available to donate. There are also types of acid called monoprotic and polyprotic. These types of acid donate protons instead of hydrogen ions. Monoprotic donate one while polyprotic donate more than one.
Bibliography:
Planning
Hypothesis
The higher the concentration of acid the faster it reacts and the faster carbon dioxide is produced in a certain period of time, as it is less diluted. This is because the rate of reaction is higher. This is stated in the collision theory, which says that the more the concentration the higher the rate of reaction.
Apparatus
- Samples of citric acid (10 g is suitable)
- Calcium carbonate (see notes)
- Top-pan balance for measuring quantities
- Range of measuring cylinders
- Gas syringes
- Bungs with delivery tubes
- Large beakers
- Syringe or pipettes, including those measuring 1 cm3
- Small conical flasks
- Boiling tubes and test-tubes
- Thermometers
- Distilled water
- Glass rod
- Watch glass
- Stopwatch or stop clock.
- Candidates may also request the use of:
- Top-pan balance (reading to 0.01 g) for measuring change in mass.
Controlled Variable
- Same amount of citric acid being used for each reaction.
- Duration of the time the carbon dioxide is measure for
- The temperature of the water that is used to dilute the acid. The temperature will be room temperature
- Same amount of calcium carbonate
Independent Variable
- The variable that I am going to change is the amount of the water that is used to dilute the citric acid
Dependent Variable
- The rate that the carbon dioxide that is produced by the reaction
Method
- First start off by measuring 50ml of water in a measuring cylinder
- After that pour the distilled water into a large beaker and wait for the temperature to drop/increase to room temperature, using a thermometer. We will do this by leaving the water on the table for a couple of minutes
- Next measure the weight of the watch glass
- Then pour some citric acid onto the watch glass and measure until you get 10g of citric acid. To find an accurate weight we will use the top-pan balance and take away the weight of the watch glass from the end result
- After that pour the 10g of citric acid into the distilled water and mix thoroughly using the glass rod. This is so that the solution is diluted
- Next, measure the 2g of calcium carbonate by using the watch glass and the top-pan balance. This is how we measured the citric acid.
- The pour the calcium carbonate into a small conical flask
- The pour the citric acid solution into a small conical flask
- Immediately afterwards, insert a bung with a delivery tube into the conical flask.
- Next insert the syringe to the end of the delivery tube
- When the syringe is in place , start the stopwatch
- For every five seconds for the next thirty seconds, measure the amount of carbon dioxide that is released and record the information in a table
Minimising Errors
Minimising errors is reducing the amount of potential errors and mistakes during an experiment. By doing this, we can make the results of the experiment more accurate. First, we are using a pipette to get accurate volumes of water and the standard solution. This is so that all the experiments are fair. The amount of citric acid was also measured using the pipette. The amount of citric acid and calcium carbonate were kept the same. This is, again, because we wanted to keep it a fair test.
We also made sure to close the conical straight away with the rubber bung. This would prevent any carbon dioxide from escaping after the reaction had started to happen. We kept the syringe perfectly sideways and not tilted. This was so that gravity did not move the syringe around, changing the values.
We made sure that the timings were recorded correctly and we used a scale with two decimal places. This gave us an accurate measure of time and how much substance we were using.
Risk assessment
Table of results
Test 1
Test 2
Test 3
Table of averages
Analysis
There are many patterns that can be found in the graph. One pattern I found is that the higher the concentration the fast carbon dioxide is produced in a certain time period. This is back up by the graph as it shows that when the concentration was 0.231, there was 4cm3 of carbon dioxide released after 45 seconds. If we look at the amount released when the concentration was the highest, at 0.417, the amount produced in the same time is 11.37. This supports my hypothesis as it says that the higher the concentration the faster the rate of reaction, producing more CO2 in a certain amount of time.
In addition, a pattern I found was that as time passed the rate at which carbon is produced generally slows down. We can see this in the graph as the line slowly starts becoming less steep as time passes. For example, for the 0.417 concentration, from 0-15, it goes from 0cm3 to 6.67cm3 whereas it only rises 1.3 at 45-60 seconds, from 11.33 to 12.67.
Comparing
I compared my results with Tahmid Rahman’s results. The first thing I notice is that the results were very different as we used varying concentrations. However there is the general pattern that both results share, which is the higher the concentration, the faster carbon dioxide was produced and the volume of the gas. For example, in my results, if we look at the lowest concentration, 0.231, there was 4cm3 of carbon dioxide released after 45 seconds. If we look at Tahmid’s lowest concentration, 1cm3 was produced. Then when we look at the highest concentrations on both, 0.417 for my results and 5.208 for Tahmid’s, 11.33 cm3 and 10.67cm3 f carbon dioxide was produced respectively. Both of the graphs had a positive correlation, the total of carbon dioxide growing over time.
Evaluation
We managed the risks of the test in various ways. We made sure all the equipment was away from the edge of the table, as to ensure that tools such as the syringe and glass beakers would not fall and break.
We also made sure that the distilled water did not get near any sockets so that no one would have the risk of being electrocuted. We also tried to not spill any water. This would make sure that we did not slip or fall, especially with equipment in our hands.
When we handled calcium carbonate we wore goggles so that it would not make contact with our eyes and washed our hands before touching our eyes with our hands. This made sure that it would not irritate our eyes. We did the same thing with citric acid, except we used a glass rod to weigh it as it is irritate to the skin and could have caused rashes amongst other things.
As we started the test, we had to refine the method. We first reduced the amount of water used to give a higher concentration. We also had to try a find a way to minimise the loss of gas that happens at the beginning of the experiment. When we pour the citric acid solution into the conical flask with the calcium carbonate in it, they start to react before the bung has been put on to capture the gas. To minimise as much loss as possible, we tried to put the bung as soon as the liquid had finished pouring.
We also tried to improve the reliability of the test in other ways. One way was that we did the test three times and then found the averages. This gave us more accurate and reliable results. Furthermore, we also made sure that the water was the same temperature. This is because if we were to get accurate results we needed the water to be the same or the solution may have either reacted faster or slower. This would happen as the water molecules would have either less or more energy. This would increase or decrease the amount of collisions that would happen, changing the rate of reaction.
Does it Support My Hypothesis
My results from part two, which are the table of results and the table of averages, both support my hypothesis. My hypothesis says that “The higher the concentration of acid the faster it reacts and the faster carbon dioxide is produced as it is less diluted.” If look at the results you can see that as the concentration increases more carbon dioxide was produced and at a faster rate. The results of a classmate also support my hypothesis as his table shows that the more concentrated his solution was the more carbon dioxide was produced.
Citric acid is usually preferred to ethanoic acid (vinegar) as a de-scalar. Explain why. Use information from your research
Firstly, citric acid is more widely available to use as a de-scalar than ethanoic acid as it is found in many fruits making it easy to extract. Secondly, citric acid does not pose many risks to the environment around it. This cannot be said for ethanoic acid, which is a vinegar. Vinegar is usually used to as flavouring: therefore it may have an effect on the taste of coffee which is being made by the coffee machine if it is not cleaned thoroughly.
Another reason is that citric acid has lower pH than ethanoic acid. This is because citric acid is tribasic while ethanoic is monobasic, meaning citric acid has more free hydrogen ions. Having a lower pH means that it has a faster rate of reaction as it has a higher concentration, meaning that the numbers of particles which collide are higher. This reduces the time needed to get rid of lime scale.
Sulfamic acid is more expensive than citric acid. Sulfamic acid is often preferred to citric acid because it works more quickly. Explain why sulfamic acid works more quickly.
Use information from your research and your investigation. Use relevant scientific explanations in your answer.
The first reason why sulfamic acid works faster is that it has a pH of 2 while citric acid has a pH of 2.2. Having a lower pH value means that it is more acidic, therefore a more powerful de-scalar. It also means that the molar mass of sulfamic acid is higher than citric acid. This means that there are more hydrogen ions in the solution, thus the rate of reaction will be higher as there are more free ions available to collide and cause a reaction. Sulfamic also has a low toxicity and low volatility making it safe to use. It also is soluble in water making it easy to apply.