Chemistry

Chemistry Prac 1

Acids, Bases and Indicators

In this laboratory session three experiments will be performed which illustrate the properties of substances which are described as Acids, Bases or Indicators.

1. pH of Common Materials

The pH scale is a scale which rates the acidic and basic properties of a
substance relative to water (H2O). Water is considered to be neutral, i.e. it is neither acidic nor basic. The pH scale ranges from 1 to 14 and water is given the value 7. Substances with a pH value below 7 are acidic whilst substances with a pH value above 7 are basic. pH values are logarithmic, that is, each value is a power of 10 greater than the one before it. A substance with a pH value of 4 is ten times more acidic than one with a pH value of 5, and a hundred times more acidic than one with a pH value of 6, and so on. (You will be given more information on pH in lectures).

Indicators are substances which change colour depending on the pH value of
the solution they are in contact with. The most well-known indicator is litmus paper which is red when placed in acidic solutions and turns blue when placed in basic solutions. Another indicator you will come across today is phenolphthalein (pronounced FEE-NOL-THAY-LEEN). This indicator is colourless in acids, but turns a bright pink-purple in basic solutions. Universal Indicator paper undergoes a number of colour changes throughout the pH range and can be used to obtain an approximate pH value by comparing the colour of the paper after it has been dipped into the test solution with a colour chart of pH values.

In this experiment we will test the pH of a number of common materials we may find around the home, using Universal Indicator paper.

Methods:

1. Dip one pH paper strip into each of the various liquids supplied, leaving it there for about 5 seconds (or less for Bleach!).

2. Remove the strip and read the pH of the liquid by matching the colour of
the four squares on the strip with the colour chart provided on the indicator box.

3. List the substances in two columns as shown below , and note their pH as
determined by the Universal Indicator paper.

pHtable

a) Do foodstuffs tend to be more of one type than the other?

2. Indicators from Natural Sources

Indicators are compounds which change colour depending upon the pH of the
solution. Indicators are used when we want to detect if a reaction has occurred, and are commonly used during titration of an acid with a base. For example, phenolphthalein is added to an acid solution which is titrated with a base from a burette (in experiment 3). The end-point of the titration (when the acid has been completely "neutralised" by the base) is detected by the change in colour of the acid solution, since above a pH of about 8.9, phenolphthalein changes colour from colourless to pink-purple. Litmus is a common indicator and turns red in the presence of acid and blue in the presence of base. Universal indicator undergoes a number of colour changes throughout the pH range.
Indicators can be found in nature, and a number of the coloured substances
found in plant material show a colour change at different pH values. Red
Cabbage leaves contain a purple dye which undergoes a number of colour
changes throughout the pH scale. Other materials which contain indicators are cherries, grapes, black tea, lichens, beetroot juice and flower petals.

Methods:

1. Shred half a cabbage leaf into fine pieces and add to a beaker of boiling water. Turn the bunsen off and allow the cabbage to stand in the boiling water for 5 minutes, occasionally stirring with a glass rod. Now filter the solution (hot!!) through a filter funnel and paper, and collect the purple liquid in another beaker.

2. Pour about one centimeter of the purple cabbage juice into each of 12
separate test-tubes. One of the tubes will be used as a standard, to compare the original colour with the derived colours obtained from the other tubes.

3. To each of the remaining 11 tubes add sodium hydroxide (NaOH) solution
or hydrochloric acid (HCl) solution drop by drop from the bottles provided,
agitating the tube as you do so. When you get a definite colour change, put the tube aside.

4. Select another tube. Add acid and base solution to this one as before, until you get another different colour. (Note that some solutions will look paler just because you have added a lot of colourless sodium hydroxide.)

5. Now measure the pH of each tube with Universal indicator paper, noting
down the colour of the solution in the test-tube and its pH.

6. Draw up a table like the one below, to record the colour of the solution in each test-tube, and the measured pH. Arrange your data so that the pH values are in ascending or descending order.

cabbagecolourtable

3. Titration of Vinegar

Acids and bases are capable of undergoing a chemical reaction according tothe following equation:

Acid + Base ----> Salt + Water

If equal amounts of a strong acid and a strong base are used in this reaction, then the pH of the final solution after reaction has occurred is 7, i.e. it is neutral. The acid is said to have been neutralised by the base, i.e. the acidic properties of the acid have been cancelled out by the basic properties of the base. Another way to put this is to say that an equivalent number of acid molecules have reacted with an equivalent number of base molecules. In other words, when all the available acid and base molecules have paired up and reacted, we have reached the EQUIVALENCE POINT. For a strong acid and a strong base this will be at pH 7. If we use a weak acid like acetic acid and a strong base like sodium hydroxide however, the equivalence point will be about 8.9, and the final solution will be slightly basic, NOT neutral. Similarly, a strong acid and a weak base will react together to form a solution with an equivalence point below 7. The final solution will be slightly acidic rather than neutral. An indicator can be used in the acid solution to detect if such a reaction has occurred, because the colour of the indicator will change once the base has been added to the acid and the pH of the solution reaches the equivalence point.
A titration is the name of an experimental procedure used when a precise amount of a known substance is added to an unknown substance using an item
of glassware known as a burette to deliver the known substance. The burette is capable of accurately measuring how much of the known substance was added.
In this acid-base titration an unknown quantity of acid is titrated with a known amount of base which is delivered to the acid solution through a burette. "Neutralisation" of the acid (or the end-point of the titration) is detected when the pH of the solution reaches the equivalence point, and the colour of the indicator in the original acid solution changes.

In this experiment you will be titrating different brands of vinegar (which is essentially acetic acid, CH3COOH, a weak acid) with sodium hydroxide (NaOH, a strong base) to determine exactly how much acid is present in each of the vinegars. An indicator, (phenolphthalein) will be placed in the vinegar solution and NaOH will be added through a burette. When equal numbers of base and acid molecules are present in the flask, equivalence has occurred (though not necessarily neutralisation), the pH of the solution will be at the equivalence point, and the indicator will change colour.

Methods:

1. Accurately pipette 10 ml of one of the vinegar solutions into the conical flask and add 4 drops of phenolphthalein indicator. Measure the pH of the initial solution with Universal Indicator paper.

2. Titrate the vinegar by adding NaOH from the burette slowly, about 0.5 ml at a time. After each addition agitate the flask and observe the colour of the solution. A piece of white paper under the flask will assist in detecting the colour change.

3. The end-point is reached when the solution just turns a permanent pale pink colour.

4. Now repeat the titration, but when you are about 0.5 to 1.0 ml from the
change of colour reached in your first attempt, add the NaOH drop by drop,
swirling the flask after each drop, until the end-point. You should be able to detect a faint, but permanent pink colour. Use THIS value for your calculations. Now measure the pH of the solution again, using Universal Indicator paper.

b) What is the pH of the initial vinegar solution?

c) What is the pH of the NaOH solution you are adding?

d) What is the pH at the end-point of your titration?

Equivalence occurs when the amount of NaOH added equals the amount of acid
present in the flask.

The TITRE is the volume of NaOH added through the burette.

The amount of acid present can now be calculated by multiplying this titre by 0.6

NOTE: You don't need to know the mathematics behind this method of acid
content determination, but so you don't have to blindly accept my word for it, here is how I calculate the figure you multiply by the titre.

One mole of acetic acid (CH3COOH) = 60 g

If 1 litre of 0.1M NaOH contains 0.1 mole,
then 1 ml of 0.1M NaOH contains 0.1/1000 moles

Number of moles of base used = Number of moles of acid present
= TITRE x 0.1/1000 moles

Grams of acid in 10 ml of 10% vinegar solution = TITRE x (0.1/1000) x 60
Grams of acid in 10 ml of 100% vinegar = TITRE x (0.1/1000) x 60 x 10
Grams of acid in 100 ml vinegar (%w/v) = TITRE x (0.1/1000) x 60 x 10 x 10
                                                             = TITRE x 0.6

Therefore to work out the percentage weight of acetic acid per volume of
vinegar in your sample, multiply the TITRE you obtained by 0.6.

This figure (TITRE x 0.6) is the percent of acetic acid in that brand of vinegar. Write this figure up on the blackboard in the space provided.

e) Do all the brands of vinegar tested contain the legal minimum of acetic acid?

f) Which brand of vinegar gives the best value for money?

To be handed in................

A. The 3 tables you have drawn up.
                                                 ie. pH of common substances
                                                      pH of cabbage solutions
                                                      acetic acid in vinegars
B. Answers to the lettered questions in the text, (a - f).

Chemistry Prac 2

Rates of Reaction

Chemical reactions are a part of everyday life. Every time we cook food, we set in motion chemical reactions that change the molecules in the food, breaking some down so that the food is more easily digested; and making some more complex that add to the flavour. You may think you are doing something fairly ordinary when you bake a cake, or make a stew, but you are actually a chemist, bringing about chemical reactions. If you alter the amounts of the ingredients, you are performing a scientific experiment in chemistry!
     Other chemical reactions we are all familiar with involve such processes as the drying of paint, the setting of concrete, striking a match, digesting your lunch, and driving a car.
     Some of these reactions are very fast. For example, the ignition of petrol in a car's engine is so fast that it is really a small explosion. Other reactions, such as the drying of paint, may take days or weeks. In scientific terms, the speed at which a reaction occurs is called the RATE OF REACTION. The rates of nearly all chemical are controlled by four factors:
      1) the temperature of the reacting system,
      2) the concentrations of the chemicals present,
      3) the surface area of any solid chemicals present, and
      4) the presence of any chemicals called catalysts that affect the speed of a             reaction without becoming part of the reaction.

      In this practical class we shall examine the effects of temperature, concentration, and surface area on the rate of a simple reaction.

      The reaction we will be studying will be the release of hydrogen gas (H2) from hydrochloric acid (HCl) by magnesium (Mg) metal, as shown in the chemical equation below:

                          Mg   +   2HCl   ---->   MgCl2   +    H2

      There is a problem in working with an inflammable gas like hydrogen, so instead we will use the gas produced by the reaction to displace water from a flask, and measure the time it takes to collect a certain amount of this water in a measuring cylinder.
      Set up the apparatus as shown below. Make sure that the fittings are tight, and that the whole apparatus is supported where necessary by tripods and clamps. It is important that no gas escapes around the bungs, so if the rubber bungs do not fit well, you might like to try wetting them or applying a little vaseline to them. Be warned though, this could make the bungs slippery and let them pop out of the flask when the gas pressure builds up.

Chem1

Part 1 - Temperature:

1. Measure out about 50 ml of 1M HCl in a measuring cylinder (NOT the one you will use in the apparatus) and pour it into the reaction vessel.

2. Measure the temperature of the acid using the thermometer. This temperature, whatever it is, (probably about 22 C) will be your room temperature reading.

3. Place the strip of magnesium ribbon in the reaction vessel and quickly replace the bung. You may need to fold the ribbon so that it all goes into the acid at once, but do not fold it tightly. Hydrogen gas will be released from the acid as bubbles. As the gas expands it will travel through the tubing and force water out of the conical flask into the empty measuring cylinder. The amount of water
forced out is about the same as the amount of hydrogen gas produced.

4. Time how long it takes to collect 20 ml of water in the measuring cylinder. The water will start flowing slowly at first, so you should only start timing after 10 ml has been collected, and stop after 30 ml has been collected.

                    Tempsec
5. Wash out the reaction vessel (CAREFULLY: it may still contain acid), and measure out another 50 ml of the 1M hydrochloric acid.

6. Repeat the experiment, but this time, before you add the magnesium ribbon, gently heat the acid over a bunsen burner until the temperature is about 30 C - 35 C. You will need to heat the acid on the side-benches and bring it back to your own bench. Remember to check the temperature again just before you add the magnesium ribbon. Note down the temperature and the time taken to collect 20 ml of water at this temperature.

                     Tempsec
7. Repeat the experiment at another higher temperature (around 40 C or 50 C), taking care to note the exact temperature.

                     Tempsec
8. You will be supplied with 1M hydrochloric acid that has been chilled in the fridge down to about 5 C. Pour 50 ml into your reaction vessel and carry it back to your bench. Working quickly, measure the temperature, then place the magnesium ribbon into the acid as before and measure the time taken to collect the 20 ml water.

                      Tempsec
Question a:

How does the temperature of the acid affect the speed with which 20 ml of water is collected?

Part 2 - Concentration:

Concentration is measured in moles (M). This is simply a measure of the number of molecules of the acid found in each litre of solution. The actual number of molecules per litre in a 1M solution is about 600,000,000,000,000,000,000,000. Rather a large number, so it is easier
to just refer to it as 1M.

1. Set up the apparatus as for the "temperature" experiment. All of the following timings will be made at room temperature. You have already tested 1M hydrochloric acid at room temperature, so transfer the time measured for 1M acid at room temperature to the box below.

2. Measure out about 50 ml of 2M hydrochloric acid and pour it into your
reaction vessel.

3. Place the strip of magnesium ribbon in the vessel as you did for the "temperature" experiment, and measure the time taken for 20 ml of water to collect in the measuring cylinder.

4. Repeat using 3M acid . Remember to wash out the reaction vessel carefully between measurements.


5. Repeat using 0.5M acid.


Question b: How does the concentration of acid affect the rate of reaction?

Part 3 - Surface Area:

When a solid reacts with a liquid, only the molecules on the surface of the solid can react. A sphere has the smallest surface area for a given weight. If we could increase the surface area, more molecules could react with the liquid, and in theory, the reaction should proceed at a faster rate. The magnesium ribbon already has a large surface area for its weight, but we can increase the surface area by cutting it up into little pieces. If we could cut it up small enough, each piece would contain only one molecule, and all the molecules could react at the same time. In practice we cannot do this, so we must make do with magnesium powder.

1. Set up the apparatus as before. This time we will be using only 1M hydrochloric acid and performing the experiment only at room temperature. You have already measured the time at 1M concentration at room temperature, so copy your results into the box below.

                      Magsec
2. Repeat the experiment, but cut the ribbon up into very small bits. (Don't forget to add the bits to the acid all at once, not bit by bit).

                      Magsec
3. Repeat the experiment, but this time use the magnesium powder provided in the small glass vials. Tip it all in and replace the bung. (Note: you will have to act QUICKLY as the reaction is vigorous!).

                       Magsec
Question c: How does the surface area of the magnesium affect the rate of                      reaction?

To be handed in .......
Plot the class results on graph paper (3 graphs) with the variable (temperature, concentration, surface area) along the horizontal (X) axis, and time in seconds along the vertical (Y) axis.

PLEASE READ THE SAFETY PAGE