Investigating the energy change during neutralisation reactions

The concentration is measured in mol/dm3.

1 mole per dm3 = 1M solution

2 mole per dm3 = 2M solution etc.

The mole is a simple way of counting atoms. This is important when we want to know how many HCl molecules are dissolved in a litre of water. This then enables us to know how concentrated the acid is.

In this investigation I will be using Hydrochloric acid and Sodium hydroxide (alkali). I have chosen these two because they are both strong, which ensures that I have a maximum temperature read off. When a strong acid is neutralized by a strong alkali, the temperature change is at a maximum because all the ionised molecules can react. But when a weak acid is neutralized by a weak alkali, then the temperature is lower because not all the molecules are ionised therefore the heat is withheld to help other molecules to ionise therefore making the temperature change less.

When the HCl and NaOH react, bonds are formed. The bonds being made are Na-Cl and H-OH. Strong bonds such as ionic bonds need a lot of energy to break them and when they are formed they also give out a lot of energy. But weak bonds such as covalent bonds do not require as much energy to break them and when they form they do not give out a lot of energy.

The Na-Cl bond is a very strong ionic bond. Therefore when it is created it gives out a lot of energy as heat. This then makes the temperature change high.

But the H-OH bond is a weak covalent bond and therefore when it is formed not a lot of energy is given off.

This is why the concentration of the solution is one factors affecting the heat given off out of the solution.

If the solution is not concentrated or diluted, then there are more water molecules in the solution and therefore when the H2O forms there is only a small amount of heat given off.

But if the solution is highly concentrated and the volume stays the same, then there is a higher amount of heat given off because more Na-Cl bonds will form in the absence of water molecules.

FACTORS

There are certain factors that affect my results that I need to take into account. I will have to find a way to keep the factors constant in order for me to get accurate results.

. Concentration of either acid or alkali.

This factor has to be kept under control because as the concentration varies, the heat given off will also vary therefore altering my results.

As my background knowledge states, the more concentrated the acid/alkali is, the less water is present in the solution. Therefore there will be a greater amount of strong Na-Cl bonds created in the absence of the water molecules (giving that the volume remains the same) therefore giving of a greater amount of heat. So if the concentration is not kept constant for all my experiments, my results will be unreliable.

2. The Chemical nature of the acid or alkali.

The chemical nature of the acid/alkali refers to the concentration of the H+ ions and OH- ions respectively. Stronger acids have more H+ ions concentrated in the solution than weaker acids. While stronger alkali's have more OH- concentrated in the solution than weaker acids. Consequently when conducting the experiment, if the chemical natures of both the acid and alkali are not kept the same, the heat given off will also vary therefore making my results irrelevant.

* I will be investigating Factor 1.

APPARATUS

For this experiment I will require the following equipment:

* 5 test tube

* A 30ml measuring cylinder

* A polystyrene cup with it's cover

* A beaker

* A sufficient amount of cotton wool

* A thermometer

Before the experiments begin, I make sure that all the equipments needed are of good for use and beneficial in gaining accurate results.

PREDICTION

I predict that as I decrease the concentration of the solution, the amount of heat given off will decrease. This is because there will be less strong bonds being made and weaker ones in it's place. In other words there will be less Na-Cl bonds being made and more H-OH bonds being made. And because the strong bonds are less, the total heat given off will be less.

METHOD

. I gathered all the equipment needed and make sure they are perfect for use.

2. I than set up the apparatus as shown in the diagram below.

3. Then with my measuring cylinder, I measure 25ml of NaOH and pour it into the polystyrene cup.

4. Then I rinse the measuring cylinder to get rid of the alkali and then I measure 5ml of HCl and pour it into one of the test tubes. I then measure four more 5mls of HCl and pour it into the other test tubes.

5. After waiting a while for the liquids to settle, take the temperature of the acid and then the alkali.

6. I then use the mean of the two temperatures as my initial temperature.

7. I record this on my table.

8. I then pour the HCl into the NaOH and give it a slight stir. (Vigorous stirring causes extra heat).

9. I then read the temperature on the thermometer and record the highest reading in my table.

0. I then quickly add the second test tube of HCl into the solution and give it a slight stir.

1. Then by repeating procedures 8 and 9 for each test tube of HCl, I complete my table and work out the temperature difference.

2. I then repeat the whole procedure a second time and take the averages to work out my final table and graph.

RESULTS

In order to get a near accurate result, I have used the average of both my experiments to complete these tables.

Concentration

Temperature at start (oC)

Maximum temperature (oC)

Temperature difference (oC)

25cm3 of NaOH with 25cm3 of HCl in parts of 5cm3

22.7

5cm3 of HCl

25.7

3

5cm3 of HCl

28.0

5.3

5cm3 of HCl

30

7.3

5cm3 of HCl

30.7

8

5cm3 of HCl

29.9

7.2

Concentration

Temperature at start (oC)

Maximum temperature (oC)

Temperature difference (oC)

25cm3 of NaOH with 20cm3 of HCl in parts of 5cm3and 5cm3 of water

23.3

5cm3 of water

23.3

0

5cm3 of HCl

26.2

2.9

5cm3 of HCl

28.9

5.6

5cm3 of HCl

30

6.7

5cm3 of HCl

30.5

7.2

Concentration

Temperature at start (oC)

Maximum temperature (oC)

Temperature difference (oC)

25cm3 of NaOH with 15cm3 of HCl in parts of 5cm3and 10cm3 of water

23.5

5cm3 of water

23.4

0.1

5cm3 of water

23.5

0

5cm3 of HCl

26

2.5

5cm3 of HCl

28

4.5

5cm3 of HCl

29.8

6.3

Concentration

Temperature at start (oC)

Maximum temperature (oC)

Temperature difference (oC)

25cm3 of NaOH with 10cm3 of HCl in parts of 5cm3and 15cm3 of water

23.8

5cm3 of water

23.8

0

5cm3 of water

23.7

-0.1

5cm3 of water

23.8

0

5cm3 of HCl

26

2.2

5cm3 of HCl

28.7

4.9

Concentration

Temperature at start (oC)

Maximum temperature (oC)

Temperature difference (oC)

25cm3 of NaOH with 5cm3 of HCl in parts of 5cm3and 20cm3 of water

24

5cm3 of water

23.9

-0.1

5cm3 of water

24

0

5cm3 of water

24

0

5cm3 of water

24

0

5cm3 of HCl

26.7

2.7

ANALYSIS AND CONCLUSION

My results fit my prediction. As you can see from the tables, as the concentration of the acid decreases, the heat given off decreases as well. This is because there are more weak H-OH bonds being made than strong Na-Cl bonds. And strong bonds give off a greater heat. This results to an overall decrease in the temperature given off.

I also noticed that my temperature to start with kept on increasing as I proceeded with the experiment. This is a flaw because of the rising temperature due to the weather at the time.

Another temperature flaw occurred in my third, fourth, and fifth graph. When I added some water to the solution of Alkali, the temperature changed. This should not occur because the water should not react with the alkali and therefore not give off any heat. This fault might be because of human error or probably the temperature of the water forces the overall temperature of the solution to fall.

EVALUATION

Although my experiment was quite successful to a certain degree, there were a number of faults that could be minimised.

For example the temperature change when water was added to the alkali.

This fault could be minimised if I conducted my experiment in an enclosed and well-insulated environment.

The procedure I used to conduct my experiment was quite reliable to my understandings and I believe that it was the best way to conduct the experiment. The only change I would make is the apparatus. Instead of using a normal thermometer, I would prefer to use an electrical thermometer so that human error is limited.