27 Mar - Reflections about Term 1 Lab Lessons

Lesson 1: Properties of Acids and Alkalis

In this lesson, I learned about some basic properties of acids and alkalis, as stated in the title. I learned that universal indicators and natural indicators change colour based on the pH of the solution they are in. I also learned about the effect of acids and alkalis on Methyl Orange and Phenolphthalein. I read that such indicators have a sharp change in their colours and are used in a kind of volumetric analysis known as titration. Universal indicator is not good for titration because its change in colour at the end point of titration is not sharp. One of the 16 Habits of Mind states the need for accuracy and precision. If the change in colour is not sharp, we cannot get an accurate end point. I read that the error for titration is only 0.1ml thus a sharp change is needed.

Lesson 2: Strength of Acids and Alkalis

In this lesson, I learned about how the strength of acids and alkalis affect its electrical conductivity. Strong acids and alkalis disassociate and ionize completely in water. For example, the gas hydrogen chloride ionizes completely in the presence of water:

HCl (aq) à H⁺ (aq) + Cl^- (aq)

Weak acids and alkalis disassociate and ionize partially in water. Such acids and alkalis prefer remain as molecules rather than ions. Organic acids in particular behave this way. For example, hydrogen ethanoate ionizes partially and some of hydrogen ethanoate remain as molecules:

CH₃COOH (aq) ↔ CH₃COO^- (aq) + H⁺ (aq)

It is quite easy to grasp the concept that strong acids and bases ionize more easily. However, how do ions such as the hydrogen ion (H⁺) and hydroxide ion (OH^-) conduct electricity. I would like to find out more.

Lesson 3: pH Changes

Here we performed a down-sized form of titration on hydrochloric acid using sodium hydroxide. The concentration of both hydrochloric acid and sodium hydroxide is 0.1M or 0.1mole/litre. I read that one mole is about 6 × 10²³ molecules. Another experiment was performed: copper (II) oxide was added in access to hydrochloric acid. This time, an interesting observation was made. The pH increased till pH 7 where no increased was observed no matter how much copper (II) oxide was added. Later, I learned that copper (II) oxide is not soluble in water and thus is not an alkali. It does not release hydroxide ions. However, it does “absorb” H⁺ ions by reacting to form water.

Lesson 4: When Dilute Acid Meets Metals…

Finally after all of the basics, comes the most interesting part – chemical reactions. We know that acids react with metals (metals which are more reactive than hydrogen) to form a salt and hydrogen gas. But how does it work? This is basically a Redox reaction. A Redox reaction is also known as an oxidation-reduction reaction. Oxidation is the loss of electrons by a substance while reduction is a gain of electrons by a substance. A Redox reaction basically has both happening at the same time. For example, hydrochloric acid reacts with magnesium metal to form magnesium chloride and hydrogen gas.

Firstly, we can express the reaction in a chemical equation:

2HCl (aq) + Mg (s) à MgCl₂ (aq) + H₂

Then, we convert it into an ionic equation (Cl^- ions are omitted because they do not change in their oxidation state):

Mg (s) + 2H⁺ (aq) à Mg²⁺ (aq) + H₂ (g)

In the reaction, magnesium loses its electrons and thus is oxidized:

Mg à Mg²⁺ + 2e^-

Hydrogen gains electrons and is oxidized:

2H⁺ + 2e^- à H₂

The next interesting part is with the testing of hydrogen gas. Place a burning splint (yes burning and not just lighted) into the test-tube containing the gas. If the flame is extinguished with a ‘pop’ sound, the gas is hydrogen gas. The reason is because when hydrogen gas mixes with oxygen gas found in the atmosphere, it forms an explosive mixture. The burning splint ignites the explosive mixture and the shockwave from the explosion puts out the flame. At the same time, one hears the ‘pop’ sound.

Finally, the last interesting part is about why copper does not react with acids. What I was told was that copper is less reactive than hydrogen, that is copper is “not willing” to “take over” the chloride ions from hydrochloric acid. But I am a little unsure if we use a more concentrated acid like 4mol/dm³. I have read that in the past, alchemists used aqua regia or ‘royal water’ (which is in fact a mixture of concentrated hydrochloric acid and nitric acid) to dissolve gold. And we all know gold is even less reactive than copper.

Lesson 5: When Dilute Acid Meets Carbonates…

In this lesson, we perform one of the oldest science experiments – the volcano. I recall that when I was young, I mixed vinegar and baking soda together to produce a messy fizz. Today, I know the vinegar is a kind of acid – ethanoic acid – and baking soda is actually sodium hydrogen carbonate. Below is their reaction:

CH₃COOH (aq) + NaHCO₃ (aq) à CH₃COONa (aq) + H₂O (l) + CO₂ (g)

Lesson 6: When Acid Meets Alkali…

I find this one of the more boring experiments as there isn’t much reaction happening – no effervescence, no precipitation and no colour change. In fact, not many of the ions change during neutralization:

H⁺ (aq) + OH^- (aq) à H₂O (l)

However, no matter how pathetic it looks, it is the basis of a more complex volumetric analysis. Titration. I believe that everything we learn today is in preparation of what we would be learning tomorrow. That is why I always learn the most I can every day.

Lesson 7: When Alkali Meets Ammonium Salts…

This experiment is in fact somewhat similar to that of adding acid to carbonate salts. Both of these reactions liberate a gas – one of them is acidic and the other is alkaline. Both the gases form weak acids/alkalis as both gases prefer to remain as covalent compounds instead of forming ionic compounds. It seems that these two reactions are quite similar. In fact, I am considering a kind of salt – ammonium carbonate. Water is an amphoteric substance (it behaves both like an acid and an alkali) and when mixed with ammonium carbonate, will it liberate carbon dioxide and ammonia gas? Below is the possible equation:

H₂O (l) + (NH₄)₂CO₃ (aq) à 2H₂O (l) + 2NH₃ (g) + CO₂ (g)

Then again, this could just be my imagination running wide.

Lesson 8: Preparation of Soluble and Insoluble Salts

These two methods use the idea of filtration either to remove the insoluble reactant or to extract the insoluble product. The first method works when the reactant (metal, metal oxide and metal carbonate) is insoluble. The second method works when the salt produced is insoluble. However, what happens if both the reactants and salts are soluble? Then we need to use titration to determine the end point or such that both reactants are used up completely and not leftover. For example, what if we want to prepare sodium nitrate? We know that we need sodium cations and nitrate anions. The sodium cations can come from sodium hydroxide while the nitrate anions can come from nitric acid. Below is the reaction:

NaOH (aq) + HNO₃ (aq) à NaNO₃ (aq) + H₂O (l)

At the end point of titration, sodium hydroxide and nitric acid would have been completely used up leaving only sodium nitrate solution left, which can be obtained through crystallization. Salts are very useful substances. Salts such as sodium chloride can be obtained from the sea easily. However, not all salts are available in nature. For such salts, we need to prepare them ourselves.