Acids, bases and salts - The characteristic properties of acids and bases (3)
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1.
Describe the characteristic properties of acids in terms of their reactions with bases. Include in your description the concept of neutralization and the products formed.
Acids react with bases in a reaction called neutralization. Neutralization is the reaction between an acid and a base to form a salt and water. The products of this reaction are a salt and water. For example, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to produce sodium chloride (NaCl) and water (H2O). The general equation is: Acid + Base -> Salt + Water.
The reaction is exothermic, meaning it releases heat. A strong acid reacts with a strong base to produce a neutral solution (pH 7). A weak acid reacts with a strong base to produce a basic solution (pH > 7), and vice versa. The rate of neutralization depends on the concentrations of the acid and base. A higher concentration of acid or base will lead to a faster reaction.
2.
A student is asked to explain why a dilute solution of hydrochloric acid is corrosive. Using the information that aqueous solutions of acids contain H+ ions, explain the chemical process that causes this corrosiveness.
Hydrochloric acid (HCl) is corrosive because it reacts with many materials, particularly metals. The hydrogen ions (H+) in the hydrochloric acid solution react with the metal atoms. This reaction typically involves the displacement of H+ ions, leading to the formation of hydrogen gas (H2) and a metal salt. For example, with zinc (Zn):
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
The hydrogen gas produced can accumulate within the material, causing it to weaken and eventually corrode. The H+ ions also contribute to the corrosion process by attacking the surface of the material.
3.
A student carries out a series of tests using different indicators to determine whether a solution is acidic or alkaline. They test a solution with litmus, thymolphthalein, and methyl orange. The results are shown in the table below. Explain the results in terms of the pH of the solution and the properties of the indicators.
| Indicator | Colour Observed |
| Litmus | Red |
| Thymolphthalein | Colorless |
| Methyl Orange | Red |
The results indicate that the solution is acidic. Here's a breakdown:
- Litmus: The litmus paper turns red, indicating that the solution is acidic. This is because the H+ ions in the acid react with the hydroxide ions in the litmus dye, causing a colour change.
- Thymolphthalein: Thymolphthalein remains colorless. This is because the acidity of the solution is not high enough to deprotonate the thymolphthalein molecule and cause it to turn blue. The pH is below 8.3.
- Methyl Orange: The methyl orange turns red, which is characteristic of acidic solutions. This is because the concentration of H+ ions is high enough to protonate the methyl orange molecule, leading to a red colour. The pH is below 4.4.
Therefore, the combination of all three indicators provides strong evidence that the solution has a pH less than 7, making it acidic. The different indicators are sensitive to different ranges of pH, allowing for a more comprehensive assessment of the solution's acidity.