Neutralization Science 9.3 PDF

# Neutralization ## 9.3 Neutralization ### Flipped classroom Watch a video and answer the questions. ### Let's begin A housewife's hand smell fishy after cutting a raw fish. She washes her hands with lemon juice to remove the smell. Why does this work? In Let's begin, the fishy smell on the hous...

# Neutralization ## 9.3 Neutralization ### Flipped classroom Watch a video and answer the questions. ### Let's begin A housewife's hand smell fishy after cutting a raw fish. She washes her hands with lemon juice to remove the smell. Why does this work? In Let's begin, the fishy smell on the housewife's hands comes from certain alkaline substances in the raw fish. Washing her hands with lemon juice helps remove the smell because the acid in the lemon juice neutralizes the alkaline substances. Do you know what neutralization is? Let us find out in this section. ### What happens when neutralization occurs? #### 1 Change in pH value Acids have pH values lower than 7 while alkalis have pH values higher than 7. If an acid and an alkali are mixed, how will the pH value of the resulting solution change? Let us find out in Practical 9.5. ## Practical 9.5 Change in pH value during neutralization ### Aim To observe the change in pH value when dilute hydrochloric acid is mixed with dilute sodium hydroxide solution. ### Apparatus and materials per group (for part I) - measuring cylinder (10 cm3): 2 - white tile: 1 - conical flask (100 cm3): 1 - pH colour chart: 1 - syringe: 1 - universal indicator: 1 - dilute hydrochloric acid: 1 - dilute sodium hydroxide solution: 1 ### Apparatus and materials per class (for Parts II and III) - beaker (glass or plastic) (100 cm3): 1 - data logger with temperature sensor: 1 - burette: 1 - stand and clamp: 2 sets - data logger with pH sensor: 1 - magnetic stirrer: 1 - dilute hydrochloric acid - dilute sodium hydroxide solution - stir bar: 1 ### Part I Measuring change in pH value during neutralization using a universal indicator **Wear safety spectacles!** 1. Use a measuring cylinder to put 10 cm3 of dilute hydrochloric acid into a conical flask. Refer to Skill reminder on p. 255 for how to take readings from a measuring cylinder. 2. Add three drops of universal indicator to the acid. Shake the flask gently. 3. Put the flask on a white tile. Match the colour of the solution with a pH colour chart. Record the pH value of the solution in the table on the next page. 4. Add 1 cm3 of dilute sodium hydroxide solution into the flask using a syringe. Shake the flask gently to mix the solutions well. **Caution:** Dilute sodium hydroxide solution is irritant. Avoid contact with skin. 5. Match the colour of the resulting solution with the pH colour chart. Record the pH value of the resulting solution. 6. Repeat steps 4 and 5 until a total of 15 cm3 of dilute sodium hydroxide solution has been added to the flask. ### Results | Volume of dilute sodium hydroxide solution (alkali) added (cm3) | pH value of resulting solution | |---|---| | 0 | 1 | | 1 | 1 | | 2 | 1 | | 3 | 1 | | 4 | 2 | | 5 | 2 | | 6 | 2 | | 7 | 2 | | 8 | 2 | | 9 | 2 | | 10 | 7 | | 11 | 12 | | 12 | 13 | | 13 | 13 | | 14 | 13 | | 15 | 13 | **Draw a graph to show** the results on the graph paper below. You have learnt how to draw a line graph in BK 1A, Unit 2. **Title:** Change in pH value when the dilute sodium hydroxide solution added. ### Discussion 1. How does the pH value of the resulting solution change when dilute sodium hydroxide solution is added to dilute hydrochloric acid at the beginning? The pH value increases slowly. 2. Just before and after the resulting solution reaches pH 7? The pH value increases sharply. 3. When more dilute sodium hydroxide solution is added after the resulting solution reaches pH 7? The pH value further increases and levels off at about pH 13. 2. When the pH value of the resulting solution reaches 7, the solution becomes neutral. In other words, the acid and the alkali neutralize each other. How much dilute sodium hydroxide solution is needed to neutralize 10 cm3 of dilute hydrochloric acid? 10 cm3 of dilute sodium hydroxide solution is needed to neutralize 10 cm3 of dilute sodium hydroxide solution. ### Part II Measuring change in pH value during neutralization using a data logger connected to a pH sensor **(Teacher demonstration)** 1. Your teacher will set up the apparatus as shown. A data logger connected to a pH sensor works like a pH meter. It can even record the changes in the pH value of a solution over a period of time. 2. He/she will slowly add dilute sodium hydroxide solution to dilute hydrochloric acid in the beaker until a total of 40 cm3 of dilute sodium hydroxide solution has been added. Thermal energy is released during neutralization. 3. Observe the graph displayed on the screen of the computer. ### Part III Measuring change in temperature during neutralization using a data logger connected to a temperature sensor 1. Your teacher will repeat Part II steps 1 and 2 using a temperature sensor instead of a pH sensor. 2. Observe the graph displayed on the screen of the computer. ### Discussion 1. How does the temperature of the resulting solution change? The temperature increases to a maximum and then decreases. 2. Is thermal energy absorbed or released during neutralization? Thermal energy is released during neutralization. In Practical 9.5, we can see that when an alkali is added to an acid, the pH value of the resulting solution increases. In the other way round, when an acid is added to an alkali, the pH value of the resulting solution decreases. When the solution has a pH value of 7, the solution is neutral. The acid and the alkali neutralize each other. Moreover, we can see that the temperature of the resulting solution increases when neutralization occurs. This indicates that thermal energy is released during neutralization. ## 2 Products of neutralization When an acid and an alkali neutralize each other, will any new substances form? Let us carry out Practical 9.6 to get some ideas. ## Practical 9.6 Substances formed during neutralization ### Aim To find out if new substances form during neutralization ### Apparatus and materials per group - measuring cylinder (25 cm3): 2 - Bunsen burner: 1 - gas lighter: 1 - conical flask (100 cm3): 1 - tripod: 1 - dilute hydrochloric acid: 1 - beaker (250 cm3): 1 - wire gauze: 1 - dilute sodium hydroxide solution: 1 - dropper: 1 - insulating mat: 1 - watch glass: 1 - tongs: 1 pair **The acid and the alkali used are the same as those in Practical 9.5.** 1. Use a measuring cylinder to put 10 cm3 of dilute hydrochloric acid into a conical flask. 2. In Practical 9.5 Part I (p. 29), we found that 10 cm3 of dilute sodium hydroxide solution was needed to neutralize 10 cm3 of dilute hydrochloric acid. Use another measuring cylinder to add this amount of dilute sodium hydroxide solution into the flask. Shake the flask gently to get a neutralized solution. **Caution:** Wear disposable gloves. Dilute hydrochloric acid and dilute sodium hydroxide solution are irritant. Avoid contact with skin. Wash your hands thoroughly after the practical. 3. Use a dropper to transfer a small amount of the neutralized solution to a watch glass. 4. Put the watch glass over a hot water bath. Heat the watch glass until all the liquid has evaporated. Observe whether there is residue left on the watch glass. **Caution:** The water bath is very hot. Do not touch it! **Refer to Skill reminder on p. 254 for how to use a Bunsen burner.** ### Results Is there any residue left on the watch glass? If yes, describe the appearance of the residue on the watch glass. Yes, there are small white solids left on the watch glass. They look like table salt. ### Discussion Suggest the substance that evaporated when the solution was heated. Water In Practical 9.6, we observed that some small white solids were left on the watch glass after all the liquid in the neutralized solution had evaporated. These solids were a salt (Fig 9.24). The liquid that evaporated was water. During neutralization, salt and water are formed. The neutralization reaction between an acid and an alkali can be represented by the word equation below: **acid + alkali → salt + water** ## Go further ### Mixtures and compounds The neutralized solution is a mixture. In the mixture, water particles and particles of salt are not chemically joined together. Upon evaporation, only the salt is left on the watch glass. The salt is a compound which is a pure substance. A compound is made up of two or more elements chemically joined together. Details will be discussed in BK 3B, Unit 13. ## More about word equations for neutralization When hydrochloric acid and sodium hydroxide neutralize each other, a salt and water are formed. As new substances are formed in the reaction, neutralization is a chemical change. The chemical name of the salt formed is sodium chloride. The reaction between hydrochloric acid and sodium hydroxide can be represented by the word equation below: **hydrochloric acid + sodium hydroxide → sodium chloride + water** Neutralization of different acids and alkalis may produce different salts. How do we write word equations for neutralization? Do these word equations follow a pattern? Let us learn about this in Skill builder 9.1 on the next page. ## Go further ### Physical change and chemical change Neutralization is a chemical change in which new substances (salt and water) are formed. A change in which no new substance is formed is called a physical change. Melting of ice is an example. Details will be discussed in Bk 3B, Unit 13. ## Skill builder 9.1 ### Writing word equations for neutralization The general format of the word equation for neutralization is as follows: **acid + alkali → salt + water** Let us learn how to write a word equation for a reaction between an acid and an alkali with example below. ### Example Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. **Steps of writing a word equation for this reaction:** 1. Write down the reactants of the reaction. 2. Write down the other product, 'water.' **hydrochloric acid + sodium hydroxide → sodium chloride + water** 3. Write the salt. In general, the name of a salt consists of two parts. - The first part (sodium) comes from the alkali used. - The second part (chloride) comes from the acid used. ### More examples **Sulphuric acid reacts with magnesium hydroxide to form magnesium sulphate and water.** **Word equation:** **sulphuric acid + magnesium hydroxide → magnesium sulphate + water** ### Nitric acid reacts with aluminium hydroxide to form aluminium nitrate and water. **Word equation:** **nitric acid + aluminium hydroxide → aluminium nitrate + water** ## Skill practice 9.1 **Write the word equation for the neutralization between:** a. **nitric acid and calcium hydroxide.** **nitric acid + calcium hydroxide → calcium nitrate + water** b. **sulphuric acid and potassium hydroxide.** **sulphuric acid + potassium hydroxide → potassium sulphuric + water** c. **hydrochloric acid and zinc hydroxide.** **hydrochloric acid + zinc hydroxide → zinc chloric + water** **Check your answers (p. 76)** ## After-class activity Search for card games to help you identify patterns in word equations of neutralization. ## Key point - During neutralization, salt and water are formed, and thermal energy is released. - The type of salt formed in neutralization depends on the types of acid and alkali that react. - In general, the name of the salt formed in neutralization consists of two parts: - The first part comes from the alkali used. - The second part comes from the acid used. ## Did you know? There are many types of salts. Some of them are very useful. For example: - **Potassium nitrate** (a salt) is commonly used to make fertilizers. - **Calcium sulphate** (a salt) is commonly used as a coagulant to make tofu. - **Calcium chloride** (a salt) is commonly used as a desiccant to absorb moisture from the air. ## Quick check **Questions 1 and 2:** A student mixes dilute nitric acid and dilute magnesium hydroxide solution. 1. **Which of the following statements are correct?** (1) Neutralization occurs. (2) Magnesium nitrate is formed. (3) Water is formed. **A (1) and (2) only** **B (1) and (3) only** **C (2) and (3) only** **D (1), (2) and (3)** **B** 2. **Write the word equation for the reaction.** **nitric acid + magnesium hydroxide → magnesium nitric + water** **Check your answers (p. 76)**