Chapter 8 part 2: Bases Flashcards
What is a base?
- A base is a metal oxide/a metal hydroxide.
- Most bases are insoluble in water.
- E.g. Magnesium oxide, magnesium hydroxide, copper (II) oxide, and copper (II) hydroxide do not dissolve in water.
What is an alkali?
- An alkali is a soluble base that produces hydroxide ions (OH-) when dissolved in water.
What are the names and chemical formula(s) of some common alkalis?
- Sodium hydroxide:
Chemical formula: NaOH - Potassium hydroxide
Chemical formula: KOH - Calcium hydroxide
Chemical formula: Ca (OH)2 - Aqueous ammonia
Chemical formula: NH3
Do all alkalis produce hydroxide ions (OH-) in aqueous solutions?
- Yes, all alkalis produce hydroxide ions (OH-) in an aqueous solution.
- E.g. KOH (aq) –> K+ (aq) + OH- (aq)
- E.g. NaOH (aq) –> Na+ (aq) + OH- (aq)
- E.g. Ca (OH)2 (aq) –> Ca2+ (aq) + 20H- (aq)
Does ammonia contain any metals like other alkalis?
- No, ammonia does not contain any metals like other alkalis
- NH3 (g) + H2O (l) –>/<– OH- (aq) + NH4+ (aq)
What are some properties of alkalis?
Alkalis:
- Have a bitter taste.
- Feel soapy.
- Conduct electricity due to the mobile ions produced when they dissolve in water.
- Turn red litmus paper blue.
What is a Alkali + acids
–> salts + water reaction?
- Alkalis react with acids to form salts and water.
Alkali + acid –> salt + water - E.g. Sodium hydroxide + nitric acid –> sodium nitrate + water
NaOH (aq) + HNO3 (aq) –> NaNO3 (aq) + H2O (l)
What is the reaction between an acid and an alkali?
- The reaction between an acid and an alkali is known as a neturalisation reaction.
- The hydrogen ions from the acid react with the hydroxide ions from the alkali to form water.
- The ionic equation for a neutralisation reaction:
H+ (aq) + OH- (aq) –> H2O (l)
What is the reaction between an alkali and ammonia salts?
- Alkalis react with ammonium salts on heating to form salts, water and ammonia gas.
Alkali + ammonium salt –> salt + water + ammonia - E.g. Sodium hydroxide + ammonia chloride –> sodium chloride + water + ammonia
NaOH (aq) + NH4 Cl (aq) –> NaCl (aq) + H2O (l) + NH3 (g) - E.g. Potassium hydroxide + ammonia sulfate –> potassium sulfate + water + ammonia
2KOH (aq) + (NH4)2 SO4 (aq) –> K2SO4 (aq) + 2H(square 2)O (l) + 2NH3 (g)
How can the ammonia gas produced during the reaction of an alali with an ammonium salt be identified?
- The ammonia gas produced during the reaction of an alkali with an ammonium salt can be identified using damp (moist) red litmus paper.
- Ammonia gas turns damp red litmus paper blue.
What is the factor that affects acidity?
- Acidity depends on the concentration of hydrogen ions (H+) relative to that of hydroxide ions (OH-) in a solution.
How can we tell if the solution is acidic?
- If the concentration of hydrogen ions in a solution is higher than the concentration of hydroxide ions, the solution is acidic.
What is the factor that affects alkalinity?
- Alkalinity depends on the concentration of hydroxide ions (OH-) relative to that of hydrogen ions (H+) in a solution.
How can we tell if the solution is alkaline?
- If the concentration of hydroxide ions in a solution is higher than the concentration of hydrogen ions, the solution is alkaline.
Is a neutral solution acidic/alkaline?
- In a neutral solution, the concentrations of hydrogen ions and hydroxide ions are the SAME.
What are the functions of a pH scale?
- The pH scale is used to indicate the acidity and alkalinity of solutions.
- The lower the pH value, the higher the concentration of hydrogen ions compared to that of hydroxide ions in the solution, and higher the acidity.
- The higher the pH value, the higher the concentration of hydroxide ions compared to that of hydroxide ions in the solution, and the higher the alkalinity.
- pH value: < 7
Nature of solution: Acidic - pH value: (=) 7
Nature of solution: Neutral - pH value: > 7
Nature of solution: Alkaline
What is a universal indicator?
- A universal indicator is an indicator that shows a variety of colours at different pH values.
What are the colours of the universal indicator at pH 1-14?
- pH: 1
Colour: Red - pH: 2
Colour: Red-orange - pH: 3
Colour: Orange - pH: 4
Colour: Beige - pH: 5
Colour: Yellow - pH: 6
Colour: Lime-green - pH: 7
Colour: Green - pH: 8
Colour: Dark green - pH: 9
Colour: Turquoise - pH: 10
Colour: Pale blue - pH: 13
Colour: Violet - pH: 14
Colour: Purple
What are the different indicators and their colour changes in acidic and alkaline solutions?
- Methyl orange:
Colour in acidic solution:
- Red
pH range at which indicators changes colour:
- 3-5
Colour in alkaline solution:
- Yellow - Screened methyl orange:
Colour in acidic solution:
- Violet
pH range at which indicators changes colour:
- 3-5
Colour in alkaline solution:
- Green - Bromothymol blue:
Colour in acidic solution:
- Yellow
pH range at which indicators changes colour:
- 6-8
Colour in alkaline solution:
- Blue
What are the functions of soil?
- Soil of appropriate pH provides sufficient amount of the required nutrients to plants.
- Different plants thrive on soil of different pH values, and in general, plants grow better in soil with pH range of 5-7.
Can soil be more acidic over time (gradually)?
- Yes, soil tends to become more acidic overtime due to different environmental conditions such as acid rain.
What is liming (process)?
- Liming is the process of adding slaked lime to soil, and bases such as calcium hydroxide (slaked lime) can be added to soil that is too acidic.
** Slaked lime: Calcium hydroxide**
What happens during liming?
- During liming, the base reacts with the acids present in the soil to raise the pH of the soil.
- To ensure that the right amount of the base is added, the pH of the soil should be measured before, and during the process (of liming).
