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chem ‘24 > acids & bases > Flashcards

acids & bases Flashcards

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1
Q

What is a base?

A
  • a base is a substance that reacts with an acid to form salt and water only
  • bases are usually metal oxides or metal hydroxides (exception: aqueous ammonia)
  • Most metal oxides and metal hydroxides are insoluble in water but there are those that are soluble in water
    -If a metal oxide dissolves and reacts with water, a solution of metal hydroxide (an alkali) is formed
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2
Q

What is an alkali?

A
  • Alkalis are bases that dissociate in water to produce hydroxide ions, OH -
  • alkalis are usually metal hydroxide solutions with the exception of aqueous ammonia
  • all alkalis are bases but not all bases are alkalis
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3
Q

Strength of alkalis

A
  • A strong alkali completely dissociates in water to give hydroxide ions
    e.g. KOH (aq) → K+ (aq) + OH- (aq)
  • a weak alkali partially dissociates in water to give hydroxide ions (reaction occurs with water)
    e.g. NH4 (aq) + H2O (l) <=> NH4+ (aq) + OH- (aq)
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4
Q

Why does ammonia gas form a weak alkali when dissolved in water?

A
  • When ammonia gas dissolves in water, only a small fraction of ammonia molecules react with water to produce ammonium, NH4+ (aq) and hydroxide ions, (aq)
    -Most ammonia still remains as simple molecules in the solution
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5
Q

Properties of alkalis (physical and chemical)

A

Physical: bitter, soapy, turns red litmus paper blue, pH >7, can conduct electricity
Chemical: alkalis react with acids, ammonium salts and some metal cations to form precipitate

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6
Q

Properties of alkalis - neutralization

A

Bases react with acids to form water and salt only
- base + acid → salt + water

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7
Q

Properties of alkalis - ammonium

A

On warming, alkalis react with ammonium salt to form salt, ammonia and water
- alkali+ ammonium salt → salt + ammonia gas + water
Test for ammonia gas
- colorless and pungent gas is produced
- gas turns moist red litmus paper blue
- the gas is ammonia
Note: warming is necessary as ammonia is very soluble in water. Solubility of gas decreases with increase in temperature

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8
Q

Why did the moist red litmus paper turn blue when ammonia gas is in contact with it?

A

Ammonia gas reacts with water to produce hydroxide ions which turns the red litmus paper blue

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9
Q

Properties of alkalis - test for cations (precipitation)

A
  • The reaction produces insoluble metal hydroxide, which appears as precipitate. The reaction is called a precipitation reaction.
  • Solution containing metal ion + alkali → insoluble metal hydroxide (precipitate) + salt
  • Note: precipitation is a chemical reaction between which an insoluble product (precipitate) is formed when two aqueous solutions are mixed
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10
Q

Indicators and the pH scale - pH scale

A
  • pH is a measure of acidity and alkalinity in aqueous solution
  • pH measures the concentration of hydrogen ions in the aqueous solution
  • pH scale is numbered between 0-14 (0 being most acidic, 7 being neutral, 14 being most alkaline)
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11
Q

Indicator and pH scale-comparing H+ vs OH- concentration within a solution using the pH scale

A
  • In a neutral solution, the concentration of H+ is equal to the concentration of OH-
  • in an acidic solution, the concentration of H+ is more than the concentration of OH-
  • in an alkaline solution, the concentration of H+ is less than the concentration of OH-
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12
Q

Indicator and pH scale-comparing H+ vs OH- concentration across solutions using the pH scale

A
  • The smaller the pH, the more acidic the solution, the higher the concentration of hydrogen ions
  • the larger the PH, the more alkaline the solution, the higher the concentration of hydroxide ions
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13
Q

Indicators

A
  • An indicator is an organic compound which changes in color in accordance with the pH of the solution
  • indicators are substances that have different colors in acidic and alkaline solution. They can be made from natural living organisms.
  • examples: litmus, phenolphthalein, methyl orange
  • universal indicator: The universal indicator contains a mixture of dyes and can come in the form of a solution or pH paper. It is able to give a spectrum of different colors across pH values. The pH of the substance can be ascertained by comparing the color obtained against color chart
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14
Q

pH meter

A
  • An electrical method of measuring the pH of a solution. it consists of a pH electrode connected to a meter. The pH electrode is dipped into solution and pH value is then shown on the meter
  • advantages: more reliable and accurate. Can be used in a data logging to record rapid changes in pH.
  • variation of PH during a neutralization reaction can be studied using a data logger and a computer
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15
Q

pH and agriculture

A
  • Most plants grow best at a specific soil pH
  • problem: soils tend to became acidic from acid rain and extensive use of chemical fertilizers (eg ammonium sulfate)
  • solution: adding slaked lime (Calcium hydroxide) to the soil to neutralize the acids aka liming the soil
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16
Q

Classes of acids

A

Inorganic acids - prepared in the laboratory
- examples: HCL, HNO3, H2SO4
Organic acids-obtained from plants and animals
- examples: CH3COOH, citric acid

17
Q

What are acids?

A
  • An acid is a substance which dissociates in water aqueous solutions to give hydrogen ions. All acid contain H+ ions.
  • pure acids exist as simple covalent molecules. These acid molecules react with water to produce hydrogen ions (ie acid molecules ionises/dissociates in presence of water)
  • equation to show the dissociation/ionization of acid molecules: HCL (g) → HCL (aq) → H+ (aq) + CL- (aq)
  • an acid only exhibits acid properties when dissolved in water. The presence of hydrogen ions cause the acidity.
18
Q

Basicity of acids

A
  • Basicity of an acid is the maximum number of hydrogen ions produced by one molecule of the acid when the acid molecule dissociates in water.
  • monobasic (one H+ ion): hydrochloric acid, nitric acid, ethnic acid
  • dibasic (two H+ ion): sulfuric acid, carbonic acid
  • tribasic (three H+ ion): phosphoric acid
19
Q

Strength of acids

A
  • Strength of an acid depends on the extent of dissociation/ ionization of the acid molecules in water/aqueous solution
  • A strong acid is one that completely dissociates in water to give hydrogen ions (eg hydrochloric acid, sulfuric acid, nitric acid)
  • A weak acid is one that partially dissociates in water to give hydrogen ions (eg ethnic acid, carbonic acid)
20
Q

Strength vs concentration of acid

A
  • Strong and weak refer to the extent of dissociation of the acid in water
  • concentrated and dilute refer to the amount of solute in the solution
21
Q

Concentration of acids

A
  • Concentration of a solution is a measure of how much solute has dissolved in 1 dm3 of the solution
    mol/dm3 is the unit for concentration
22
Q

Properties of acid

A

Physical: sour, turns blue litmus paper red, pH <7, can conduct electricity
Chemical: acids react with metals above hydrogen in the reactivity series, metal carbonates/metal hydrogen carbonates, bases

23
Q

Solubility table (solubility of salts and bases in water)

A
  • All sodium, potassium, ammonium and salts of other group 1 metals are soluble
  • all nitrate salts are soluble
  • all sulfate salts are soluble (except Ba, Ca, Pb, Ag2SO4 is sparingly soluble)
  • all chloride salts are soluble (except Ag and Pb)
  • all carbonate salts are insoluble ( except Na, K, NH4 and other carbonates of group 1 metals)
  • all oxides are insoluble (except Na, K, oxides of other group 1 metals, BaO and CaO. Cao is sparingly soluble)
  • all hydroxides are insoluble (except Na, K, hydroxides of other group 1 metals, Ba(OH)2 and Ca(OH)2. Ca(OH)2 is sparingly soluble)
24
Q

Chemical properties of acids - reaction with metals above hydrogen in reactivity series

A

Acids react with metals above hydrogen in the reactivity series to form salt and hydrogen gas only
- acid+ metal → salt + hydrogen
- observations: effervescence of a colorless, odorless gas observed. Gas extinguishes a lighted /burning splint with a ‘pop’ sound. The gas is hydrogen.
- some reactive metals have no apparent reaction with acids. Lead metal appears not to react with hydrochloric acid or sulfuric acid. The initial reaction produces a layer of lead(ii) chloride or Lead (ii) sulfate. This layer is insoluble in water and forms a layer around the metal, which prevents further reaction between the metal and acid.

25
Q

Reactivity series

A

Most reactive → least reactive
K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, H, Cu, Ag, Au, Pt

26
Q

Chemical properties of acids - metal carbonates

A

Acid read with metal carbonates (and hydrogen carbonates) to form salt, carbon dioxide and water
- acid + metal carbonate → salt + carbon dioxide + water
- acid + metal hydrogen carbonate → salt + carbon dioxide + water
- observations: effervescence of colorless, odorless gas observed. Gas forms white precipitate when passed into limewater. The gas is carbon dioxide.
- Carbon dioxide gas (acidic) reacts with limewater (alkaline) to give calcium carbonate, an insoluble salt which appears as a white precipitate

27
Q

Chemical properties of acids - neutralization

A

Acids react with bases to form salt and water only
- acid + bases → salt + water
Neutralization

chem ‘24 (8 decks)

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