acids & bases Flashcards
What is a base?
- 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
What is an alkali?
- 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
Strength of alkalis
- 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)
Why does ammonia gas form a weak alkali when dissolved in water?
- 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
Properties of alkalis (physical and chemical)
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
Properties of alkalis - neutralization
Bases react with acids to form water and salt only
- base + acid → salt + water
Properties of alkalis - ammonium
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
Why did the moist red litmus paper turn blue when ammonia gas is in contact with it?
Ammonia gas reacts with water to produce hydroxide ions which turns the red litmus paper blue
Properties of alkalis - test for cations (precipitation)
- 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
Indicators and the pH scale - pH scale
- 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)
Indicator and pH scale-comparing H+ vs OH- concentration within a solution using the pH scale
- 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-
Indicator and pH scale-comparing H+ vs OH- concentration across solutions using the pH scale
- 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
Indicators
- 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
pH meter
- 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
pH and agriculture
- 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
Classes of acids
Inorganic acids - prepared in the laboratory
- examples: HCL, HNO3, H2SO4
Organic acids-obtained from plants and animals
- examples: CH3COOH, citric acid
What are acids?
- 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.
Basicity of acids
- 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
Strength of acids
- 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)
Strength vs concentration of acid
- 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
Concentration of acids
- 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
Properties of acid
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
Solubility table (solubility of salts and bases in water)
- 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)
Chemical properties of acids - reaction with metals above hydrogen in reactivity series
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.
