Acids and bases 1

Acids Turn blue litmus red Neutralise bases React with active metals e.g. zinc, releasing hydrogen e.g. HCl (hydrochloric acid)      H2SO4 (sulfuric acid)      HNO3 (Nitric acid)e.g. Vinegar (weak ethanoic acid - CH3COOH)       Lemon juice (citric acid)       Car batteries (dilute sulfuric acid - H2SO4 - strong acid)All acids contain hydrogen. Acids can be qualified as monobasic, dibasic, or tribasic, depending on the number of hydrogen atoms per molecule that are removable by reaction with a base.e.g. hydrchloric acid (HCl) monobasic      sulfuric acid (H2SO4) dibasic

Bases Turn red litmus blue  Neutralise acids. e.g. Sodium hydroxide (NaOH)       Sodium Carbonate (NA2CO3)       Ammonia (NH3)e.g. Ammonia (for cleaning - NH3)      Sodium hydroxide (oven cleaners - NaOH)      Magnesium hydroxide (laxitive/antacid in milk of magnesia)A base that dissolves in water is called an alkali e.g. sodium hydroxide is an alkali ; a solution of sodium hydroxide is said to be alkaline.

NEUTRALISATION

NEUTRALISATIONWhen an acid reacts with a base, a salt is formed e.g. when hydrochloric acid reacts with sodium hydroxide

the salt sodium chloride is formed. Hydrochloric acid when neutralised by a base forms salts called chlorides.When sulfuric acid reacts with ammonia,

the salt ammonium sulfate is formed. Sulfuric acid, when neutralised by a base forms salts called sulfates.

*Neutralisation of an acid by a base has many everyday applications, e.g. indigestion (too much acid in the stomach) --> magnesium hydroxide or sodium hydrogencarbonate (bases). The excess HCl is neutralised.

HCl + NaHCO3 → NaCl + H2CO3

*Sodium hydrogen carbonate can also be used to treat bee stings, which are acidic.Lime (calcium hydroxide) is a base used in agriculture to neutralise acidic soil.

Most acids commonly used are in aqueous solutions where the acid molecules are dissolved in water. Acids usually need water before they can show their acidic properties. In 1887, the Swedish chemist, Arrhenius, developed a theory of acids and bases. The theory helps to explain how water allows acids to show their acidic properties.It was known at the time that pure acids, such as anhydrous sulfuric acid and glacial ethanoic acid, are non-conductors of electricity, while their aqueous solutions conduct electricity. It was also known that acids contain at least one hyrdogen atom per molecule of acid.

Arrhenius stated that:An acid is a neutral molecule (HX), which dissolves in water to form a hydrogen ion and an anion: HX --> H+ + X-The acidic properties of the solution are due to the presence of the H+ ions.--------------------------------------------------------------------------A base dissociates in water to form a hydroxide ion and a cation: MOH --> OH- +M+The basic properties of the solution are due to the presence of the OH- ions.--------------------------------------------------------------------------In solution, strong acids and bases are fully dissociated.in solution, weak acids and bases are only slightly dissociated.

AN ARRHENIUS ACID is a substance that dissociates in aqueous solution forming hydrogen ions.AN ARRHENIUS BASE is a substance that dissociates in aqueous solution forming hydroxide ions.

According to the Arrhenius theory, neutralisation of a solution of a strong acid such as HCl by a strong base such as NaOH, occurs as follows: In solution, the strong HCl is fully dissociated into H+ and Cl- ions, while the strong base NaOH is fully dissociated is fully dissociated into OH- and NA+ ions. When the two solutions are mixed, the H+ ions react with the OH- ions. The two other ons present, Na+ and Cl-, do not react as they are more attracted to water molecules than to each other.

Evidence of the Arrhenius theory...The Arrhenius theory explains why strong acids conduct electricity only when dissolved in water; there are no ions present in the pure acid.It also explains why a solution of a weak acid does not conduct electricity as well as a a solution of a strong acid of a similar concentration; there are fewer ions present in the solution of the weak acid.For similar reasons, a solution of a weak base will not conduct electricity as well as a solution of a strong base of similar concentration.Acids usually need water before they can show their acidic properties.

Limitations of the Arrhenius theory...The Arrhenius theory is limited to acid-base reactions that take place in water.When an acid dissolves in water, the hydronium ion, H30+ is formed, rather than the hydrogen ion.The Arrhenius theory does not explain how certain substances can be amphoteric, i.e. act as an acid with a base and as a base with an acid.

According to this theory:An ACID is a PROTON DONORA BASE is a PROTON ACCEPTORAn acid-base reaction involves the transfer of a proton (H+) from the acid to the base.The stronger the acid, the more readily it transfers a proton.The stronger the base, the more readily it accepts a proton.The weaker the acid, the less readily it transfers a proton.The weaker the base, the less readily it accepts a proton.Acid-base reactions, according to this theory, involve proton transfer. When HCl is added to water, the acid, HCl, transfers a proton to water, which in this case, being a proton acceptor is the base.

Advantages of this theoryThe Bronsted-Lowry theory shows how the hydronium ion is formed when an acid reacts with water.The Arrhenius theory only deals with reactions in water. The Bronsted-Lowry theory broadens the range of reactions that can be regarded as acid-base reactions. e.g. ammonia reacts with hydrogen chloride gas in the absence of water.

Ammonia acts as a Bronsted-Lowry base, accepting a proton from hydrogen chloride.The Bronsted-Lowry theory explains how substances can be amphoteric e.g. when water reacts with ammonia:it acts as an acid, but when it reacts with nitric acidit acts as a base, accepting a proton from nitric acid.

*An amphoteric substance is one that can as an acid with a base and as a base with an acid.

Species that differ by the presence or absence of a proton are called conjugate acid - base pairs e.g. NH4+ is the conjugate acid of NH3 while OH- is the conjugate base of H20.

A conjugate acid is formed when a proton (H+) is donated to a Bronsted-Lowry base.A conjugate base is formed when a proton  (H+) is removed from a Bronsted-Lowry acid.A conjugate acid-base pair is an acid and  base that differ by the presence or absence of a proton.

Relative strengths of acids and bases:When a strong acid, such as HNO3, donates a proton, the conjugate base that is formed has little tendenct to accept a proton and form HNO3 again, and so is a weak base. In the case of a weak acid e.g. CH3COOH, it donates a proton, forming a conjugate base CH3COO-, this has a much greater tendency to accept a proton and form CH3COOH again. Because of this, CH3COO- is a relatively strong base.IN GENERAL, the stronger an acid, the weaker its conjugate base, and the stronger the base, the weaker its conjugate acid.

Introduction

The Arrhenius Theory Of Acids And Bases

The Bronsted-Lowry Theory Of Acids And Bases

Conjugate acids and bases

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