Noadswood Science, 2012

NeutralisationTo understand neutralisation reactions

Wednesday, April 19, 2023

Acid Or AlkaliAre the following acids or alkalis?

Hydrochloric (HCl)Sodium hydroxide (NaOH)Sulphuric (H2SO4)Nitric (HNO3)Ammonium hydroxide (NH4OH)ToothpasteLemon juiceOven cleanerStomach fluid Indigestion tablets

Alkali

Acid (Hydrochloric acid)

Acid (Sulphuric acid)Acid (Nitric acid)

Acid

Acid (Stomach acid)

AlkaliAlkali

Alkali

Alkali

Acids & Alkalis

NeutralStrong acid Weak acid Weak alkali Strong alkali

Stomach acid

Skin

Lemon juice

Fizzy drinks

Vinegar

Milk

Pure wate

rBlood

Indigestion

tabletsToothpas

te

Washing powder

Oven cleaner

Acid + AlkaliWhat happens when we mix and acid and alkali?

Mixing an acid and alkali results in a chemical reaction, forming new products…

Acid Alkali

NeutralisationThe chemical reaction between an acid and alkali is called

neutralisation

If you add just the right amount of acid and alkali together a neutral solution is formed (the pH value gets close to pH 7 - neutral)

Acid Alkali Salt + Water

ExperimentYour task is to see if you can undertake a neutralisation

reaction

In a boiling tube add a small amount of acid

Using a pipette add one drop of alkali at a time, until you produce a neutral solution

Chemical ReactionThe chemical reaction between an acid and alkali is: -

Acid Alkali Salt Water

AcidsSubstances with a pH of less than 7 are acids

The stronger the acid, the lower the pH number

Acids turn blue litmus paper red and turn universal indicator red if they are strong, and orange or yellow if they are weak

Acid Formula Strength

Sulphuric H2SO4 Strong

Hydrochloric HCl Strong

Nitric HNO3 Strong

Ethanoic (vinegar) CH3COOH Weak

AcidsWhen acids dissolve in water they produce hydrogen ions,

H+

For example, looking at hydrochloric acid: -

HCl(aq) → H+(aq) + Cl-(aq)

Acids & BasesWhen acids react with bases, a salt and water are made –

neutralisation

Acid + Metal Oxide → Salt + Water

Acid + Metal Hydroxide → Salt + Water

Acids & CarbonatesWhen acids react with carbonates a salt, water and carbon

dioxide are made

Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

* Carbon dioxide causes bubbling during the reaction, and can be detected using limewater

Acids & MetalsAcids will react with reactive metals to make a salt and

hydrogen

Acid + Metal → Salt + Hydrogen

*Hydrogen causes bubbling during the reaction, and can be detected using a lighted splint

BasesSubstances that can react with acids and neutralise them

to make a salt and water are called bases

Bases are usually metal oxides or metal hydroxides, e.g. copper oxide and sodium hydroxide are bases

Type Of Compound Formula Strength

Metal Hydroxides OH- Water + Salt

Metal Oxides O2- Water + Salt

Metal Carbonates CO32- Water + Salt + CO2

AlkalisBases that dissolve in water are called alkalis

Alkaline solutions have a pH of more than 7 (the stronger the alkali, the higher the pH number) – alkalis turn red litmus paper blue and they turn universal indicator dark blue or purple if they are strong, and blue-green if they are weak

Alkalis Formula Strength

Sodium Hydroxide NaOH Strong

Potassium Hydroxide KOH Strong

Calcium Hydroxide Ca(OH)2 Strong

Ammonium Hydroxide NH4OH Weak

AlkalisWhen alkalis dissolve in water they produce hydroxide ions,

OH-

For example, looking at sodium hydroxide: -

NaOH(aq) → Na+(aq) + OH-(aq)

NeutralNeutral solutions have a pH of 7

They do not change the colour of litmus paper, but they turn universal indicator green (e.g. water is neutral)

When the H+ ions from an acid react with the OH- ions from an alkali, a neutralisation reaction occurs to form water: -

H+(aq) + OH-(aq) → H2O(l)

In the reaction between sodium hydroxide and hydrochloric acid Na+ ions and Cl- ions left over – these form sodium chloride, NaCl

StingsA bee sting is acidic A wasp sting is alkaline

Why can bicarbonate of soda (pH 9) be used to treat a bee

sting?

Why can vinegar (pH 3) be used to treat a wasp

sting?

Adding just the right amount of acid or alkali to the sting causes it to neutralise!

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