Acids and Alkalis

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Neutralisation

The Making of a Salt

When an acid reacts with an alkali it produces a salt and water.

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This reaction is called neutralisation. The alkali has neutralised the acid by removing its H+ ions, and turning them into water.

Neutralisation always produces a Salt

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Soil Treatment - Farming

The majority of plants grow best at pH 7. If the soil is acidic or alkaline the plant may grow badly. Therefore, chemicals can be added to the soil to change its pH.

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If the soil is too acidic - the most common complaint - it is treated with a base (chemicals opposite to an acid) in order to neutralise it. Common treatments use quicklime (calcium oxide) or chalk (calcium carbonate).

Indigestion

We all have hydrochloric acid in our stomach - it helps breakdown food! However, too much acid leads to indigestion. Therefore, to cure this ailment we need to neutralise the acid with a base such as, sodium hydrogen carbonate (baking soda), or an indigestion tablet.

Insect Stings

A bee sting contains acid. In order to relieve the painful symptoms of the sting we need to neutralise the acid. By rubbing on calamine lotion (zinc carbonate) or baking soda the acid can be neutralised.

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Wasp stings are alkaline, hence acid is needed to neutralise and remove the painful sting. Vinegar (ethanoic acid) is used.

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Waste from Factories

Waste from many factories are often acidic. If this acidic solution is not treated and enters rivers it can kill fish. Slaked lime (calcium hydroxide) is often used to neutralise the acid.

Exam-style Questions

  1. Complete the following table:

    pH of solution: Colour of universal indicator: Description:
    1 (ii) strong acid
    (i) green (iv)
    14 (iii) (v)

    (5 marks)

    (Marks available: 5)

    Answer

    Answer outline and marking scheme for question: 1

    (i) 7

    (ii) red

    (iii) blue/purple

    (iv) neutral

    (v) strong alkali

    (Total = 5 marks)

  2. The following equation shows the reaction that takes place between an acid and an alkali:

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    a) Name this type of reaction.

    (1 mark)

    b) Write a word equation that shows the reaction between nitric acid and potassium hydroxide.

    (1 mark)

    c) Write a symbol equation for the same reaction.

    (1 mark)

    (Marks available: 3)

    Answer

    Answer outline and marking scheme for question: 2

    a) Neutralisation.

    (1 mark)

    b) nitric acid + potassium hydroxide → potassium nitrate + water

    (Mark is gained for writing potassium nitrate as the salt).

    (1 mark)

    c) HNO3(aq) + KOH(aq) → KNO3(aq) + H20(aq)

    (1 mark)

    (Total = 3 marks)

  3. Complete the following equations:

    a) calcium carbonate + nitric acid =

    (1 mark)

    b) sodium carbonate + sulphuric acid =

    (1 mark)

    c) Ethanoic acid + sodium hydroxide =

    (1 mark)

    (Marks available: 3)

    Answer

    Answer outline and marking scheme for question: 3

    a) = calcium nitrate + carbon dioxide + water

    (1 mark)

    b) = sodium sulphate + water + carbon dioxide

    (1 mark)

    c) = sodium ethanoate + water

    (1 mark)

    (Total = 3 marks)

  4. Some salts are made in industry for use as fertilizers in farming.

    Complete the table showing acid and alkali used to make these salts

    Salt: Acid: Alkali:
    Ammonium nitrate
    Potassium phosphate

    (4 marks)

    (Marks available: 4)

    Answer

    Answer outline and marking scheme for question: 4

    Salt: Acid: Alkali:
    Ammonium nitrate Nitric acid Ammonium hydroxide
    (1 mark) (1 mark)
    Potassium phosphate Potassium hydroxide Phosphoric acid
    (1 mark) (1 mark)

    (Total = 4 marks)

Acids, Alkalis and Neutral Substances

1. They are liquids.

2. They are solutions of compounds in water.

3. If concentrated they can be corrosive.

4. Acids taste sour (for example, vinegar).

5. Turn blue litmus paper red - this is an easy test for an acid!

6. Usually react with metals to form salts.

7. Acids contain hydrogen ions.

8. Turn Universal Indicator from green to red, and have a pH less than 7.

Examples of acids: are vinegar (ethanoic acid) and lemon juice (citric acid)

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magnesium + hydrochloric acid -> magnesium chloride + hydrogen gas

Some common acids used in your laboratories at school will be:

1. Hydrochloric acid, HCl(aq)

2. Nitric acid, HNO3(aq)

3. Sulphuric acid, H2SO4(aq)

1. They feel soapy to touch.

2. They are soluble bases.

3. Like acids, they can burn the skin.

4. They turn red litmus blue - this is how you test for an alkali!

5. Alkalis contain hydroxide ions (OH-).

6. They taste bitter.

7. Turns Universal Indicator from green to blue or purple.

Some common alkalis used in your laboratories at school will be:

1. Sodium hydroxide, NaOH(aq)

2. Ammonia, NH3NH4OH(aq)

3. Calcium hydroxide, Ca(OH)2(aq)

1. Litmus paper is not affected by neutral paper.

2. Tend to be harmless

3. Universal Indicator stays green.

Common examples of neutral substances:

1. Water

2. Sodium chloride solution, NaCl(aq)(common salt)

3. Sugar solution C6H12O6(aq)

The Strength of an Acid

Acids and alkalis can be strong or weak!

So how can we measure their strength?

The strength of an acid or alkali is shown using a scale of numbers called the pH scale. The numbers go from 0-14.

PH Table

On the scale it follows that:

An acidic solution has a pH number less than 7

An alkaline solution has a pH number greater than 7

A neutral solution has a pH number of exactly 7.

You can find the pH of any solution using universal indicator. Universal indicator is a mixture of dyes. It comes as a solution or in paper.

Universal indicator will change from green to a different colour depending on the pH of the solution you place it in.

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Note:

In a strong acid, nearly all the acid molecules form ions.

In a weak acid, only some of the acid molecules form ions.

The more OH- ions (hydroxide ions), the more alkaline an alkali will be.

In other words, the more OH- ions there are the higher the pH number.

S-Cool Revision Summary

Acids are a group of chemicals that taste sour, turn  litmus paper red and react with metals to form salts.

Acids release hydrogen ions, H+ in solution.

Bases are a group of chemicals that feel soapy to touch. They behave in an opposite manner to acids.

Alkalis are soluble bases.

Alkalis turn red litmus blue.

Alkalis release hydroxide ions, OH-, in solution.

Neutral substances, such as water, are neither acidic or alkaline.

The strength of an acid is measured using a scale called the pH scale. The numbers go from 0 to 14.

An acidic solution has a pH number less than 7.

An alkaline solution has a pH number greater than 7.

A neutral solution has a pH number of exactly 7.

To find the pH number of any solution you use universal indicator. Universal indicator is a mixture of dyes that change colour depending on what they have been placed in.

A neutralisation reaction occurs when you add an acid to an alkali - they cancel one another out. A salt and water are the two products of neutralisation.

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