pH and Neutralisation

When universal indicator (UI) is added to solutions, it will change colour depending on what ions are present. UI can be used to estimate the pH of a substance using a colour chart, however pH probes give a much more accurate reading of pH.

  

The coloured pH scale can only be used with indicators like UI, because these are the colours UI turns. Other indicators like methyl orange, and phenolphthalein, do not turn these colours - so this scale cannot be used.

Acids are made up of hydrogen ions (H⁺), which are released when acids are dissolved in water. The more hydrogen ions there are in solution, the lower the pH of that solution.

  

A base is a substance that can neutralise an acid to form salt and water only. The most common bases you will come across are metal oxides, metal hydroxides, and metal carbonates. Most bases are insoluble in water, and those that are soluble are called alkalis. Alkalis are made up of hydroxide ions (OH).

  

When an acid and an alkali react, we can write an ionic equation to show how they make water:

  

H⁺(aq) + OH⁻(aq) → H₂O(l)

  

When a base and an acid react, they produce a substance that is neutral. We call this a neutralisation reaction. A salt and water are always made during a neutralisation reaction.

  

acid + base → salt + water

Higher Only

The higher the concentration of hydrogen ions in an acidic solution, the lower the pH.

  

The higher the concentration of hydroxide ions in an alkaline solution, the higher the pH.

  

As hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution will decrease by 1.

Core Practical 2: Investigating pH (neutralisation)

This practical focuses on recording the pH at intervals when calcium hydroxide or calcium oxide reacts with dilute hydrochloric acid.

  

An initial mass of the solid must be added to a fixed volume of the acid, and the pH recorded each time more of the solid is added to the acid. The pH can be recorded using a pH meter, or universal indicator paper with a glass rod used to take a pH measurement at each interval. 

Method

  1. use a measuring cylinder to add 20 ml of dilute hydrochloric acid to a beaker

  2. dip a clean glass rod into the contents of the beaker, then use it to transfer a drop of liquid to a piece of universal indicator paper on a white tile

  3. wait 30 seconds, then match the colour to a pH colour chart - and then record the estimated pH

  4. add 0.3 g of calcium hydroxide powder to the beaker

  5. stir thoroughly, then repeat steps 2 and 3

  6. add another 0.3 g of calcium hydroxide powder to the beaker, and repeat steps 2-4 until you have added a total of 3.0 g of calcium hydroxide powder

Dilute/Concentrated and Weak/Strong (Higher Only)

solution forms when a solute dissolves in a solvent. The more concentrated the solution, the more particles it contains in a given volume.

When solutions are described as dilute or concentrated:

  • a dilute solution contains a small amount of dissolved solute

  • a concentrated solution contains a large amount of dissolved solute

  

Acids in solution are a source of hydrogen ions (H⁺). The hydrogen ions are produced when the acid dissociates (splits up) to form ions.

When acids are described as weak or strong:

  • weak acids only partially dissociate into ions in solution, for example ethanoic acid

  • strong acids completely dissociate into ions in solution, for example hydrochloric acid

26 weak, strong acids-01.png

General Reactions

When acids react with different chemicals, they can make different products. Using different acids will create salts that have different names. In the table below you can see how each of these acids affects the name of the possible salt:

  • sulfate salts form when using sulfuric acid

  • nitrate salts form when using nitric acid

  • chloride salts form when using hydrochloric acid

Testing for Gases

Hydrogen

The test for hydrogen uses a lit (burning) splint held at the end of a container of the gas. If you hear a 'squeaky pop' then hydrogen is present.

   

This test requires you to allow a second, or two, for oxygen gas from the air to mix with the hydrogen first, as the test is actually showing the chemical reaction between hydrogen and oxygen to make water.

  

Carbon Dioxide

The test for carbon dioxide involves bubbling the gas through limewater (calcium hydroxide solution). If the limewater turns milky/cloudy then carbon dioxide is present.

  

This test is actually a reaction between the carbon dioxide and limewater, to make limestone (calcium carbonate).

26 gas test-01.png

Making Soluble Salts

From acids and insoluble reactants

  • excess of the reactant (a metal, metal oxide, or metal carbonate) is added

    • the reactant is added in excess to make sure all of the acid reacts

    • it may have to also be gently heated to speed up the reaction

  • the excess reactant has to be removed

    • because we added more reactant than we need, we have to remove any left over by filtration

  • the solution remaining is only salt and water

    • the acid is used up and the insoluble reactant has been removed

    • evaporating the water leaves the pure, dry crystals of salt

28 making copper sulfate-01.png

From acids and soluble reactants

  • a titration must be used

    • this is because there is no insoluble excess reactant that we can remove using filtration

  • the acid and the soluble reactant (usually a dilute alkali) are mixed in the correct proportions

    • because we want to exactly neutralise the acid, adding any less, or more, alkali than needed will not result in a neutral solution of salt

  • the solution remaining is only salt and water

    • the reactant has been added in correct proportions to fully react with the acid

    • evaporating the water leaves the pure, dry crystals of salt

Core Practical 3: Preparing Copper Sulfate

Excess copper oxide must be added to warm dilute sulfuric acid (warmed using a water bath), which will react to produce a blue solution of the salt copper(II) sulfate.

  

The solution then needs to be filtered using filter paper and evaporated using an evaporating basin and Bunsen burner, followed by final drying using a watch glass to allow all the water to evaporate.

coppersulfate-02.png
coppersulfate-03.png

Method

  1. use a measuring cylinder to add 40 ml of sulfuric acid in a beaker

  2. warm the beaker in a water bath for 5 minutes

  3. add a spatula of copper oxide powder to the beaker

  4. stir the solution with a glass rod, and add more copper oxide powder, until it no longer disappears (add in excess)

  5. filter the mixture to remove the excess copper oxide, then pour the filtrate (the copper sulfate solution) into an evaporating basin

  6. place the evaporating basin above a water bath, and heat the copper sulfate solution to evaporate off half of the water

  7. pour the solution into a watch glass and leave on the side to allow all of the water to evaporate

Acid-Alkali Titration

Apparatus

  • A glass pipette is used to accurately measure the volume of a reactant before transferring it to a conical flask

  • burette is used to add small, exact volumes of one reactant to the other reactant

  • A suitable indicator, usually either methyl orange or phenolpthalein (as these give exact colour changes, rather than UI that has too many colours and is difficult to tell when neutral)

  

Method

  1. Using the pipette and pipette filler, add 25 ml of alkali to a clean conical flask

  2. Add a few drops of suitable indicator to the alkali, and put the conical flask on a white tile

  3. Fill the burette with acid and record the starting volume

  4. Slowly add the acid from the burette to the conical flask, swirling it to make sure they mix

  5. Stop adding the acid when the end-point is reached (this is when the indicator permanently changes colour), and record the final volume reading

  6. Repeat steps 1 to 5 until you get concordant titres. You will achieve more accurate results if acid is added drop by drop near to the end-point (you will be able to calculate this from your rough titre

29 titrations-01.png

Results and analysis

Readings should be recorded to two decimal places (where the final digit is either a 0 or 5).

  

You need to work out a mean titre, and you must only include results that are concordant (within 0.10 ml of each other) in your calculation. 

Making Insoluble Salts

An insoluble salt can be prepared by reacting two soluble salts together to form a precipitate (insoluble salt).

  

Method

  1. Mix together two suitable solutions (see solubility rules table to determine which soluble salts will produce an insoluble salt)

  2. Use filtration to separate the precipitate as a residue from the solution

  3. Wash the precipitate with distilled water whilst it is still on the filter paper

  4. Leave the precipitate in a warm oven, or on the side, to dry

30 making lead iodide-01.png
 
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