GCSE OCR Gateway Chemistry C3.3g–j Neutralisation Titration curves

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 

In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification. 

I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least. 

That written in italics is for the higher tier paper only.

C3 Chemical Reactions

C3.3 Neutralization Reactions

C3.3g use and explain the terms dilute and concentrated (amount of substance) and weak and strong (degree of ionization) in relation to acids: ratio of amount of acid to volume of solution.

Dilute and concentrated acid or alkali refer to the number of moles of acid or alkali in a litre of solution.

So1M hydrochloric acid refers to a solution of hydrochloric acid in which there are exactly one mole of acid molecules in a litre of the solution. (not in water) i.e 1 mole.dm^-3   called one mole per decimetre cubed.

Equally a 1M solution of ethanoic acid is a solution of the acid in which there are exactly one mole of acid molecules in a litre of the solution.

But these two solutions do not have the same pH.

1M Hydrochloric acid is a strong acid in which all (100%) of the 1 mole of acid (HCl) molecules are split into H+ ions and Cl^— ions.

Here is the equation for this split:

HCl (aq)   →    H+ (aq)     +    Cl^—  (aq) 

However, when one mole of ethanoic acid molecules dissolve in water only a small percentage of the molecules of the acid split up into ions H+ and ethanoate CH₃COO^— . 

Here is the equation for this different type of split:

CH₃COOH (aq)    ⇌     CH₃COO^— (aq)       +       H+ (aq)

Note the reversible arrow in the second equation.

The reactions are called ionization.

The percentage of molecules split up is called the degree of ionization.

The effects of ionization are obvious since a completely ionised solution of acid or alkali will contain many more ions than a partly ionized solution. 

So strong acids (like HCl) are better conductors of electricity than weak acids (like CH₃COOH).

Strong acids are also better oxidizing agents than weak acids and so produce much more hydrogen and faster on reaction with a reactive metal like magnesium. 

Here is a summary of the effects of concentration and acid strength on the properties of a strong acid versus a weak acid: 

C3.3h recall that relative acidity and alkalinity are measured by pH

pH measures how acidic a solution is.

The pH scale was first developed by Carlsberg in Denmark for measuring lager acidity.

The scale runs from 1-14.

An increase in one pH unit means a 10 fold increase in alkalinity or acidity i.e. hydrogen ion concentration.

pH	Colour of full range indicator (approx)	Acidity decreasing	Alkalinity decreasing
1
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14

C3.3k describe techniques and apparatus used to measure pH

pH is measured using Full Range indicator or a pH probe attached to a datalogger/pH meter.

How to measure the strong acid strong base titration curve

Here is a diagram of a titration apparatus

Here is a lab to use to determine the pH curve for the titration of a strong alkali using a strong acid.

The result looks like this:

There is a rapid change in pH at the end point of the titration (25.00ml)

Within adding a 2 or 3 drops of the alkali the pH changes by 10 units.

Either methyl orange or phenolphthalein indicators change colour over this pH range and so can be used to mark the end point.

Methyl orange will turn from red to yellow with orange the end point colour.

Phenolphthalein will just turn magenta from colourless at the end point.

If acid is run into alkali then the results look like this:

C3.3i describe neutrality and relative acidity and alkalinity in terms of the effect of the concentration of hydrogen ions on the numerical value of pH (whole numbers only)

C3.3j recall that as hydrogen ion concentration increases by a factor of ten the pH value of a solution decreases by a factor of one