Chemistry C3 Higher Tier (Topic 2) Acids and Bases.

Topic 2.

(kill me now. i hate chemistry more than the thought of naked old men..)

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  • Created by: Lawraa
  • Created on: 25-05-09 15:23

Acids.

When acids dissolve in water they produce hydrogen ions (H+).

H+ ions are protons, so acids are proton donors.

In aqueous solution, water molecules surround the protons to keep them in solution. This is called 'hydration'.

Hydrated ions are represented by (aq) eg. H+(aq).

Strong acids (hydochloric, nitric and sulfric acid)

- completely ionise in an aqueous solution

- very low pH (1 - 3)

Weak acids (ethanoic and citric acid)

- only party ionise in an aqueous solution

- low pH (4 - 6)

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Alkalis.

Alkalis are bases which dissolve in water to produce hydroxide ions (OH-).

These ions easily combine with hydrogen ions to form water:

OH-(aq) + H+(aq) => H2O(l)

Because bases act like this they are called proton acceptors.

Strong alkalis (sodium hydroxide, potassium hydroxide)

- completely ionise in an aqueous solution

- very high pH (11 - 14)

Weak alkali (ammonia solution)

- high pH (8 - 10)

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Titrations.

Acid solution + alkali solution => salt + H2O (this is called a neutralisation reaction.)

If too much acid/alkali is added the solution that is formed (neutralised) will be slightly more acidic/alkaline.

Thus, precise volumes must be added to ensure no excess acid or alkali is left over at the end point (when all the acid + alkali has reacted).

A chemical indicator is used to check the if the solution formed is neutral (pH 7):

- strong acid + strong alkali = any indicator

- weak acid + strong alkali = phenolphthalein

- strong acid + weak alkali = methyl orange

This can be done with the technique, titration.

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How to carry out a titration.

1) Measure known volume of alkali using a pipette (which has been cleaned with distilled water)

2) Now add an indicator to the alkali

3) Then add the acid slowly using a burette (which has been cleaned using distilled water) at 0.05 ml intervals

4) Do this until the solution of alkali and indicator change colour (goes colourless) and record readings from the burette

5) Repeat three times and discard annomalous results

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Comparing strengths.

To compare strengths, we dissolve a weak and strong acid in water which have the same concentrations, the strong acid will have a higher pH as it has completely ionised. (concentration of H+ ions in the water = 1 mol/dm3)

pH measures the concentration of hydrogen ions in a solution. The higher the pH, the lower the concentration of hydrogen ions.

Another way to distinguish strength of acids is to observe the rate of reaction at which an acid reacts with a metal such as magnesium or zinc.

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Titration Calulations. Question 1.

15cm3 of CaOH2(aq) was neutralised by 22cm3 of 0.5 HCl. What was the concentration of the CaOH2? (2HCl + CaOH2 => CaCl2 + 2H2O)

- Volume of acid (HCl) = 22cm3

- Concentration of acid = 0.5m

- Number of moles = 2

- Volume of base (CaOH2) = 15cm3

- Concentration of base = ?

- Number of moles = 1

(Vol A x Conc A / Num of moles A = Vol B x Conc B / Num of moles B) (Rearrange so Conc B is the subject..)

- Concentration of base (Conc B) = Vol A x Conc A x Num of moles B / Num of moles A x Vol B => 22 x 0.5 x 1 / 2 x 15 = 0.37M

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Titration Calcs continued. Question 2.

What volume of 0.5 NaCO3 is needed to neutralise 50cm3 of 2m HCl?

2HCl + Na2CO3 => 2NaCl + H2O + CO2

- Volume of acid = 50cm3

- Concentration of acid = 2m

- Number of moles = 2

- Volume of Base = ?

- Concentration of Base = 0.5m

- Number of moles = 1

Rearrange the equation (previously given) to make Vol B the subject.

- Volume of base (Vol B) = Vol A x Conc A x Num of moles B / Num of moles A x Conc B => 50 x 2 x 1 / 2 x 0.5 = 100cm3

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Titration Calcs continued. Question 3 & 4.

How many moles?:

(moles = concentration x (volume / 1000))

(moles of a gas = volume / 24000)

How many moles of H2SO4 are there in 25cm3 of 2.0m H2SO4?

- moles H2SO4 = 0.2 x (25 / 1000) = 5 x 10-3

Using 'how many moles?':

20g of NaOH was added to 100cm3 of water, what is its concentration now?

(1st step) How many moles is 20g NaOH?

- moles = mass / Relative Formula Mass (RFM) = 20 / 23 + 16 + 1 = 0.5

(2nd step) (moles = concentration x (volume / 1000))

- conc = moles x 1000 / vol = 0.5 x 1000 / 100 = 5M

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