Enthalpy changes of reaction for reactive metals with HCl

Sodium carbonate can be used as a standard to determine acid concentration, because it is readily available in a pure and dry form. Here 7.5g is used to make up a 250cm solution with a concentration of 0.283 moldm .The calculations show this;

The exact concentration of hydrocloric acid, is found by titrating it against the standard solution.

Apparatus Chemicals

2 decimal place balance; HCl (exact

weighing jar; concentration not known

teat pipette; between 2.0 and 2.5moldm);

pipette;

burette; screened methyl orange;

conical flask; anhydrous sodium carbonate.

250cm measuring flask

and bung;

100cm measuring cylinder;

white tile;

clamp stand.

Safety points.

Solutions of HCl greater than or equal to 2moldm are irritant to the skin and eyes. A lab coat should be worn and direct contact with acid avoided. Na CO is also irritant, same preucautions should be taken.

Preparation of the standard solution.

Weigh the exact mass of sodium carbonate required using table 1

Transfer Na CO into a 250cm measuring flask, rinse the weighing bottle in distilled water and add these washings.

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Add about 200cm of distilled water

Bung and shake vigorously until all Na CO has dissolved

Allow solution to drain down sides of flask and debung.

Move down to eye level to avoid parralax and make up to 250cm using a teat pipette for the last few drops.

Ensure meniscus is correctly adjusted as shown.

Replace bung and shake for a few minutes to ensure the solution is of uniform concentration.

Standardisation of hydrochloric acid

Very accurately fill a measuring cylinder to the 250cm mark, with the HCl.

Make up to 100cm with distilled water.

Mix solution then transfer 50cm to the burette ensuring the jet is filled.

Record the burette reading on table.

Rince pipette through with the Na CO solution and drain away.

Fill the pipette again to above the mark, then at eye level control the release of the liquid until the meniscus is level with the 250cm line.

Transfer to the conical flask.

Add a few drops of screened methyl orange.

Place on white tile below burette as shown.

My Trial Results.

Add acid rapidly into the conical flask until 18cm has been added.

Very slowly add further acid swirling the flask continously.

Stop at the neutral grey colour before the solution becomes purple.

Take final burette reading.

Repeat titration (using same standard) until 3 results within 0.1cm are obtained.

Record all results but only use the values which are in agreement to calculate the mean volume used.

Calculation of hydrocloric acid concentration.

When chemical bonds are broken energy is required, when new bonds are formed, energy is given out.

An exothermic reaction is a reaction which overall gives out more energy than it takes in.

We can calculate this enthalpy change in a reaction by measuring the temperature change.

The five metals in this method are potassium, magnesium, lithium, calcium and zinc.

They are selected because they are all fairly reactive and are all safe to react with 2moldm of HCl. The come from a variety of groups from the periodic table.

I am using 0.3g of each metal as in larger quantities the reaction could be dangerous, and the hydrogen released explosive.

For a fair test all metals should react in the same quantaty of acid. To decide this volume I have performed the following calculations, using a concentration of 2moldm because it gives the maximum volume that could be required.

For a fair test all metals should react fully, and I have calculated the maximum volume of acid required for any of the five metals is 21.4 cm. Therefore I will choose a volume of acid that is in exess for all these reactions. I have decided on 50cm because it is more accurate to measure heat change in a larger volume.

Apparatus Chemicals

Polystyrene cup with lid and hole; Magnesium ribbon;

Glass beaker; Calcium granules;

Cotton wool; Zinc granules or powder;

Thermometer; Aluminium granules or

Clamp stand. powder;

Lithium granules.

Hydrochloric acid of

determined concentration.

Method

Set up as shown in the diagram.

Weigh exactly 0.3g of metal using the table below.

Record initial temperature of the acid.

Add the metal and immediately replace lid.

Monitor temperature and record the maximum temperature reached.

The metals used are not all powders- the Mg is in ribbon form, however this should still be a fair test. This is because this reaction is not dependant on rate. However in these conditions, during the reaction unavoidably some heat will be lost in heating the cup and the surroundings. A reaction which occurs faster will have less heat loss. This may slighly effect the results.

Safety points.

The greatest precautions must be taken during the reactions and when the products are formed. Hydrogen is a very flammable gas and is produced during all the reactions. There should be no contact with naked flames. Some of the products formed are corrosive and cause burns to the skin so contact must be avoided with all chemicals e.g. Aluminium chloride is corrosive. A lab coat should be worn at all times.

Enthalpy change = mc t

The HCl is made up mainly from water, so we can use water's data:

The specific heat capacity C of water =4.2JK g , and the mass 1cm=1g.

m = mass of HCl + mass of metal

=50g +0.3g

=50.3g

T is the temperature change in K or C

In my trial I obtained a T value of 17 C for magnesium

Enthalpy change = 50.3 x 4.2 x 17

= -3890 J

To convert to moles:

moles of Mg = mass = 0.3 = 0.0125

RFM 2.4

0.0125moles of Mg give an enthalpy change of -3590 J

so 1 mole of Mg gives - 287000 J mol = -287KJ mol

To simplify for each metal, in this particular reaction only

enthalpy change = mc t x mol/1000

= 50.3 x 4.2 x t x 80 /1000

=(16.9008 x t) KJmol