C3 – Unit 6

Thermochemistry

Thermochemistry is about studying heat changes during chemical reactions

Hey S3 click on ‘formal note’ below to get a copy of the unit notes. I’ve now coloured it and made it bold. I’ve put it between the dashed lines. What more can I do. Surely you can get it now! Doc F

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formal note

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1. Magnesium burning

A piece of magnesium was set alight.

It burned with a bright white flame.

The reaction is obviously producing heat and light energy.

 

2. Heat changes when substances dissolve

Orsay

Orsay

Orsay                           Orsay

20cm3 of water were measured into a polystyrene cup and the temperature measured.

Two spatulas of solid were dissolved in it and the temperature measured again.

solid temperature 1 (ºC) temperature 2 (ºC) change (ºC)
sodium hydroxide 20 36 +16
ammonium chloride 20 17 -3
sodium nitrate 20 17 -3
sodium chloride 20 20 0
ammonium carbonate 20 18 -2

 When substances dissolve they can give out, or take in heat.

 

3. The best fuel

An investigation was carried out to see which of two fuels had the greater heat output – petrol or methylated spirits.

 

A burner containing methylated spirits was weighed.

Orsay

The burner was used to warm some water by about 40°C.

Orsay

The burner was reweighed.

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Using the difference in temperature and the difference in mass the heat produce by the burner was calculated.

Fuel Mass (g) Temperature (°C) Heat
start finish change start finish change (°C/g)
petrol 169.06 167.83 1.23 15 66 51 41.5
spirit 159.86 157.79 2.07 19 59 40 19.3

fuel_graph

4. The bomb calorimeter

The bomb calorimeter is used to measure heat produced from combustion reactions.

 

5. Exothermic and Endothermic Reactions – an investigation

The reactions were carried out in a poly-cup as shown. The temperature change was followed using a temperature probe connected to a Logit Datameter 1000. This was linked to a PC and the data captured.

Orsay

Here is sodium bicarbonate (baking soda) being reacted with citric acid.

Orsay

And a screen shot of the data being captured.

SYYST5~E

The experiment was repeated reacting magnesium powder with hydrochloric acid. A printout was obtained and temperatures worked out.

mandtsgraph

 

Reaction Temperature (°C)
Start Finish Change
Sodium bicarbonate + citric acid 18 12 -6
Magnesium + hydrochloric acid 18 45 +27

 

So the reaction between sodium bicarbonate and citric acid is endothermic. The temperature falls as heat is taken in from the surroundings.

The reaction between magnesium and hydrochloric acid is exothermic. The temperature rises as heat is given out.

 

6. Chemiluminescence

Chemiluminescence is the production of light using a cold chemical reaction.

Luminol produces light in the presence of certain oxidisers.

Experiment 1

Here luminol is reacted with sodium hypochlorite.

Solution 1 – 0.02 g of luminol and 15 ml of 0.1 M sodium hydroxide are made up to 200 ml with water.

Solution 2 – 7 ml of stock sodium hypochlorite are made up to 25 ml with water

3 ml of solution 2 are added with a dropper to 50 ml of solution 1 in a beaker.

 

Experiment 2

This time luminol is reacted with hydrogen peroxide

Solution 1 – 4 g sodium carbonate, 0.2 g luminol, 24 g sodium bicarbonate, 0.5 g ammonium bicarbonate and 0.4 g copper II sulphate made up to 1 l with water

Solution 2 – 50 ml of 10 vol hydrogen peroxide made up to 1 l with water

20 ml of solution 1 are added to 20 ml of solution 2 in beakers

 

An internet research was carried on Chemiluminescence by answering some questions

 

CFE Outcomes

Level 4

I can collect and analyse experimental data on chemical reactions that result in an obvious change in energy. I can apply my findings to explain the significance of the energy changes associated with chemical reactions.

SCN 4-19a

National 4

Energy changes of chemical reactions

Recognition and uses of exothermic and endothermic reactions.

Reactions can be exothermic or endothermic. This is dependent on the overall energy change taking place.

National 5

Energy from fuels

Different fuels provide different quantities of energy and this can be measured experimentally and calculated