Reversible reactions and analysing substances

1. Reversible reactions and analysing substances

2. What is the % of carbon in CO2? • What is the mass of 4 moles of H2O? • Calculate the % yield of a reaction to make iron when you could make 86Kg but actually make 56Kg. • How many moles are in 22g of CO2?

3. Answers • Ar of carbon/Mr of CO2 12/44= 27.27 • Mass= moles x Mr 4 x 18 = 72g • Actual yield/ theoretical yield x 100 56/86 x 100 = 65.1% • Moles= mass / Mr 22/ 44 = 0.5 moles

4. What are reactions? • How can we tell if reactions are taking place?

5. Irreversible reactions Most chemical reactions are considered irreversible – the products that are made cannot readily be changed back into their reactants. For example, when wood burns it is impossible to turn it back into unburnt wood again! Similarly, when magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen, it is not easy to reverse the reaction and obtain the magnesium.

6. What are reversible reactions? + + A B C D (reactants) (products) Reversible reactions occur when the backwards reaction (products  reactants) takes place relatively easily under certain conditions. The products turn back into the reactants. For example, during a reversible reaction reactants A and B react to make products C and D. However, products C and D can also undergo the reverse reaction, and react together to form reactants A and B.

7. Reversible biochemical reactions + Hb 4O2 Hb.4O2 Many biochemical reactions (those that take place inside organisms) are reversible. For example, in the lungs, oxygen binds to haemoglobin (Hb) in red blood cells to create oxyhaemoglobin. When the red blood cells are transported to tissues, the oxyhaemoglobin dissociates back to haemoglobin and oxygen. There are also some very important industrial reactions, like the Haber process, that are reversible.

8. The reversible reactions of copper sulphate • Place 2/3 spatulas of copper sulphate powder in a boiling tube. • Heat strongly with a blue flame and note down any change. • Allow to cool for 1 minute. • Pipette 4/5 drops of water onto the powder. • What do you notice? • Why is this a reversible reaction?

9. Heating copper sulfate

10. ammonium chloride hydrogenchloride + ammonia + HCl (g) NH3(g) NH4Cl (s) NH4Cl reforms in the cooler part of the test tube NH4Cl decomposes back into NH3 and HCl gases when heated Heating ammonium chloride An ammonium salt can be made by reacting ammonia with an acid. Some of the salt will decompose back into the reactants when heated.

11. reactant A reactant B product + What is dynamic equilibrium? In some reversible reactions, the forward and backward reactions largely occur in the same conditions and at the same rate. These reactions are said to be in dynamicequilibrium – there is no overall change in the amount of products and reactants, even though the reactions are ongoing. Dynamic equilibrium can only take place in a closed system, otherwise the products would escape.

12. What happens in dynamic equilibrium? What is special about the forward and backward reactions at dynamic equilibrium?

13. Discuss the following questions: How do we know what elements are present in the sun? How do we know that objects in the universe are moving away from us? How do we know what type of paint to use for restoring the works of the Old Masters? How do we detect miniscule traces of chemicals in drinking water? How do Trading Standards obtain evidence that e.g. children’s toys do/do not contain harmful heavy metals? How do forensic scientists confirm the exact identity of a white powder?

14. Instrumental analysis

15. Identifying chemicals in dyes and food additives.

16. Paper Chromatography • Basic technique • Stationary Phase – Paper • Mobile Phase – Usually water • The solvent (water) moves up the paper carrying the solute. • Most soluble travels further, distance travelled depends on the compound. • Good for separating water soluble inks • Limited types of mobile phases can be used therefore limit to what can be separated • Can be slower • Detection – naked eye or UV light

17. Paper Chromatography Finish Start How many colours in a) red dye b) green dye c) purple dye d) overall

18. How many dyes were used in a) Black, b) Green? How many dyes were used overall? How many blues were used? Are the yellows in brown and orange the same? How do you know?

19. A B C D Who are these people and what is the link between them? What links them to today’s lesson?

20. Instrumental analysis • Analysing substances using machines. • Why would we do this? What advantages does it have over methods for detecting chemicals experimentally?

21. More accurate, • More sensitive • Quicker • Useful when the amount of a sample is very small.

22. Example: Gas chromatography Sample is injected into oven where it is vaporised. Mixtures are split up, some compounds are carried through quicker. Compounds are detected at the end, time taken help tells us what the compound is.

23. Mass spectrometry Some gas chromatographs are linked to a mass spectrometer. This helps give the relative molecular mass of the compound. (Mr)

24. Explain what a reversible reaction is, giving an example of a reaction that is reversible. (3) • Explain why instrumental analysis like gas chromatography is better for sampling mixtures than chemical analysis like paper chromatography. (3) • Describe what these parts of a gas chromatograph do. Oven, injector, mass spectrometer. (3)