IGCSE CHEMISTRY SECTION 4 LESSON 4

Content The iGCSE Chemistry course Section 1 Principles of Chemistry Section 2 Chemistry of the Elements Section 3 Organic Chemistry Section 4 Physical Chemistry Section 5 Chemistry in Society

Content Section 4 Physical Chemistry a)Acids, alkalis and salts b)Energetics c)Rates of reaction d)Equilibria

Lesson 4 d) Equilibria 4.22 understand that some reactions are reversible and are indicated by the symbol ⇌ in equations 4.23 describe reversible reactions such as the dehydration of hydrated copper(II) sulphate and the effect of heat on ammonium chloride 4.24 understand the concept of dynamic equilibrium 4.25 predict the effects of changing the pressure and temperature on the equilibrium position in reversible reactions.

Reversible Reactions OK, let’s introduce you to a new symbol and concept.

Reversible Reactions X

This is the symbol for a reversible reaction

Reversible Reactions A reversible reaction is one which can go in both directions.

Reversible Reactions This means that in a chemical reaction the products of the reaction can be turned back into the original reactants.

Reversible Reactions For example: A + B C + D

Reversible Reactions For example: A + B C + D A and B react to produce C and D, but also C and D can react to produce A and B.

Reversible Reactions Let’s look at an example

Reversible Reactions Heating blue copper sulphate Heat Water vapour

Reversible Reactions Heating blue copper sulphate Heat Water vapour White anhydrous copper sulphate

Reversible Reactions Blue hydrated copper sulphate White anhydrous copper sulphate Add a few drops of water

Reversible Reactions Blue hydrated copper sulphate White anhydrous copper sulphate Add a few drops of water CuSO 4.5H 2 O CuSO 4 + 5H 2 O

Reversible Reactions Let’s look at another example – favourite one in the exams!

Reversible Reactions Let’s look at another example – favourite one in the exams! The Thermal Decomposition of ammonium chloride, NH 4 Cl

Reversible Reactions NH 4 Cl NH 3 + HCl Heat Water in and out Solid ammonium chloride Ammonia and HCl gases

Reversible Reactions NH 4 Cl NH 3 + HCl Heat Water in and out Solid ammonium chloride Ammonia and HCl gases 1. When heated. Ammonium chloride splits up into ammonia gas and hydrogen chloride gas.

Reversible Reactions NH 4 Cl NH 3 + HCl Heat Water in and out Solid ammonium chloride Ammonia and HCl gases 1.When heated. Ammonium chloride splits up into ammonia gas and hydrogen chloride gas. 2.As these gases cool on the bottom of the flask, they recombine to form solid ammonium chloride again.

Reversible Reactions NH 4 Cl NH 3 + HCl Heat Water in and out Solid ammonium chloride Ammonia and HCl gases 1.When heated. Ammonium chloride splits up into ammonia gas and hydrogen chloride gas. 2.As these gases cool on the bottom of the flask, they recombine to form solid ammonium chloride again. 3.This is a good example of a reversible reaction because the products recombine to easily form the original reactants.

Reversible Reactions and Closed Systems

Try to imagine a closed system where no more reactants are added, and no products are removed.

Reversible Reactions and Closed Systems

A + B C + D A and B will be combining to form C and D, while at the same time C and D will be combining to form A + B.

Reversible Reactions and Closed Systems A + B C + D A point will eventually be reached when the rate of the forward reaction is equal to the rate of the backward reaction.

Reversible Reactions and Closed Systems A + B C + D This point is know as the DYNAMIC EQUILIBRIUM

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is ENDOTHERMIC and heat energy is taken in ……..

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is ENDOTHERMIC and heat energy is taken in …….. Then if the temperature is increased.

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is ENDOTHERMIC and heat energy is taken in …….. Then if the temperature is increased. The yield of products is increased, and more heat is taken in.

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is ENDOTHERMIC and heat energy is taken in …….. Then if the temperature is decreased

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is ENDOTHERMIC and heat energy is taken in …….. Then if the temperature is decreased. The yield of products is decreased, and less heat is taken in.

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is EXOTHERMIC and heat energy is given out……..

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is EXOTHERMIC and heat energy is given out…….. Then if the temperature is increased.

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is EXOTHERMIC and heat energy is given out…….. Then if the temperature is increased. The yield of products is decreased, and less heat is given out.

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is EXOTHERMIC and heat energy is given out…….. Then if the temperature is decreased.

Reversible Reactions and Closed Systems Temperature and equilibria A + B C + D If the forward reaction (i.e. going from left to right) is EXOTHERMIC and heat energy is given out…….. Then if the temperature is decreased. The yield of products is increased, and more heat is given out.

Reversible Reactions and Closed Systems What about gases and equilibria?

Reversible Reactions and Closed Systems Pressure and equilibria

Reversible Reactions and Closed Systems Pressure and equilibria

Reversible Reactions and Closed Systems Pressure and equilibria

Reversible Reactions and Closed Systems Pressure and equilibriaIncreasing the pressure will push the reaction in the direction of fewer molecules

Reversible Reactions and Closed Systems Pressure and equilibriaIncreasing the pressure will push the reaction in the direction of fewer molecules Increased pressure

Reversible Reactions and Closed Systems Pressure and equilibriaIncreasing the pressure will push the reaction in the direction of fewer molecules Decreased pressure

End of Section 4 Lesson 4 In this lesson we have covered: Reversible reactions Reversible reactions in closed systems