Chemistry (14 - 16)

Exothermic & Endothermic

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Energy Level Diagrams The energy changes that occur during a chemical reaction can be represented by energy level diagrams.

Exothermic reactions give out energy to their surroundings, and the reaction vessel may feel warm or hot when touched.

Energy Level Diagrams The energy changes that occur during a chemical reaction can be represented by energy level diagrams.

Exothermic reactions give out energy to their surroundings, and the reaction vessel may feel warm or hot when touched.

Energy Level Diagrams The energy changes that occur during a chemical reaction can be represented by energy level diagrams.

Exothermic reactions give out energy to their surroundings, and the reaction vessel may feel warm or hot when touched.

Examples of exothermic reactions are:

§  Combustion reactions

§  Neutralisation reactions

§  Oxidation reactions

Energy Level Diagrams

Endothermic reactions take in energy from their surroundings, and the reaction vessel may feel cold when touched.

The energy changes that occur during a chemical reaction can be represented by energy level diagrams.

Energy Level Diagrams

Endothermic reactions take in energy from their surroundings, and the reaction vessel may feel cold when touched.

The energy changes that occur during a chemical reaction can be represented by energy level diagrams.

Energy Level Diagrams

Normally the heat given out or taken in is measured when 1 mole of a substance reacts.

Examples of endothermic reactions are:

§  Thermal Decomposition

§  Photosynthesis

§  Electrolysis

Endothermic reactions take in energy from their surroundings, and the reaction vessel may feel cold when touched.

The energy changes that occur during a chemical reaction can be represented by energy level diagrams.

Exothermic Reactions

The energy difference between the reactants and the products represents the energy given out by a reaction. This energy value is known as rH.

Exothermic Reactions

In exothermic reactions, rH is negative.

In exothermic reactions the products are at a lower energy level than the reactants.

The initial increase in the energy level (activation energy) represents the energy required to break the bonds in the reactants and start the reaction off.

Example When 1 mole of of carbon (12g) burns, 394kJ of heat is given out.

Endothermic Reactions In endothermic reactions, rH is positive.

In endothermic reactions the products are at a higher energy level than the reactants.

The difference in the energy levels of the reactants and the highest value in the energy level diagram is the activation energy. Again, the activation energy represents the energy required to break the bonds in the reactants and complete the reaction.

Making Quicklime - An Endothermic Reaction Quicklime can be made in the laboratory by heating calcium carbonate...

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Making Quicklime in the Laboratory

limestone + quicklime carbon dioxide CaCO3 CaO CO2

In this experiment, the limestone is converted to quicklime plus carbon dioxide...

⇒

If the heating is stopped then the yellow glow disappears indicating that the reaction has stopped.

The continued input of heat implies that the products (CaO and CO2) are at a higher energy level than the reactant CaCO3.

Overall, more energy is required to break the chemical bonds in the reactant(s) than is released by making new chemical bonds in the products.

Endothermic Reactions

Example When 1 mole of ammonia decomposes to give nitrogen and hydrogen, 93kJ of heat is taken in from the surroundings.

⇔

Reversible Reactions If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. An example relates to the hydration and dehydration of copper sulfate.

copper sulfate hydrated (blue)

The amount of energy transferred in both directions of the reaction is the same. Hydrated copper sulfate is crystalline and has water of crystallisation - CuSO4 (5H2O) Androus copper sulfate is a white powder - CuSO4

endothermic ⇒

exothermic ⇐

copper sulfate + water anhydrous (white)

Reversible Reactions

If the temperature is increased for an endothermic reaction, the yield for the reaction is increased, e.g.

copper sulfate ⇒ copper sulfate + water hydrated (blue) anhydrous (white)

endothermic

Temperature increase

More of the product is produced

When a reversible reaction takes place in a closed system, equilibrium is reached once the reactions take place at exactly the same rate in each direction. At equilibrium the relative amounts of products and reactants is dependant upon the conditions, e.g.

Reversible Reactions

If the temperature is decreased for an exothermic reaction, the yield for the reaction is increased, e.g.

copper sulfate ⇐ copper sulfate + water hydrated (blue) anhydrous (white) exothermic

Temperature decreased

Less of the product is produced

When a reversible reaction takes place in a closed system, equilibrium is reached once the reactions take place at exactly the same rate in each direction. At equilibrium the relative amounts of products and reactants is dependant upon the conditions, e.g.

Reversible Reactions

If the pressure is increased for a gaseous reaction, such as the Haber Process, the yield for the reaction which produces the least number of molecules is increased, e.g.

nitrogen + hydrogen ⇔ ammonia

N2(g) + 3H2(g) ⇔ 2NH3(g)

molecules = molecules = 4 2 If the pressure is increased, this reaction moves towards the production of ammonia.

When a reversible reaction takes place in a closed system, equilibrium is reached once the reactions take place at exactly the same rate in each direction. At equilibrium the relative amounts of products and reactants is dependant upon the conditions, e.g.

Reversible Reactions

If the pressure is decreased for a gaseous reaction, such as the Haber Process, the yield for the reaction which produces the highest number of molecules is increased, e.g.

nitrogen + hydrogen ⇔ ammonia

N2(g) + 3H2(g) ⇔ 2NH3(g)

molecules = molecules = 4 2 If the pressure is decreased this reaction moves towards the production of nitrogen and hydrogen.

When a reversible reaction takes place in a closed system, equilibrium is reached once the reactions take place at exactly the same rate in each direction. At equilibrium the relative amounts of products and reactants is dependant upon the conditions, e.g.

Sustainable Development In industrial processes such as the Haber Process, considerations about equilibrium play a vital role in determining the optimum conditions under which the processes are carried out.

It is of vital importance for the future of the planet, as well as for economic reasons, that industrial processes are developed in a sustainable manner. This means minimising the energy requirements of industrial processes.

Industrial processes have to be designed with non-vigorous conditions, which means less energy is used and less energy is released into the environment.

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