Enthalpy ChangesChapter 13

LEARNING OUTCOMES

Distinguish between exothermic and endothermic reactions in terms of energy content of products and reactants

Draw an energy profile diagram to illustrate endothermic and exothermic change

Calculate energy changes from experiments or experimental data

Exothermic reactions When a piece of magnesium ribbon burns

in air or oxygen, heat and light energy are produced.

This type of chemical reaction in which heat energy is produced and given out to the surroundings is called an exothermic change or reaction.

An exothermic reaction is a reaction in which heat energy is given out to the surroundings.

Enthalpy ChangesChapter 13

Exothermic reactions During an exothermic reaction,

the temperature of the surroundings rises since heat energy is given out by the reaction.

Enthalpy ChangesChapter 13

Other exothermic reactions Freezing and condensation (physical changes) Combustion of fuels Neutralisation of acids and alkalis Dissolving solid sodium hydroxide, anhydrous sodium

carbonate and anhydrous copper(II) sulphate in water Adding water to concentrated sulphuric acid

Enthalpy ChangesChapter 13

Endothermic reactions

An endothermic reaction is a reaction in which heat energy is absorbed from the surroundings.

In an endothermic reaction, the temperature of the surroundings drops, since heat energy is absorbed.

Enthalpy ChangesChapter 13

When ammonium chloride crystals are dissolved in water, there is a drop in temperature. This shows that heat energy is absorbed from the surroundings.

Other Endothermic reactions Melting and boiling (physical changes) Thermal decomposition of compounds such as calcium

carbonate (limestone) and copper(II) carbonate Photosynthesis Taking a photograph with film (decomposition of

silver bromide into silver and bromine) Dissolving ionic crystals in water, e.g.

dissolving ammonium nitrate, ammonium chloride and sodium carbonate crystals in water

Enthalpy ChangesChapter 13

Heat of Reaction The amount of heat energy given out or taken in during a chemical reaction is

called the heat of reaction or enthalpy change. Enthalpy change is represented by the symbol, ∆H. Matter contains both potential and kinetic energies. During an exothermic reaction, heat

is given out by the reactants to the surroundings. Thus, enthalpy change is negative. During an endothermic reaction, heat energy is absorbed by from the surroundings.

Reactants gain energy, so enthalpy change is positive.

Enthalpy ChangesChapter 13

Heat of Reaction In an exothermic reaction, the

reactants are at a higher energy level than that of the products.

The excess heat energy is given out by the reactants when the products are formed.

Since the reactants have lost heat energy, we take ∆H as negative.

Exothermic reaction

Enthalpy ChangesChapter 13

Heat of Reaction In an endothermic reaction,

the reactants are at a lower energy level than that of the products.

Heat energy is absorbed by the reactants from the surroundings in order to change to the products. Endothermic reaction

Since the reactants have gained heat energy, we take ∆H as positive.

Enthalpy ChangesChapter 13

Heat of Reaction

The negative value of ∆H means that the reaction is exothermic. “890 kJ” means that when 1 mole of methane burns completely

in oxygen, 890 kJ of heat energy is given out.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) [∆H = - 890 kJ]

The heat of reaction can be included in a chemical equation as:

Enthalpy ChangesChapter 13

Heat of Reaction

The positive value of ∆H means that the reaction is endothermic.

“222 kJ” means that when 1 mole of calcium carbonate is decomposed, 222 kJ of heat energy is absorbed.

CaCOCaCO33 CaO + CO CaO + CO22 [∆ H = + 222 kJ] [∆ H = + 222 kJ]

Enthalpy ChangesChapter 13

Bond breaking and bond making The reason why reactions are endothermic or exothermic is

because energy is taken in or given out when chemical bonds are broken or made.

Heat energy is absorbed to break bonds, so bond breaking is endothermic.

Enthalpy ChangesChapter 13

Bond breaking and bond making Heat energy is given out (released) when bonds

are formed, so bond forming is exothermic.

Enthalpy ChangesChapter 13

Bond breaking and bond making So, the overall heat change in a reaction is given by:

If more energy is absorbed for breaking bonds than released in forming bonds, the reaction will be endothermic.

If more energy is released in forming bonds than absorbed for breaking bonds, the reaction will be exothermic.

Heat of reaction, ∆H = Heat absorbed Heat given out in bond breaking in bond forming

Enthalpy ChangesChapter 13

Worked example

H+ H

OH H O

H HO

H H O

Since ∆H is negative, the overall reaction is exothermic.

Hydrogen burns in oxygen to form water according to the equation:

2H2(g) + O2(g) 2H2O (g)

By calculating ∆H, state whether the reaction will be endothermic or exothermic, given the following bond energies per mole:H-H = 436 kJ, O=O = 496 kJ, O-H = 463 kJ.

Solution:

Energy absorbed to break bonds = (2 x 436) + (1 x 496) = 1368 kJEnergy released to form bonds = (4 x 463) = 1852 kJ

Hence, ∆ H = Energy absorbed – Energy released = (1368 – 1852) kJ = - 484 kJ

Enthalpy ChangesChapter 13

Summary

Exothermic reaction Endothermic reaction

Heat energy is given out to the surroundings.

Heat energy is absorbed from the surroundings.

∆H is negative. ∆H is positive.

More heat is given out in making bonds than absorbed in breaking bonds.

More heat is absorbed in breaking bonds than given out in making bonds.

Enthalpy ChangesChapter 13

Quick check1. What is meant by (a) an exothermic reaction, (b) endothermic reaction?2. What is meant by enthalpy change? What symbol is used to represent enthalpy

change?3. What type of reaction is it when the enthalpy change ∆H is

(a) positive, (b) negative ?4. Sodium hydroxide reacts with nitric acid according to the equation:

NaOH(aq) + HNO3(aq) NaNO3(aq) + H2O(l) ∆H = - 57.3kJ(a) Is this reaction endothermic or exothermic? Explain your answer.(b) What would be the heat evolved if 2 moles of sodium hydroxide are completely neutralised ?

5. State whether the following are exothermic or endothermic reactions: (a) decomposing zinc carbonate, (b) reacting potassium with water, (c) forming hydrogen atoms from hydrogen molecules, (d) forming chlorine molecules from chlorine atoms, (e) respiration, (f) photosynthesis. Solution

Enthalpy ChangesChapter 13

Solution to Quick check1. (a) An exothermic reaction is a reaction which gives out heat energy to the

surroundings.(b) An endothermic reaction is a reaction which absorbs heat energy from the surroundings.

2. Enthalpy change is the amount of heat given out or taken in during a reaction. The symbol used is ∆H.

3. (a) Endothermic (b) Exothermic4. (a) The reaction is exothermic because ∆H is negative which means that heat

energy is lost from the reactants.(b) Amount of heat evolved = 2 x 57.3 = 114.6 kJ

5. (a) decomposing zinc carbonate: endothermic(b) reacting potassium with water: exothermic(c) forming hydrogen atoms: endothermic (d) forming chlorine molecules: exothermic(e) respiration: exothermic(f) photosynthesis: endothermic

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Enthalpy ChangesChapter 13

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Enthalpy ChangesChapter 13