Energy Changes in Chemical Reaction1. There is energy changes when a chemical reaction takes place.

2. The energy is in the form of heat, could be released or absorbed during the chemical reaction.

3. There are 2 types of reaction with change in heat energy:

a. a. Exothermic reaction

b. b. Endothermic reaction

4. The chart below shows the difference between this 2 reactions.

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Exothermic Reaction1. Exothermic reaction is the chemical reaction that releases heat to the surrounding.

2. When energy is given off during a chemical reaction, the temperature of the surrounding will increase.

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Example of Exothermic Reaction

Reaction Equation

Neutralization Diluted sulphuric acid is neutralized by sodium hydroxide aqueous 

H2SO4 + 2NaOH → Na2SO4 + 2H2O

Acid + carbonate Reaction between hydrochloric acid and sodium carbonate 

2HCl + Na2CO3 → 2NaCl + CO2 + H2O

Calcium oxide + waterCaO + H2O → Ca(OH)2

Water + dehydrated copper(II) sulphate CuSO4 + xH2O → CuSO4•xH2O

Dissolve NaOH and KOH in water NaOH + water → Na+ + OH-

KOH + water → K+ + OH-

Reaction between alkali metal (Li/Na/K)and water

Sodium + Water

2Na + 2H2O → 2NaOH + H2

Reaction between reactive metal(Mg/Al/Zn/Fe) and diluted acid

Magnesium + Nitric acid

Mg + 2HNO3 → Mg(NO3)2 + H2

Combustion Combustion of methane

CH4 + 2O2 → CO2 + 2H2O

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Others  Rusting Dilution of concentrated sulphuric acid/ concentrated nitric acid or others acid in water. Crystallization of sodium tiosulphate. Change of state from gaseous state to liquid state and liquid state to solid state.

Energy Level Diagram1. When a chemical reaction occurs, certain amount of heat is given off or absorb.

2. The energy change in the chemical reaction can be presented by an energy level diagram.

3. An energy level diagram it is a graph that shows the energy change in a chemical reaction.

Exothermic Reaction1. Figure below shows the general energy level diagram for exothermic reaction.

2. We can see that the energy decreases after reaction. This is because energy is given off during an exothermic reaction

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Example

Zn + 2HCl →  ZnCl2 + H2

∆H =   126 kJ mol-1

The energy level diagram of the reaction above is as below:

Relationship between Energy Change and Breaking/Formation of BondsBreaking and Formation of Chemical Bond1. During a reaction, energy must be supplied to break bonds in the reactants, and energy is give out when the bonds in the products form.

2. The amount of energy that absorbed or released depends on the strength of the bond.

3. If the amount of energy released during the process of creating bond is higher than the amount of energy that taken in during process of breaking bond, the reaction is an exothermic reaction.

4. If the amount of energy been absorbed to break the bond is higher than the amount of energy been released during the formation of chemical bond, the reaction is an endothermic reaction.

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Heat of Reaction1. The Heat of Reaction is the heat absorbed in a reaction at standard state condition between the numbers of moles of reactants shown in the equation for the reaction. 

2. The Heat of reaction is represented by the symbol ∆H. 

3. The unit of ∆H is kJmol-1. 

4. If the reaction is exothernic, ∆H shows a value of negative. If the reaction is endothernic, ∆H shows a value of positive.

5. The 4 heat of reaction that you need to know in the syllabus

a. Heat of PrecipitationThe heat of precipitation of a substance is the heat change when 1 mole of precipitate formed from its solution of ion under standard state condition.

b. Heat of DisplacementThe Heat of Displacement of an element is the heat change when 1 mole of the element was displaced from its compound under standard state condition.

c. Heat of NeutralisationHeat of neutralisation is the heat change when 1 mol of water is formed by the neutralisation of hydrogen ions by hydroxide ions with measurements made under standard conditions.

d. Heat of CombustionThe Heat of Combustion of a substance is the heat energy evolved when 1 mole of the substance is completely burnt in oxygen.

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Calculating Heat ChangeHeat change in a chemical reaction is directly proportional to the number of mole of reactant that take part in a reaction or number of mole of product been produced.

Example 1:N(g) + ½ O2(g) → NO2(g) ∆H = +66 kj mol-1

Calculate the heat change when 0.1 mole of nitrogen dioxide is formed in the reaction that shown above.

Answer:Heat change = 0.1 x 66kJ = 6.6 kJ

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Example 2:C(s) + O2(g) → CO2 (g)   ∆H=  393.5kJ mol-1

How much energy is released when 4g of carbon combust completely in excess oxygen. (Relative atomic mass of carbon = 12 )

Answer:Number of mole of carbon = 4g12gmol−1=13mol 

Total heat been released =13×393.5kJ=131.2kJ

Example 3:CH3OH(l) +  O2 (g) → CO2 (g) + 2H2O (l)     ∆H =  560 kJ mol-1

Find the mass of alcohol that need to be combusted, in excess of oxygen, to release 140 kJ of heat energy.[Relative atomic mass: H=1; C=12; 0=16] 

Answer:Number of mole of alcohol = 140kJ560kJmol−1=0.25mol 

Relative molecular mass of CH3OH = 12 + 3(1) + 16 + 1 = 32

Mass of alcohol = 0.25mol x 32g/mol = 8g

Example 4:Calculate the heat change when excess zinc powder is added into 50cm³ of copper(II) sulphate solution 0.2 mol dm-3

CuSO4(ak) + Zn(p) → ZnSO4(ak) + Cu(p)∆H =  190 kJ mol-1 

Answer:Number of mole of copper(II) sulphate solution

n=MV1000n=(0.2)(50)1000=0.01mol

Heat change = 0.01 x 190kJ = 1.9kJ

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Example 5:The heat of combustion of carbon to CO2 is -393.5kJ/mol. Calculate the heat released upon formation of 35.2g of CO2 from carbon and oxygen gas.

Answer:Relative molecular mass of carbon dioxide = 12 + 2(16) = 44

Number of mole of carbon dioxide = 35.2/44 = 0.8 mol

C + O2 → CO2

Number of mole of carbon = 0.8 mol

Heat released = 0.8 x 393.5kJ = 314.8 kJ

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