chemistry calculations c.pdf

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IGCSE Chemistry

Chemistry Calculations C

25 Energy Calculation

To break something, obviously you need energy; hence breaking of bonds is

endothermic

When substances form bonds, it's usually to become more stable, and thus

they release energy; hence making bonds is exothermic

Bond Energy

Bond energy is the energy required to break a particular bond. Some bond energies

are given in the table below:

Bond Bond Energy (KJmol-1)

C-H +413

C-Cl +346

C-I +234

Cl-Cl +243

I-I +151

H-Cl +432

H-I +298

H-H +436

F-F +158

H-F +568

C-C +346

C=C +611

The same amounts of energy is released when each of the above bonds are formed.

When writing energy released when bonds form, we use a negative sign (-) in front

of the value to show that bond making is exothermic.

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How to find the enthalpy change of a reaction using bond energy

H = Energy required to break bonds + Energy released when bonds are made

Q Calculate the enthalpy changes for the following reactions and state whether

they are endothermic reactions or exothermic reactions.

a) CH4 (g) + Cl2 (g) CH3Cl(g) + HCl (g)

b) H2 (g) + F2 (g) 2HF (g)

c) C2H4 (g) + HI (g) C2H5I

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Q Draw fully labeled energy profile diagrams for the above 3 reactions

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Measuring energy changes involving solutions

1 Write the formula which is used for calculating the total amount of heat

absorbed or given out.

2 What is Specific Heat Capacity and state its units?

3 What is Specific Heat Capacity of water?

4 Write the formula which is used for calculating the enthalpy change of a reaction

5 Suggest a method that can be used to find the heat of a reaction

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6 Describe an experiment to estimate the heat energy transferred during the

combustion of ethanol.

7 Calculate the enthalpy of combustion of ethanol in KJmol-1 using below data:

Initial temperature of water : 17.7C

Final temperature of water : 41.2C

Mass of water in the can : 101 g

Mass of lamp + ethanol before burning : 29.97 g

Mass of lamp + ethanol after burning : 29.59 g

Specific Heat capacity of water : 4.18 J g-1 c-1

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8 The accepted value for combustion of ethanol is 1370 KJmol-1 (to 3 s.f.) Give

four reasons as to why this accepted value is different from the value you

calculated in the previous question.

9 Describe an experiment to estimate the heat energy transferred when

magnesium reacts with dilute sulphuric acid.

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10 Calculate the enthalpy of the above reaction in KJmol-1 using below data:

Initial temperature of water : 17.4C

Final temperature of water : 27.5C

Mass of weighing bottle + Magnesium : 10.810 g

Mass of weighing bottle after adding Mg : 10.687 g

Specific Heat capacity of water : 4.18 J g-1 c-1

11 State 2 assumptions you have made during the above calculation.

chemistry calculations c.pdf

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