Worksheet

Name: ______

Class : 11 ______

Day/date: ______

MADANIA (Highschool)

Grade 11ChemistryWorksheet 3.1

THERMOCHEMICAL EQUATION

22 / 9 / 2010

An equation that includes the amount of heat produced or absorbed by a reaction is a thermochemical equation. As with other equation, charge and mass must be balance. However, they must also include the enthalpy change.

In writing a thermochemical equation, the following points should be remembered.

  • A positive or negative sign must be included with the H value.
  • Since enthalpy is measured in the units kJ mol-1, the coefficients in the equation represent the amount of each substance, in moles, which react. Thus, the reaction:

2H2(g) + O2(g)  2H2O(l) H= -572 kJmol-1

Can be read as, when 2 moles of hydrogen react with 1 mole of oxygen, 2 moles of water form and 572 kJ of energy is produced.

  • The physical state of matter must be shown, since the change of state require energy changes:

2H2(g) + O2(g)  2H2O(l) H= -572 kJmol-1

2H2(g) + O2(g)  2H2O(g) H= -484 kJmol-1

In one case the product is liquid, in the other case the product is gas. Thus the condensation of 2 moles of water vapor to 2 moles of liquid water at 250C produces 88 kJ of energy.

  • If the coefficients are doubled, the H value must be doubled.

2H2(g) + O2(g)  2H2O(l) H= -572 kJmol-1

4H2(g) + 2O2(g)  4H2O(l) H= -1144 kJmol-1

The amount of energy produced by a chemical reaction is directly proportional to the amount of the substance initially present. If, as in the above example, twice as much reactant is used, then twice as much energy can be produced.

  • If the reaction is reversed, the H value is equal to, but opposite in sign, to that of the forward reaction:

2H2O(l)  2H2(g) + O2(g) H= +572 kJmol-1

Sample problem 1 / The air pollutant sulfur trioxide reacts with water in the atmosphere to produce sulphuric acid according the equation:
SO3 (g) + H2O (l)  H2SO4 (aq) H= -129.6 kJ
Calculate the heat change, in kJ, when 0.5 Kg SO3 (g) reacts with water! (Ar of: S = 32, O = 16)
Answer /
  • Step 1: describe the thermochemical equation given.
From the equation, it is known that a reaction between 1 mole of SO3 and 1 mole of H2O will release 129.6 kJ of heat. We need to find the heat that will be released if the amount of SO3 used in a reaction is 0.5 Kg.
  • Find the mole of SO3
Mole of SO3 / = / Mass of SO3 / = / 500 g / = / 6.25 mol
Mr of SO3 / 80 g/mol
  • Calculate the amount of heat release from a reaction between 6.25 mole of SO3 and water.
H = -129.6 kJ x 6.25 mole = -810 kJ/mole
  • So, a reaction between 0.5 kg SO3 and water release 810 kJ/mole of heat.

Answer these questions.

  1. Referring to the reaction below, how much heat is released if 11.7 g of potassium metal reacts? (Ar K = 39)

2 K(s) + 2 H2O(l) H2(g) + 2 KOH(aq) ΔH = -160 kJ

  1. How much heat is transferred when 5 g of ammonia (NH3) reacts with excess oxygen according to the following equation? (Ar: N = 14, H = 1)

4 NH3 (g) + 5 O2 (g)  4 NO (g) + 6 H2O (g) ΔH = -905 kJ

  1. How much heat is absorbed in a decomposition of 67.2 L (at STP) of H2O2?

2 H2O2 (l)  2 H2O (l) + O2 (g) ΔH = +196 kJ

  1. What is the mass of Si needed to produce 328.5 kJ of heat of a reaction below: (Ar Si = 28)

Si (s) + 2 Cl2 (g)  SiCl4 (g) ΔH = -657 kJ

  1. 2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(l) H = -2372 kJ

Calculate the heat released from the combustion of 5.6 L of C2H2 (STP).

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S1/W9/WS-3.1/PAU/Chemistry/2010-2011