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The following conventions apply when we use Δ H :

  1. Chemists use a thermochemical equation to represent the changes in both matter and energy. In a thermochemical equation, the enthalpy change of a reaction is shown as a Δ H value following the equation for the reaction. This Δ H value indicates the amount of heat associated with the reaction involving the number of moles of reactants and products as shown in the chemical equation . For example, consider this equation:

    H 2 ( g ) + 1 2 O 2 ( g ) H 2 O ( l ) Δ H = −286 kJ

    This equation indicates that when 1 mole of hydrogen gas and 1 2 mole of oxygen gas at some temperature and pressure change to 1 mole of liquid water at the same temperature and pressure, 286 kJ of heat are released to the surroundings. If the coefficients of the chemical equation are multiplied by some factor, the enthalpy change must be multiplied by that same factor (Δ H is an extensive property):

    (two-fold increase in amounts) 2 H 2 ( g ) + O 2 ( g ) 2 H 2 O ( l ) Δ H = 2 × ( −286 kJ ) = −572 kJ ( two-fold decrease in amounts ) 1 2 H 2 ( g ) + 1 4 O 2 ( g ) 1 2 H 2 O ( l ) Δ H = 1 2 × ( −286 kJ ) = −143 kJ
  2. The enthalpy change of a reaction depends on the physical state of the reactants and products of the reaction (whether we have gases, liquids, solids, or aqueous solutions), so these must be shown. For example, when 1 mole of hydrogen gas and 1 2 mole of oxygen gas change to 1 mole of liquid water at the same temperature and pressure, 286 kJ of heat are released. If gaseous water forms, only 242 kJ of heat are released.

    H 2 ( g ) + 1 2 O 2 ( g ) H 2 O ( g ) Δ H = −242 kJ
  3. A negative value of an enthalpy change, Δ H , indicates an exothermic reaction; a positive value of Δ H indicates an endothermic reaction. If the direction of a chemical equation is reversed, the arithmetic sign of its Δ H is changed (a process that is endothermic in one direction is exothermic in the opposite direction).

Measurement of an enthalpy change

When 0.0500 mol of HCl( aq ) reacts with 0.0500 mol of NaOH( aq ) to form 0.0500 mol of NaCl( aq ), 2.9 kJ of heat are produced. What is Δ H , the enthalpy change, per mole of acid reacting, for the acid-base reaction run under the conditions described in [link] ?

HCl ( a q ) + NaOH ( a q ) NaCl ( a q ) + H 2 O ( l )

Solution

For the reaction of 0.0500 mol acid (HCl), q = −2.9 kJ. This ratio −2.9 kJ 0.0500 mol HCl can be used as a conversion factor to find the heat produced when 1 mole of HCl reacts:

Δ H = 1 mol HCl × −2.9 kJ 0.0500 mol HCl = −58 kJ

The enthalpy change when 1 mole of HCl reacts is −58 kJ. Since that is the number of moles in the chemical equation, we write the thermochemical equation as:

HCl ( a q ) + NaOH ( a q ) NaCl ( a q ) + H 2 O ( l ) Δ H = −58 kJ

Check your learning

When 1.34 g Zn( s ) reacts with 60.0 mL of 0.750 M HCl( aq ), 3.14 kJ of heat are produced. Determine the enthalpy change per mole of zinc reacting for the reaction:

Zn ( s ) + 2 HCl ( a q ) ZnCl 2 ( a q ) + H 2 ( g )

Answer:

Δ H = −153 kJ

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Be sure to take both stoichiometry and limiting reactants into account when determining the Δ H for a chemical reaction.

Another example of the measurement of an enthalpy change

A gummy bear contains 2.67 g sucrose, C 12 H 22 O 11 . When it reacts with 7.19 g potassium chlorate, KClO 3 , 43.7 kJ of heat are produced. Determine the enthalpy change for the reaction

C 12 H 22 O 11 ( a q ) + 8 KClO 3 ( a q ) 12 CO 2 ( g ) + 11 H 2 O ( l ) + 8 KCl ( a q ) .

Solution

We have 2.67 g × 1 mol 342.3 g = 0.00780 mol C 12 H 22 O 11 available, and 7.19 g × 1 mol 122.5 g = 0.0587 mol KClO 3 available. Since 0.0587 mol KClO 3 × 1 mol C 12 H 22 O 11 8 mol KClO 3 = 0.00734 mol C 12 H 22 O 11 is needed, C 12 H 22 O 11 is the excess reactant and KClO 3 is the limiting reactant.

The reaction uses 8 mol KClO 3 , and the conversion factor is 43.7 kJ 0.0587 mol KClO 3 , so we have Δ H = 8 mol × −43.7 kJ 0.0587 mol KClO 3 = −5960 kJ . The enthalpy change for this reaction is −5960 kJ, and the thermochemical equation is:

C 12 H 22 O 11 + 8 KClO 3 12 CO 2 + 11 H 2 O + 8 KCl Δ H = −5960 kJ

Check your learning

When 1.42 g of iron reacts with 1.80 g of chlorine, 3.22 g of FeCl 2 ( s ) and 8.60 kJ of heat is produced. What is the enthalpy change for the reaction when 1 mole of FeCl 2 ( s ) is produced?

Answer:

Δ H = −338 kJ

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Questions & Answers

what's kinetic energy
Charity Reply
what is anion?
Hussniz Reply
an anion is a positively charge ion
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Thnkz
Hussniz
an anion is a negatively charged ion
Johnson
And anion is a negatively charged ion which move towards the anode which is positively charged.
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what is matter
Henry Reply
what is pressure
Henry
what is water?
Scarcies Reply
hydrogen gas burned into oxygen gas which forms a polar molecule, has a defenate shape volume but not shape. at 20 degrees Celsius exists as an aqueous solution. Is a solvent that dissolves many solutes into solution. And can act as an acid or a base.
David
HOW CAN ETHANOL BE PREPARED FORM CASSAVA.... (WITH EQUATION)
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by mixing
Henry
it is less dense than water
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mowete
no
PRINCESS
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What is happening
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As it cools, olive oil slowly soldifies and froms a solid over a range of temperatures. Which best describes the solid?
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definition of a sample
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a small part or quantity intended to show what the whole is like
Abdull
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Katr Reply
kilojoules or Kelvin
Annie
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divinegift Reply
atom is anything that has weight and can occupy space
Neutons
atom is the smallest particle of an element that can neitherbe divided nor destroyed
clin
atom is the smallest indivisible particle of an element.
Igbayima
atom is the smallest particle of an element that can neither be created,subdivided or destroyed
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All wrong
Gift
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Go and read 📘 up Dalton atomic theory and who disproved.. Read 📘 up atomic theory
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an atom is the smallest particle of an element that can take part in a chemical reaction
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good@anyebem
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Hussniz
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Deren Reply
WATS atomic structure?
mowete
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is the force of attraction that exist between two or more molecules
Johnson
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a known solution
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Characteristic of a primary standard solution
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pauli's exclusion is based on what?
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quantum theory
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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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