introduction

All matter posses energy in one form or another. Energy can neither be created nor destroyed but only transformed from one form to another. Some substances have energy due to movement of the particles and this is referred to as kinetic energy. The energy that a substance possesses due to its position or due to the arrangement of its component parts is the potential energy. Due to the fact that energy can be present in many forms makes it difficult to determine the magnitude of the total energy possessed by a substance, it is easier to determine the energy changes which take place when the substance interacts with other substances.


Energy change
Energy change in a reaction is the energy given out or taken in by a substance during a reaction. Energy changes can be recognized in many different forms such as:

  • Light e.g. when magnesium burns in air
  • Sound e.g. explosion of hydrogen and oxygen
  • Electricity e.g. in cell reaction (electrochemical cells)
  • Heat e.g. burning of fuel like charcoal and wood.
    In some cases a reaction may involve a mixture of several energy forms, for example the reactions of potassium with water.

In thermo chemistry, we mainly deal with heat change (enthalpy change) as it is the most common form of energy change.
Heat (enthalpy) change
Many reactions are accompanied by marked heat changes. There are basically two types of heat changes or enthalpy changes.

  1. An exothermic reaction
    Is a reaction in which heat is liberated to the surrounding. It is indicated by a rise in temperature. In such reactions, the products have less energy content than the reactants.
    Energy profile for exothermic reactions
image 407

Reaction path

  1. An endothermic reaction
    Is a reaction in which heat is absorbed from the surrounding. In this reaction, the products are richer in energy than the reactants.
    Energy profile for endothermic reactions
image 409

Units for heat changes: KJ/mol (KJmol-1)


Heat of reaction and ΔH notation
This is the heat change which takes place when substances react to form one mole of the product.
The heat content of a reacting system is denoted by H and we normally consider the changes in heat content-delta H, (ΔH) since it is not easy to precisely determine H.

The heat content of a reacting system is denoted by H and we normally consider the changes in heat content-delta H, (ΔH) since it is not easy to precisely determine H.
ΔH=H(product)-H(reactants)
If heat is lost by the reacting system (in exothermic reaction), the ΔH is negative. For example;

image 410

This means that when 1 mole of carbon reacts with 2 moles of sulphur, it forms I mole of carbon disulphide with absorption of 106 KJ of energy. (106 KJ are absorbed when 12g of carbon combines with 64g of sulphur).


Sources of heat changes
The source of heat change in a chemical reaction is the potential energy which substances posses. Bonds (electrovalent, covalent e.t.c.) between atoms or ions represent a form of stored potential energy and a source of heat changes in chemical reactions. It is always necessary to provide energy to break bonds in chemical compounds and in the same way, energy is evolved when a new bond is created from the constituent atoms.
Bond breaking requires supply of heat from the surrounding (endothermic process) while bond formation liberates heat to the surrounding (exothermic process).


Types of heat (enthalpy) changes
Enthalpy of combustion
This is the heat change when one mole of a substance is completely burnt in oxygen. All combustion reactions are exothermic hence heat is always evolved and ΔH value is always negative. For example;

image 411

Expt. To determine the heat of combustion of ethanol
An ethanol lamp with a wick inserted in it is used.
Set up of apparatus

image 412

Procedure

  • A known mass of ethanol is put in the ethanol lamp and the mass of ethanol lamp and ethanol is weighed, M1 g.
  • A known mass of water is Mw is put is a thin metal container.
  • The initial temperature of the water, T1 is recorded.
  • Light the wick and stir the water constantly until when the temperature of the water rises by about 30˚C. Extinguish the flame and record the temperature T2 of the water.
  • Reweigh the ethanol lamp and its content and record the mass M2 g.
    In this experiment,
  1. A wire gauge is not used as it would absorb some of the heat.
  2. The shield reduces heat loss to the surrounding.
  3. It is assumed that all the heat produced by the burning ethanol is absorbed by the water. This is actually not true as some of the heat is usually lost to the surrounding and the container holding the water. The values obtained are usually lower than the actual values.
    Treatment
    Initial mass of ethanol lamp and ethanol before burning =M1 g
    Mass of ethanol lamp and ethanol after burning =M2 g
    Mass of ethanol burnt =(M1-M2)g
    Mass of water = Mw g
    Temperature rise of water = (T2-T1)
    Specific Heat Capacity of water (Cw) = 4.2J/g/K
image 413

Ethanol is an example of fuel. The following are qualities of a good fuel.

  1. It must produce a lot of heat.
  2. It must be cheap and must burn easily.
  3. It should not burn with much smoke (pollutant gas)
  4. Is should be easily transported with little or no fire risks.
  5. It should be easily safely stored.
  6. Example
    Use the following specimen results to calculate the heat oif combustion of ethanol.
    Initial mass of ethanol lamp and ethanol before burning =29.974 g
    Mass of ethanol lamp and ethanol after burning =29.592 g
    Initial temperature of water = 17.7˚C
    Final temperature of water = 41.2˚C
    Volume of water in can= 100 cm3
image 414
image 415

a) Write an equation for the complete combustion of carbon
b) 80 kg of charcoal costs 4000/=. Calculate the cost of charcoal required to produce 16375 KJ.

image 417

Heat of solution
Heat of solution is the heat change which takes place when one mole of a substance is dissolved in a specified amount of solvent. The enthalpy change can either be negative if the reaction is exothermic of positive if it is endothermic.
Expt. To determine the heat of solution of sodium hydroxide


Procedure

  • Place a known volume of water, V cm3 in a plastic beaker.
  • Stir well with a thermometer and record the temperature of the water, T1
  • Carefully add a known mass, M of sodium hydroxide pellets in the water.
  • Stir well until there is no change in temperature; record the final temperature T2 of the solution.
    Treatment of results
    Volume of water used = V cm3
    Mass of sodium hydroxide = M g
    Initial temperature = T1
    Final temperature = T2
    (Given, density of water=1g/cm3; specific heat capacity of solution (Cs)=4.2J/g/˚C)
    Mass of water= density x volume, =1xV =Vg
    Mass of solution = (M+V)g
    Temperature change =(T2-T1)
    Heat liberated by solution =mass of solution x Cs x Δtemperature
    =(M+V) x Cs x (T2-T1) J
    M g of sodium hydroxide liberates (M+V) x Cs x (T2-T1) J
image 418

Example

  1. When 10g of ammonium chloride was dissolved in 100 cm3 of water, the temperature changed from 21˚C to 19˚C. Determine the molar heat of solution of ammonium chloride (SHC of solution=4.2J/g/˚C; density of water= 1 g/cm3; N=14, H=1, Cl=35.5)
image 419
image 420
image 421

The reaction is endothermic as it was accompanied by heat absorption and rise in the temperature of the product.


Enthalpy of neutralization
This is the heat change that takes place when an acid reacts with a base to produce one mole of water. The heat change is as a result of the reaction of hydrogen ions of the acid and hydroxyl ions of the base.

image 422

For strong acid and strong alkalis, the heat of neutralization is almost constant. This is because, they are fully ionized in their aqueous solutions and the solution consists of only ions, therefore when the solutions are mixed, there is no bond breaking as the ions are already existing freely. E.g. a solution of hydrochloric acid consists of only H+ and Cl- ions and sodium hydroxide solution consists of only Na+ and OH- ions, when the two solutions are mixed, there is no bond breaking as the ions are already separated.
Weak acids and weak bases are not fully ionized and their heats of neutralization are not constant. This is because some of the energy is first used in the dissociation of the molecules of the weak acid/base before neutralization can take place. Hence the value is usually less than that of strong acids and bases.
Expt. To determine heat of neutralization of an acid and a base

Set up

image 423

Procedure

  • Put a known volume of acid VA of known concentration MA in a plastic beaker
  • Stir and record the temperature TA of the acid.
  • Put in another plastic container a known volume VB of the alkali of known concentration MB.
  • Stir and record the temperature TB of the alkali
  • Quickly mix the two solutions, stir well using a thermometer and record the maximum temperature T2 of the mixture.

Treatment of results

image 424
image 425
image 426

Enthalpy of displacement
Elements that are more reactive displace those that are less reactive than them during chemical reactions. For example zinc displaces copper from its compound say copper(II)sulphate. During such reactions, heat is normally liberated.

image 427

Expt. To determine the heat of displacement of reaction between copper (II) sulphate solution and zinc
Procedure

  • Put a known volume of copper(II)sulphate solution V1 of known concentration M1 in a plastic beaker.
  • Stir and record the temperature T1 of the solution.
  • Add a known mass of zinc powder to the copper (II) sulphate solution, stir the mixture and record the maximum temperature of the mixture, T2.
    Treatment of results
    Volume of copper (II) sulphate solution = V1
    Concentration of copper (II) sulphate solution = M1
    Initial temperature =T1
    Final temperature = T2
    Temperature change = T2-T1
    Mass of solution= V1 x density of solution. Assuming density=1g/cm3
image 428
image 429
image 430
image 431

Heat changes during physical processes
Melting

The particles in a solid are packed close to each other and do not move from one place to another. When a solid is heated, the particles gain more kinetic energy and vibrate more vigorously. Eventually they are able to leave their positions in the structure and move past one another; they flow as a liquid. At this point, the melting point of the solid is reached. The heat supplied to melt the solid causes negligible rise in temperature and therefore referred to as ‗latent‘ heat (literally meaning hidden heat). This is because, the heat supplied at first simply weakens the strong intermolecular forces of attraction holding the particles together. Melting is an endothermic process as it involves heat absorption.

The enthalpy of melting or enthalpy of fusion of a substance is the enthalpy change when one mole of a substance changes from solid to liquid state (i.e. when it melts)
The enthalpy of freezing of a substance is the enthalpy change per mole when the substance changes from liquid to solid state (i.e. when is freezes)
These changes do not take place under standard conditions. Enthalpy changes of freezing and melting are quoted at 1 atmosphere and at the temperature at which the change of state takes place. The enthalpy of freezing is the negative of the enthalpy of melting.
ΔHfreezing = -ΔHmelting

Vaporization
The particles in a liquid are in motion and the forces of attraction keep them in the liquid phase. When a liquid is heated, the average kinetic energy of the molecules increases. Some molecules with energy above average, are able to break away from the attraction of other molecules and enter the vapor phase. Vaporization is endothermic.


The enthalpy of vaporization is the enthalpy change that takes place when one mole of substance changes state from liquid to vapor.
ΔHvaporisation = Enthalpy of vapor – Enthalpy of liquid


Condensation
This is the reverse of vaporization and it is exothermic.
ΔHcondensation= -ΔHvaporisation

image 432
image 433
image 434