Types of Reaction
Chemical reactions can generally be grouped as follows:
Combination reaction: This is a reaction in which two or more substances combine to form a single compound. E.g.
Fe(s) + S(s) → FeS(s)
PbO2(s) + SO2(g) → PbSO4(s)
Polymerization of ethyne is also an example of a combination reaction.
Decomposition reaction: this occurs when a single compound splits to two or more substances. When
heat is applied to bring about the splitting, the reaction is called thermal decomposition. E.g. 2Pb (NO3)2(s)heat → 2PbO(s) + 4NO2(g) + O2(g)
Displacement reaction: This is a reaction in which one element or radical replaces another element or radical in a compound. More electropositive metals displace the less electropositive metals from their salts while the more electronegative non-metals displace the less electronegative ones from their salts. E.g. Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l)
Double decomposition: this is a reaction in which the reactants decompose to form new substances by
exchange of radicals. The two reactants must be soluble, while only one of the products is soluble. The
reaction is used to prepare an insoluble or volatile product, since it can be separated from the other substances in the reaction system. E.g.
AgNO3 (aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Precipitate
KNO3(aq) + H2SO4(aq) → HNO3(g) + KHSO4(aq)
Volatile
Catalytic reaction: This is any type of chemical reaction that uses catalyst. A catalyst is a substance that
alters the rate of chemical reactions.
Reversible reaction: this is a chemical reaction that takes place both in forward and backward direction.
The condition for the forward reaction may be (or may not be) the same as the condition for the backward reaction. E.g.
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
Thermal dissociation: in this reaction, each molecule of a substance dissociates into two or more simpler molecules or atoms on the application of heat. Unlike thermal decomposition, thermal dissociation is a reversible process e.g. NH4Cl(l) heat → NH3(g) + HCl(g)
cold
ENERGY CHANGE IN A REACTION
Energy is defined as the ability to do work. It exists in various forms that are interconvertible. Whenever a chemical reaction occurs, an energy change takes place because the reactants and the products possess different amounts of chemical energy. The amount of chemical energy present in a reaction cannot be measured but the energy change accompanying chemical reactions can be measured in form of:
– Heat e.g. when a strong acid and a strong base are mixed
– Light and heat e.g. when magnesium burns in air
– Sound and heat e.g. when a mixture of hydrogen and oxygen is ignited
Of the three only the heat energy can be measured easily.
Heat content and heat of reaction
Every substance possesses a characteristic internal energy which is due to its structure and physical state. The energy is known as heat content or enthalpy. It is represented by H. The energy change that accompanies a chemical reaction is known as the heat of reaction. It is denoted by ΔH.
ΔH = H product – H reactant
EXOTHERMIC AND ENDOTHERMIC REACTIONS
During a chemical reaction, heat can either be evolved or absorbed depending on the relative heat contents of the reactants and the product.
An exothermic reaction is one during which heat is liberated to the surroundings. The total heat content of the product is less than that of the reactants.
An endothermic reaction is the one during which heat is absorbed from the surrounding. The total heat content of the product is more than that of the reactants.
In a chemical reaction, the reactant particles collide with one another. The collision results in bond-breaking and bond formation. The atoms and/or ions of the reactant particles are first broken and then regrouped to form the products. The bond-breaking process requires energy (endothermic) while the bond forming process releases energy (exothermic). The overall energy change in a chemical reaction results from the combination of the bond-breaking and bond forming energy.
For exothermic reaction: bond–breaking energy < bond-forming energy
For endothermic reaction: bond-breaking > energy bond-forming energy
Measurement of heat of reaction
If the heat of reaction is large, we can detect it by feeling the temperature of the reacting vessel before and after the reaction. An exothermic reaction makes the vessel to feel warm or hot while an endothermic reaction makes the vessel to feel cool. However, for an accurate determination of ΔH, the use of a calorimeter (glass calorimeter is mostly used) is adopted. The heat of reaction ΔH, depends on the following factors:
– Mass of the reactants
– Temperature and pressure of the reacting system
– Physical states of the substances involved in the reaction
The standard conditions for determining ΔH are:
– Temperature of 298K (250C)
– Pressure of 1.01 x 105Nm-2 or 1 atm or 760mmHg.
– Concentration of 1 mol/dm3.
The ΔH obtained under these standard conditions is denoted as ΔHѳ.
HEAT OF NEUTRALIZATION
The standard heat of neutralization ΔHnѳ is the amount of heat evolved when one mole of hydrogen ion H+, from an acid reacts with one mole of hydroxide ion OH– from an alkali to form one mole of water under standard condition. For strong acids and strong alkalis, ΔHn ѳ = -57.4KJ/mol but the value is less for weak acid or weak alkalis because of the partial dissociation of the weak acids or bases/alkalis.
HEAT OF COMBUSTION
The standard heat of combustion of a substance, ΔHcѳ is the heat evolved when one mole of a substance burns completely in oxygen under standard condition. It is determined by the use of bomb calorimeter.
HEAT OF SOLUTION
The standard heat of solution ΔHsѳ is the amount of heat absorbed or evolved when one mole of a substance is dissolved in so much water that further dilution results in no detectable heat change. Whenever an ionic substance dissolves in water, two things happen:
– The water molecules split up the ions in the crystals into free ions. This process needs
energy known as lattice energy.
– The free ions are hydrated by water molecules. This process evolves energy known as
hydration energy.
The resultant heat change of the two steps is known as heat of solution. Generally;
If lattice energy < hydration energy, the dissolution is exothermic
If lattice energy > hydration energy, the dissolution is endothermic
Bond energy: This is the average amount of energy associated with making or breaking of one mole of a particular bond in its gaseous state. It is higher in stable compounds than in reactive compounds.
The lattice energy of an ionic crystal is the heat of formation of one mole of ionic compound from widely separated gaseous ions under standard conditions.
The enthalpy change of atomization of an element is the enthalpy change when one mole of gaseous atom is formed from the element. E.g
½ H2(g) H(g); ΔHa = +218KJ
Assignment
- Define and give examples of: A.– Surrounding B.– Open and close system
- Distinguish between a catalyst inhibitor and a catalyst promoter
Tutorial questions
- Define rate of a chemical reaction
- What are the factors that affect the rate of a chemical reaction?
- State five properties of a catalyst
- State the collision theory
- List two factors that affect collision
- Consider the following reaction equation:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
List three ways by which rate of the reaction can be increased
7. Define heat of formation