Chemical Reactions

Molecular Mass

• The molecular mass or formula mass of any compound is the sum of atomic masses of all atoms in one molecule of that compound

• The molecular mass of sodium bromide, NaBr,is 103 which represents the sum of the atomic masses of Na 23 and bromide 80.

Molecular mass ctd

• To find the molecular mass of a compound, add the atomic masses of all the atoms that make up the compound

• For example in the compound H20,the molecular mass is adding the mass of 2 atoms of Hydrogen and one atom of oxygen

• 2 hydrogen atoms ( atomic mass 1) 2 X 1=21 oxygen atom (atomic mass 16) 1 X

16=16 molecular mass=2+16=18

Molecular mass

• Molecular mass of glucose,C6H12O6 can be calculated as follows:

• 6 carbon atoms (atomic mass 12) 12 X 6=72• 12 hydrogen atoms(atomic mass 1) 12 X 1=12• 6 oxygen atoms(atomic mass 16) 6 X 16=96• Molecular mass is therefore 180• Calcium phosphate Ca3(po4)2…MM is 310..• 3 ca atoms(atomic mass 40) 3 X 40 =120• 2 phosphorus atoms(atomic mass 31) 31 X 2=62• 8 oxygen atoms(atomic mass 16) 8 X 16=128• Molecular mass =120 + 62 + 128 =310

Percentage composition

• Can be calculated from the relative atomic masses of the elements present in that compound.

• Ca3po4 whose molecular mass is 310.

• Of this mass,120 is ca,62 is p, and 128 oxygen. Then

• % ca =mass of ca in compound X100

mass of compound

Reaction rates.

• Some chemical reactions proceed slowly e.g. iron rusts while others are rapid and some are instantaneous.

• For a chemical reaction to occur, their molecules must collide(interact) with sufficient energy to overcome any force of repulsion between electron clouds.

• The minimum amount of energy required for a collision to occur is known as the “activation energy”

Chemical reaction

• What determines the speed of a chemical reaction?

1)The nature of the reacting compounds

2)The temperature

3)Concentration of reacting substance

4)Presence of a catalyst

5)Surface area of reacting substance

Nature of reacting substances

• Some reactions occur rapidly e.g. the mixture of Na2SO4 + BaCl2→2NaCl +BaSO4

• As opposed to the reaction between H2 and O2 to form H2O which is usually slow because the bonds between the H atoms has to be broken and also the bonds between the O2 atoms have to be broken

Temperature

• As the temperature rises, the speed of a chemical reaction increases.

• At higher temperature the molecules move faster• So a slight change in temperature can alter the speed of

a reaction • Every 10º rise in temperature doubles the rate of a

reaction• A patient who has fever with only a few degrees has an

increased pulse rate and also an increased respiratory rate

• This signifies an increase in the rate of reactions taking place in the body

Concentration of reactants.

• The concentration of a reactant is the amount present in a given unit of volume.

• Greater concentration produces faster reactions because there are more molecules that can react.

• Patients with respiratory disease can breathe more easily with nasal oxygen catheter because of the increased concentration of oxygen in the lungs, increasing speed of Oxygen uptake, making breathing easier for the patient

Catalyst

• Any substance that increases the speed of a chemical reaction without itself being changed chemically

• E.g. are the enzymes found in the human bodies

• The body uses catalysts to increase the pace of the chemical reactions

• Inhibitors act in contrast to catalysts.

Surface Area

• The surface area of the reacting substances affect the rate of reactions.

• Increased surface area increases the rate of the reaction

• E.g. many medications are given in the form of finely divided solids which increases the surface area and so increases the rate of the absorption of the drugs in the body

Equilibrium reactions

• Often when 2 or more reactants unite to form a number of products, these products re-unite to form the original reactants.

• These reactions are called Reversible reactions.

• May proceed in either direction depending on the conditions that exist.

• For example. formation of NH3 from N2 and H2.

Equilibrium (ctd)

• When rates of formation and decomposition are equal, a chemical equilibrium exists.

• However this does not mean that the reaction has stopped. Only means that the rate of decomposition is equal to rate of formation.

• Therefore the composition of the reaction remains constant.

Equilibrium (ctd)

• Equilibrium can be defined as a dynamic state in which the rate of the forward reaction is equal to the rate of the reverse reaction.

• 2 examples of equilibrium reactions in the body are. formation of H2CO3 and also. Hemoglobin + oxygen to give oxyhemoglobin..

Equilibrium Constant

• Law of mass action: states that the rate of a chemical reaction is proportional to the concentration of the reacting substances.

• Rate of forward reaction is the product of the concentration of the reactants.

• While the rate of the reverse reaction is the product of the prodcct concentration of the product.

• At equilibrium, the rate of forward reaction is equal to the rate of the reverse reaction.

Equilibrium constant (ctd)

• Is the product of the concentration(moles/litre) of the products divided by the product of the concentration of the reactants.