Introduction:

We are now aware that you can't rely only on enthalpy data to forecast the direction of a spontaneous change. There are as well familiar and studies reactions predicted to be spontaneous by free energy computations, however yet are not observed to take place.

H₂ (g) + 1/2 O₂ (g) → H₂O (l)

This reaction via predicted to be spontaneous via free energy computations will not take place until a spark is introduced to the mixture of hydrogen and oxygen. A few methods are found to take place instantaneously whereas a few are slow and might go on for years. Mixing the aqueous solutions of sodium chloride and silver trioxonitrate (v) yields in instantaneous precipitation of silver chloride however the transformation of one rock salt to the other is a continuous and slow method.

Rate of a Chemical Reaction:

Meaning of rate of reaction:

Whenever a chemical reaction takes place, reactant bonds are broken and new bonds form. The outcome is that the product appears as the reactant disappears. The rate of the reaction is the amount (or moles) of a reactant transformed or product made up per unit time of reaction.

Rate of reactions = Change in concentration of reaction of product/Time taken for the reaction

For the reaction Mg (g) + 2 HCl (aq) → MgCl₂ (aq) + H₂ (g), the rate can be deduced in four different manners.

1) Rate of consumption (disappearance) of Mg (s)

2) Rate of consumption (disappearance) of HCl (aq)

3) Rate of formation of MgCl₂ (aq)

4) Rate of formation of H2 (g)

Monitoring the rates of chemical reactions:

Monitoring comprises making sequence of measurement carefully via a view to establish or verify relationship. The relationship here is the consequence of concentration on the rate of a reaction. For a particular chemical reaction the choice of a process or property for monitoring the rate based on the nature of the reactants and products.

Now, consider this reaction as an illustration.

Zn (s) + 2 HCl (aq) → ZnCl₂ (aq) + H₂ (g)

In lab, two methods can be employed to monitor the reaction rate efficiently.

1) You evaluate the volume or pressure of H₂ (g) and make a plot of gas volume or gas pressure against the time of reaction. A graduated syringe can be fixed to the reaction container. Note in this situation, it is the concentration of product you are evaluating.

Whenever, concentration of product at t = 0 = C_o

Then, t₁ = C₁ and t₂ = C₂

Here, t = 0 whenever you are beginning the measurement.

t = 1 whenever you measure firstly (state) after 5 mins.

t = 2 whenever you measure secondly (state) after 10 mins.

2) Titration of HCl (aq) an acid by a standard solution of an alkali (NaOH) to check the change in the acid concentration with the time of reaction. The outcomes from these two processes are described in figure given below:

Fig: Monitoring the rates of chemical reactions

The average rate of the reaction can be computed at t₁ t₂ and t₁¹, t₂¹

Average rate = (change in product/reactant concentration)/time

By employing the first curve (a):

Average rate at t₁ = (C₁ - O)/(t₁ - O)

Average rate at t₂ = (C₂ - O)/(t₂ - O)

The product concentration is (0) zero at the starting of the reaction.

By employing the second curve (b):

Average rate at t₁¹ = (C_o - C₁¹)/(t₁¹ - O)

Average rate at t₂¹ = (C_o - C₂¹)/(t₂¹ - O)

Here, C_o is the initial concentration of the acid. We can observe from computations and the curves that the average rate is not constant and reduces as the time of reaction increases.

C₁/t₁ - C₂/t₂ and (C_o - C₁¹)/t₁ - (C_o - C₂¹)/t₂¹

Meaning that the reactant and product concentrations change most quickly by time at the starting. The graphs are steepest at the starting. The steepness of each graph reduces with the time of reaction and approaches a zero value close to the end of the reaction. This agrees by observation. Gas effervescence from a solid or liquid reaction as in the case of Zn (s) + HCl (aq) → ZnCl₂ (aq) + H₂ (g) is extremely vigorous at the starting of the reaction, becoming less and less vigorous till the reaction at last stops.

Methods of monitoring the rates of reactions:

We are familiar that the main aim of monitoring is to establish or prove a relationship.

Now we will reconsider the reaction:

Zn (s) + 2 HCl (aq) → ZnCl₂ (aq) + H₂ (g)

We are as well familiar with the methods of monitoring the rate which are applicable to numerous reactions with similar characteristics. However, there are as well numerous reactions to which such processes are not applicable.

Take for illustration:

BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2 NaCl (aq)

This is a reaction in which no gas is involved however a precipitate forms as the reactants are employed up. The amount of precipitate formed could serve as a process of monitoring its rate. A choice of process for a reaction should thus be done after cautious study of the physical and chemical nature of the reactants and products. Listed below are processes that could be employed to monitor rates in some particular conditions. The property that is changing in each and every process should change proportionately by the amount of a reactant or product in the reactions.

1) Decrease in the mass of reaction system as a gaseous product escapes.

2) Measurement of volume or pressure of a gaseous product or reactant.

3) Quantity or amount of precipitate formed.

4) Color intensity. This process is employed whenever a colored product is made up from colorless reactants or a colored reactant is transformed to a colorless product.

5) Changes in the pH of the reaction medium. This is applicable whenever an acid or base is employed up or produced in the reaction.

6) Titration of a product or reactant by a suitable reagent.

7) Increase or decrease in total pressure or volume for a gas reaction. Remember the Avogadro's law; a change in the number of molecules of gas in a reaction will lead to a change of pressure or volume. Now recommend a method each to monitor the rates of such reactions.

CaCO₃ (s) + 2 HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)

2SO₂ (g) + O₂ (g) → 2SO₃ (g)

Factors that Affect Reaction Rates:

A few reactions are extremely fast while a few are slow. The rates of numerous reactions are changed markedly if the condition of the reactions is modified. Most of the factors will influence the rate of a chemical reaction. This is significant that whenever studying the effect of one factor on the rates, the others are kept constant. In this manner the effect of each and every factor can be evaluated and if possible quantified. Here are factors which can influence the rate of a chemical reaction.

Nature of the reactant:

In a chemical reaction, bonds are broken in the reactant. Reactants that are strongly bonded are not very reactive because of the high energy needed to initiate the reaction. The reaction of the form:

M + HCl (aq) → MCl (aq) + H₂

Takes place only slowly by iron, rapidly and vigorously by zinc and magnesium and there is no verification of reaction with gold.

The reaction of solid sodium hydroxide having aqueous hydrochloric acid is not very fast however the reaction of aqueous solutions of the two reacts instantaneously. The reaction of potassium tetraoxomanganate (vii) having Fe²⁺ in acid is instantaneous at room temperature. The similar potassium tetraoxomanganate (vii) solution will not react by aqueous sodium oxalate (Na₂C₂O₄) unless the reaction container is first heated. The entire are evidences of the reactant nature on the reaction rate.

Concentration of the reactant:

The rate of a homogeneous reaction is influenced by the concentration of the reactants. Homogeneous here signifies that the reactants are in the similar physical state that is, all reactants are either in solution or they are all gases. It will be noted that the solid-solid reactions are extremely unlikely.

Temperature:

Reactions are generally faster at elevated temperature. Most of the reactions which are slow at room temperature have their rates markedly increased if the temperature is increased. For good yields, experiments to monitor the rate should for all time be at a constant temperature. For a 10°C increase in temperature the rate of a reaction might double.

Pressure/volume for gases:

Pressure only influences the rate of gas reactions. For reactions of the solids or liquids, pressure consists of little or no effect at all. For a gas at a constant temperature, a pressure increase leads to a decrease in volume resultant in higher concentration and a faster reaction. The reverse is the case whenever the pressure is reduced.

Remember that for a gas:

PV = nRT

P/RT = n/V

P/RT = C

Thus, P α C

P = Pressure, V = Volume, T = Temperature, N = Number of mole, R = Gas constant and C = Concentration

As pressure increases at constant temperature P/RT increases that is, concentration increases.

Surface area:

For a reaction taking place between reactants in different physical states, the rate based on the area of contact between the two states. If the area of contact increases the rate as well increases. A lump of lead metal for illustration reacts very slowly by oxygen gas. Whenever the lead is in powder form, its reaction with oxygen is much faster. This is due to the reason of the large area of contact of oxygen gas by the powdered lead.

Catalyst:

A catalyst is a substance that whenever added to a reaction system increases the rate of the reaction however is not used up and doesn't change its chemical nature at the end of the reaction. Catalysts are generally specific in character that is, they act on a specific reaction only a small amount of the catalyst is often required for the reaction.

Light:

A few chemical reactions are light sensitive. Some of these reactions will not take place in the absence of light whereas the rates of some are only increased in the presence of light. The decomposition of hydrogen peroxide is accelerated via light. This is why it is kept in brown bottles. The reaction of methane (CH₄) with chlorine will have not occurred devoid of light. Reactions which are sensitive to light are known as photochemical reactions. Photosynthesis reaction is the other illustration of a photochemical reaction. 

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