Effect of Temperature on Rate of Reaction

# Our Objective

The objective of this study is to investigate the influence of concentration and temperature on the rate of reaction between sodium thiosulphate and hydrochloric acid.

# The Theory

## What is chemical kinetics?

Chemical kinetics, also known as reaction kinetics, studies the speeds of reactions and how they are affected by environmental factors. It also helps in gathering and analysing information about the reaction's process and defining the features of a chemical reaction.  As a chemical reaction progresses, the number of reactants decreases while the amount of products increases. The rate of the overall reaction is determined by the rate at which the reactants are consumed or the rate at which the products are generated.

## What is rate of a reaction?

The rate of a reaction is the rate at which the concentration of the reactants or the concentration of the products changes per unit time. A chemical reaction's speed may be defined as the change in concentration of a substance divided by the time interval, which is given by

Rate = Δ concentration / Δ time

Consider a reaction, X + Y→ Z, the rate can be expressed in terms of the change in concentration of any of its components

Rate = − Δ [X] / Δ t

Rate = − Δ [Y] / Δ t

Rate = Δ [Z] / Δ t

Δ [X] is the difference between the concentration of X over the time interval t2–t

Δ [X] = [X]– [X]1

The negative sign in the first two examples indicates the decrease in the concentration of the reactants. Since negative rates do not make much sense, rates expressed as reactant concentration are always preceded by a minus sign to make the rate positive.

Consider another reaction where coefficients are different,

X + 3Y → 2Z

It is clear that [Y] decreases three times faster than [X], so we have to divide each change in concentration by an appropriate factor to avoid confusion when expressing the rate in terms of different components:

Rate = − Δ [X] / Δ t = − Δ [Y] / 3 Δ t = Δ [Z] / 2 Δ t

What is rate law?

The rate law (also known as the rate equation) of a chemical reaction is an expression that gives the relationship between the rate of reaction and the concentrations of the reactants involved.

For a reaction given by:

aA + bB → cC + dD

Rate ∝ [A] [B] y  ⇒  Rate = k [A] [B] y

where,

[A] & [B] denote the concentrations of the reactants A and B.

x & y denote the partial reaction orders for reactants A & B (which may or may not be equal to their stoichiometric coefficients a & b).

The proportionality constant ‘k’ is the rate constant of the reaction.

What is collision theory?

Collision theory states that a chemical reaction can only occur between particles when they collide (bump into each other). The collision of reactant particles is necessary but not sufficient for the reaction to occur. Collisions must also be effective. It is important to understand the exact nature of the effective collision because it determines whether the particles will react with each other and form new products.

Factors affecting rate of a reaction

• Concentration
• Temperature
• Pressure factor
• Presence of Catalyst
• Surface Area of the Reactants
• Catalyst
• Nature of the reaction

Temperature and concentration plays an important role in the rate of a reaction. As defined by the rate law and explained by collision theory, the rate of a reaction increases with concentration. As the concentration of reactants increases, the number of collisions increases. The rate of gas reactions increases with increasing pressure, similar to increasing gas concentration. Even if there are fewer moles of gas, the reaction rate increases, and if there are more moles of gas, it decreases. The pressure dependence of condensed phase reactions is poor. Increasing the temperature usually increases the rate of the reaction. An increase in temperature increases the average kinetic energy of the reacting molecules.

The temperature dependence of the rate of a chemical reaction which is known as the Arrhenius Equation.

K =  Aexp(-Ea /RT)

Where,

K = The rate constant of the reaction

A = The Arrhenius Constant

Ea = Activation Energy for the reaction (in Joules mol-1)

R = Universal Gas Constant

T = Temperature in absolute scale (in kelvins)

Here the reaction between sodium thiosulphate and hydrochloric acid provides an easy way to study the influence of reactant concentration and temperature on reaction rate.

Na2S2O3 + 2HCl → S(s)  ↓ + 2NaCl(aq) + SO2(g) + H2O (l)

Sodium thiosulfate reacts with dilute acid to produce sulfur dioxide, sulfur, and water. Sulfur dioxide is a soluble gas and dissolves completely in aqueous solution. But the sulfur formed is insoluble, and exists in the mixture as a white or pale yellow precipitate, or as a colloid, which gives a milky appearance and renders the solution opaque. Therefore, the rate of the reaction can be studied by monitoring the transparency of the reaction. This can be easily done by measuring the time (t) for the formation of a certain amount of sulfur.

In this experiment, the reaction is carried out by mixing the reactants in a conical flask and placing the conical flask on white paper with a cross. Before the reaction begins, the mark is clearly visible from the top of the conical flask through the solution. But as the reaction proceeds, sulfur precipitates, making the solution more opaque, and eventually the mark is completely masked. The time (t) for complete disappearance of the signal indicates how fast the reaction occurred.

# Learning Outcomes

• Students understand the effect of concentration and temperature of the reactant on the rate of a chemical reaction.
• Once the students understand the concept of the experiment and the different steps, they can perform the experiment in the real lab more accurately and quickly.