Effect of concentration and temperature variation on the rate of chemical reaction

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. 

 

Theory 

The reaction between sodium thiosulphate and hydrochloric acid produces a colloidal solution of sulfur, rendering the solution translucent. This colloidal property is employed to investigate the rate of sulfur precipitation. The rate of precipitation is influenced by the concentration of reacting species and the system's temperature. 

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

The ionic form of the above reaction is written as:  

S2O3 2- (aq) + 2H+ (aq) → H2O (l) + SO2 (g) + S(s) 

An escalation in concentration augments the rate of sulfur precipitation due to an increased frequency of molecular collisions. This heightened collision frequency enhances the likelihood of product formation, thereby accelerating the precipitation of sulfur. Similarly, elevating the temperature enhances the kinetic energy of the reacting species, leading to an increased number of collisions resulting in product formation. Consequently, a rise in temperature correlates with a faster rate of reaction. 

Chemical kinetics 

Chemical kinetics is the branch of chemistry that deals with the study of the rates of chemical reactions, the factors that influence these rates, and the mechanisms by which reactions occur. It provides insights into how quickly reactants are consumed and products are formed during a chemical reaction. The key aspects of chemical kinetics include: 

 

  1. Reaction Rate: The rate of a chemical reaction is the change in concentration of reactants or products per unit of time. It is often expressed as the change in moles per liter per second. 

  2. Factors Affecting Reaction Rate:  

    • Concentration: The rate of a reaction is generally proportional to the concentration of the reactants. Increasing the concentration often leads to an increase in the reaction rate. 
    • Temperature: Higher temperatures usually result in faster reaction rates as molecules have more kinetic energy, leading to more frequent and energetic collisions. 
    • Catalysts: Catalysts are substances that increase the rate of a reaction without being consumed in the process. They provide an alternative reaction pathway with lower activation energy. 
    • Surface Area: For reactions involving solids, increasing the surface area of the solid can enhance the reaction rate. 

  3. Reaction Mechanisms: Chemical reactions often occur through a series of steps, known as a reaction mechanism. Understanding the sequence of these steps helps in comprehending how reactants are transformed into products. 

  4. Rate Laws: The rate of a reaction can be expressed mathematically using rate laws. These equations relate the reaction rate to the concentrations of reactants and any catalysts. 

  5. Activation Energy: Activation energy is the minimum energy required for a reaction to occur. It represents the energy barrier that reactant molecules must overcome for the reaction to proceed. 

 

 Effect of Concentration: 

  1. Increased Concentration of Reactants: According to the rate law, the rate of a reaction is directly proportional to the concentration of the reactants. In the case of the reaction between sodium thiosulphate and hydrochloric acid, an increase in the concentration of either or both of the reactants will generally lead to an increase in the reaction rate. 
  2. Collision Theory: Higher concentrations increase the frequency of collisions between reactant molecules. This is important because only a fraction of collisions have enough energy and proper orientation to result in a reaction. 

 

Effect of Temperature: 

  1. Temperature and Reaction Rate: Generally, an increase in temperature tends to increase the rate of a reaction. This is because at higher temperatures, molecules have more kinetic energy and collisions between them are more energetic. 
  2. Activation Energy: The rate of a chemical reaction is influenced by the activation energy required for the reaction to occur. Increasing the temperature provides more molecules with energy greater than or equal to the activation energy, making them more likely to undergo a reaction. 
  3. Reaction Rate Equation: The relationship between temperature and rate of reaction is often described by the Arrhenius equation:  

k = Ae – Ea/RT 

k = rate constant of the reaction 

A = Arrhenius Constant 

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

R = Universal Gas Constant 

T = Temperature in absolute scale (in kelvins) 

In summary, increasing the concentration of sodium thiosulphate or hydrochloric acid generally leads to an increase in the rate of reaction. Similarly, raising the temperature enhances the kinetic energy of molecules, increasing the likelihood of successful collisions and, therefore, the reaction rate. 

 

Factors affecting the rate of a reaction  

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

 

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 experiment in the real lab more accurately and quickly.