Monitoring and Controlling Chemical Reactions

Importance and Objectives of Monitoring and Controlling Chemical Reactions:

• Efficiency

• Safety

• Quality

• Reduced production costs

• Minimizing waste generation

Monitoring Reactions:

• Different reactions occur at different rates; some slow, some faster.

• The rate of reaction can be measured by how fast a reactant is used up or by how fast the product is made. 

  • REACTANT: the beginning materials in a reaction that undergo a chemical change to form a product

  • PRODUCT: the new substance formed after a chemical reaction between two or more reactants.

• Formula triangle: 

• The units of the rate of reaction would therefore be g s-1 or cm3 / dm3 s-1

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A. To measure mass,

1. If any product is a gas, the reaction can be performed in an open flask on a balance to measure the mass lost of the reactant

2. Cotton wool is usually placed in the mouth of the flask. This can prevent materials from exiting the flask.

3. This method is not appropriate for hydrogen and other gases with a low relative formula mass, Mr, because the decrease in mass may be too minimal to detect accurately.

B. To measure volume of a gas, 

1. collecting a gas by using an inverted measuring cylinder filled with water (provided the gas is not soluble in water)

C. To measure the rate a precipitate forms,

1. Precipitation reactions form a solid precipitate when 2 clear solutions are mixed together.

2. The reaction mixture becomes cloudy due to the precipitate, so placing the flask over a piece of paper with a cross on it allows you to measure the time it takes for the cross to become obscured from view as the precipitate forms.

Rates:

Rate of reaction graphs are used to calculate the mean rate of a reaction, at a specific point, and the time when the reaction completes. Usually, x-axis = time; y-axis = amount of product/reactant. 

During the reaction, the concentration of the product will increase while the concentration of the reactant will decrease (as it is being consumed).

To calculate the mean rate of reaction, 

• For whole reaction, overall change in qty (y-axis) divided by total time taken (x-axis)

• Between two points in time, find slope/gradient.

Changing rates:

Rates of reaction refer to how quickly reactants are converted into products over time. This is influenced by factors such as temperature, concentration of reactants, surface area (in heterogeneous reactions), and the presence of catalysts.

• Temperature: Increasing temperature generally increases reaction rates by providing reactant molecules with more kinetic energy, leading to more frequent collisions and effective collisions that result in product formation.

• Concentration: Higher concentrations of reactants increase the frequency of collisions, thereby increasing reaction rates.

• Catalysts.

Catalysts

Catalysts are substances that increase the rate of reaction by providing an alternative mechanism with lower activation energy. They do not alter the equilibrium position or the overall thermodynamics of the reaction. Catalysts are crucial in industrial processes to enhance efficiency and reduce energy consumption.

• Role: Catalysts accelerate reactions by providing an alternative pathway that requires less energy to reach the transition state.

Usually, its transition metals that are used as catalysts as they have variable oxidation states allowing them to donate and accept electrons. For example, Iron, Manganese(IV) oxide, and Vanadium(IV) oxide are used to catalyse the Haber process, decomposition of hydrogen peroxide, and Contact process, respectively.

[Enzymes are an example of biological catalysts.]

COLLISION THEORY:

According to collision theory, chemical reactions happen only when reactant particles collide with enough energy to initiate a reaction. This required amount of energy is known as the activation energy, which varies for each reaction. Particles that collide without enough energy result in unsuccessful collisions and simply bounce off each other. Therefore, the reaction rate depends on both the energy of the collisions and the frequency of collisions. To increase the reaction rate, the number of successful collisions must be increased.

Energy profiles:

Energy Changes

Chemical reactions involve energy changes, typically in the form of heat. Understanding and controlling these changes are essential for optimizing reaction conditions and product yields.

• Exothermic Reactions: Release heat to the surroundings. Temperature control is critical to prevent overheating.

• Endothermic Reactions: Absorb heat from the surroundings. Maintaining sufficient heat input is necessary for these reactions to proceed.

There are different energy profiles for both exothermic and endothermic reactions.

To control during reactions,

1. Temperature Control

• Importance: Temperature affects reaction rates and equilibrium positions.

2.  Pressure Control

• Applications: Crucial for reactions involving gasses or where pressure influences reaction equilibrium.

3. pH Control

• Impact: pH affects reaction rates, selectivity, and stability of intermediates.

Reversible reactions:

In theory, all chemical reactions are reversible. The products can be converted back into the original reactants through an appropriate reaction. This reversibility is not always apparent when a reaction goes to completion, leaving little to no reactants remaining. For example,

1. Complete combustion of a fuel

2. Precipitation reactions

3. Gas-escaping reactions

DYNAMIC EQUILIBRIUM:

An equilibrium point is a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.

When a reversible reaction occurs in a closed system, it reaches a dynamic equilibrium. At this point:

• The forward and reverse reactions continue to occur.

• The forward and reverse reactions proceed at the same rate.

• The concentrations of all reactants and products remain constant and do not change.

Le Chatelier's Principle:

This principle asserts that when the conditions of a system at equilibrium are altered, the system will automatically adjust to counteract the change. It’s used to predict changes in equilibrium when there are changes in temperature, pressure, and concentration.