TLDR;
This lecture explores the factors that influence the rate of a chemical reaction (ROR). While only concentration and temperature truly dictate ROR, several other factors can play a role. These include the nature of reactants (ionic vs. covalent, homogeneous vs. heterogeneous, stability), physical state of reactants (gas, liquid, solid), surface area, light intensity (for photosensitive reactions), and the presence of catalysts. The lecture sets the stage for a more detailed examination of concentration and temperature in subsequent lectures.
- Only concentration and temperature truly dictate ROR.
- Other factors include nature of reactants, physical state, surface area, light intensity and catalysts.
- The lecture prepares for a detailed look at concentration and temperature in future lectures.
Introduction [0:00]
The lecture introduces the factors that affect the rate of a chemical reaction, building upon the previous lecture that defined the rate of reaction. It is mentioned that while several factors will be discussed, the primary focus in the chapter will be on concentration and temperature. The speaker encourages students to take notes and refers them to the website for downloadable PDFs.
Nature of Reactant [2:45]
The nature of the reactants significantly influences the rate of reaction. Ionic reactions, such as the reaction between NaCl and AgNO3, are very fast because ions are already present and require minimal bond breaking. In contrast, reactions involving covalent compounds, like the formation of ammonia from nitrogen and hydrogen, are slower because they require breaking existing covalent bonds before forming new ones. Homogeneous reactants react more rapidly than heterogeneous reactants due to similar molecular properties. The rate of reaction is directly proportional to the stability of the product and inversely proportional to the stability of the reactant. For example, the dissociation of HCOOH is faster than CH3COOH because the resulting HCOO- ion is more stable than the CH3COO- ion due to the electron-donating effect of the CH3 group.
Physical State of Reactant [11:23]
The physical state of the reactants also affects the rate of reaction. Reactions involving gases are generally faster than those involving liquids, which are in turn faster than those involving solids. This is because molecules in the gaseous phase have greater freedom of movement, leading to more frequent and effective collisions. The lecture uses the example of gases A and B mixing, where their ability to move freely increases the likelihood of collision and reaction, compared to solids A and B where collision opportunities are limited.
Surface Area of Reactant [13:21]
The surface area of the reactant is directly proportional to the rate of reaction, especially when the reactant is in the solid state. A sugar cube dissolves slower in tea compared to granulated sugar because the smaller particles of granulated sugar provide a larger surface area for interaction with the solvent molecules. Exposing more surface area allows for more reaction sites, thus increasing the rate of reaction.
Intensity of Light [16:29]
The intensity of light affects the rate of reaction for photosensitive or photochemical reactions. Increasing the intensity of light increases the number of photons, which in turn increases the rate of reactions like the formation of HCl from H2 and Cl2. This reaction proceeds via a free radical mechanism, where photons facilitate the homolysis of Cl2 molecules. The rate of reaction is directly proportional to the intensity of light (I) in such reactions.
Catalyst [19:54]
A catalyst is a substance that speeds up or slows down the rate of a chemical reaction. A positive catalyst increases the rate of reaction by providing an alternate pathway with lower activation energy. Activation energy is the energy barrier that reactants must overcome to form products. Catalysts lower this barrier, making it easier for the reaction to proceed. The Contact process, where sulfur dioxide is oxidised to sulfur trioxide using nitric oxide as a catalyst, is given as an example. The rate of reaction is directly proportional to the concentration of the catalyst.
Temperature [23:39]
Generally, increasing the temperature increases the rate of reaction. Higher temperatures lead to increased molecular motion and more frequent collisions. A common rule of thumb is that a 10-degree Celsius rise in temperature can double or triple the rate of reaction. This is quantified by the temperature coefficient. The Arrhenius equation (K = A.E-EA/RT) is introduced as a more detailed method for understanding the relationship between temperature and reaction rate, which will be discussed in a later lecture.
Concentration [26:27]
The concentration of reactants generally affects the rate of reaction, but the relationship is not always straightforward. While increasing the concentration of reactants often increases the rate of reaction, it can also decrease it or have no effect, depending on the specific reaction. The rate of reaction may also depend on the concentration of the products. The lecture mentions that the next 4-5 lectures will focus on how the rate of reaction depends on the concentration of reactants, introducing the concept of the rate law. The rate law equation will clarify how the rate of reaction depends on the concentration of reactants or products.
