Quenching: What is quenching? The analysis of the changes in concentration of reactants or products with time is direct measure of the rate of a reaction. Yet, since the process of analysis takes time, quenching is necessary to slow down the rate of reaction abruptly and assumed to have stopped. The methods of quenching of sample mixture include: Rapid cooling by ice Removing the catalyst Removing one of the reactants by adding another reagent which can use up the reaction rapidly Dilution with a large volume of water.
In this experiment, The addition of NaHCO3 is to neutralize the H2SO4 , with a view to removing the catalyst to lower the rate of reaction for upcoming titration process. Moreover, ice cubes were added to the reaction mixture to lower the temperature and concentration, thus minimize the reaction rate. The volume of NaHCO3 added is unimportant. It should only be added in excess in order to remove all the catalyst H2SO4. Chemical kinetics: Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes.
Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction’s mechanism and transition states, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. In 1864, Peter Waage and Cato Guldberg pioneered the development of chemical kinetics by formulating the law of mass action, which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances.
Rate of reaction Chemical kinetics deals with the experimental determination of reaction rates from which rate laws and rate constants are derived. Relatively simple rate laws exist for zero order reactions (for which reaction rates are independent of concentration), first order reactions, and second order reactions, and can be derived for others. In consecutive reactions the rate-determining step often determines the kinetics. In consecutive first order reactions, a steady state approximation can simplify the rate law.
The activation energy for a reaction is experimentally determined through the Arrhenius equation and the Eyring equation. The main factors that influence the reaction rate include: the physical state of the reactants, the concentrations of the reactants, the temperature at which the reaction occurs, and whether or not any catalysts are present in the reaction. Factors affecting reaction rate: Nature of the Reactants Depending upon what substances are reacting, the time varies. Acid reactions, the formation of salts, and ion exchange are fast reactions.
When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow. Physical State The physical state (solid, liquid, or gas) of a reactant is also an important factor of the rate of change. When reactants are in the same phase, as in aqueoussolution, thermal motion brings them into contact. However, when they are in different phases, the reaction is limited to the interface between the reactants. Reaction can only occur at their area of contact, in the case of a liquid and a gas, at the surface of the liquid.
Vigorous shaking and stirring may be needed to bring the reaction to completion. This means that the more finely divided a solid or liquid reactant, the greater its surface area per unit volume, and the more contact it makes with the other reactant, thus the faster the reaction. To make an analogy, for example, when one starts a fire, one uses wood chips and small branches—one doesn’t start with large logs right away. In organic chemistry On water reactions are the exception to the rule that homogeneous reactions take place faster than heterogeneous reactions. Concentration.
Concentration plays a very important role in reactions according to the collision theory of chemical reactions, because molecules must collide in order to react together. As the concentration of the reactants increases, the frequency of the molecules colliding increases, striking each other more frequently by being in closer contact at any given point in time. Think of two reactants being in a closed container. All the molecules contained within are colliding constantly. By increasing the amount of one or more of the reactants it causes these collisions to happen more often, increasing the reaction rate (Figure 1.
1). Temperature Temperature usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more thermal energy. Although collision frequency is greater at higher temperatures, this alone contributes only a very small proportion to the increase in rate of reaction. Much more important is the fact that the proportion of reactant molecules with sufficient energy to react (energy greater than activation energy: E > Ea) is significantly higher and is explained in detail by the Maxwell-Boltzmann distribution of molecular energies.
The ‘rule of thumb’ that the rate of chemical reactions double for every 10 °C temperature rise is a common misconception. This may have been generalized from the special case of biological systems, where the Q10 (temperature coefficient) is often between 1. 5 and 2. 5. A reaction’s kinetics can also be studied with a temperature jump approach. This involves using a sharp rise in temperature and observing the relaxation rate of an equilibrium process. Catalysts image00. png image01. png Generic potential energy diagram showing the effect of a catalyst in an hypothetical exothermic chemical reaction.
The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same. A catalyst is a substance that accelerates the rate of a chemical reaction but remains chemically unchanged afterwards. The catalyst increases rate reaction by providing a different reaction mechanism to occur with a lower activation energy. In autocatalysis a reaction product is itself a catalyst for that reaction leading to positive feedback.
Proteins that act as catalysts in biochemical reactions are called enzymes. Michaelis-Menten kinetics describe the rate of enzyme mediated reactions. In certain organic molecules specific substituents can have an influence on reaction rate in neighbouring group participation. Agitating or mixing a solution will also accelerate the rate of a chemical reaction, as this gives the particles greater kinetic energy, increasing the number of collisions between reactants and therefore the possibility of successful collisions.
Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing the rate of reaction. This is because the activity of a gas is directly proportional to the partial pressure of the gas. This is similar to the effect of increasing the concentration of a solution. A catalyst does not affect the position of the equilibria, as the catalyst speeds up the backward and forward reactions equally.
References: New Way Chemistry for Hong Kong A-Level Book 2 Ch. 13-14 by Y. C. Wong ; C. T. Wong http://en. wikipedia. org/wiki/Chemical_kinetics.