CHEMICAL REACTIONS, PERCENT YIELDS AND MATERIAL BALANCE

Percent Yield An important indicator of the efficiency of a particular laboratory or industrial reaction. Percent yield = Actual yield x 100 Theoretical yield Before performing chemical reactions, it is important to know how much product will be produced with given quantities of reactants. This is known as the theoretical yield.

Percent Yield Calculation Problem: What is the percent yield of the following reaction if 60 grams of CaCO3 is heated to give 15 grams of CaO? Heat CaCO3 → CaO + CO2 CaCO3 40+12+3x16 = 100 g 100 g 56 g 1g CaCO3 produced= 56/100 = 0.56 g 60 g CaCO3 will produce= 60 x 0.56 = 33.6 g (Theoretical Yield) % Yield = Actual yield x 100 Theoretical yield % Yield = 15 x 100 = 44.64 % 33.6

Limiting Reactants Limiting reactant – the reactant that is consumed first and therefore limits the amounts of products that can be formed. Determine which reactant is limiting to calculate correctly the amounts of products that will be formed.

Limiting Reactants Methane and water will react to form products according to the equation: CH4 + H2O  3H2 + CO The amount of products that can form is limited by the methane. Methane is the limiting reactant. Water is in excess.

MATERIAL BALANCE METHODOLOGY

Definition “Materials Balance analysis (MB) is a systematic reconstruction of the way in which a chemical element, a compound or material passes through a natural cycle and/or its economical benefits. An analysis of the material flow, usually is based on the origin of a physical balance.”

Objectives of MB Describes or illustrates any material flow present in the enterprise. Trace the waste at the point at which it is generated. Print data so that decisions may be taken. Identify the production process weaknesses. Give a detailed reasonable and quantified priorization of the results to obtain wastes and emissions minimization.

Results Identify the materials used by the enterprise. Identify the sources, the volumes and causes of the wastes and emissions. Create a base for the evaluation and prediction of future developments. Define strategies to improve the general situation of the enterprise.

Analysis Points and Technical Advice The process areas that will be studied have to be determined . Analysis as a function of: costs, risks, secure deposit of wastes, volume etc. First step is to develop an analysis of input and output of the enterprise. Detailed analysis in relation to the more extensive flows and ecological problems.

MB Overview The productive process is divided into sequencial steps according to the priority of the analysis and the study of the flows previously done. The representative period of time has to be choosen depending of the type and size of the enterprise.

Process sequence Flow diagram Detailed study of the material flow. Processes are divided according to a representative structure. What are considered processes? activities, equipments, products or utility aspects. Flow diagram Graphic interpretation of the materials flows. It shows volumes, proportions, ecological relevance among other characteristics. Indicate the sequence or steps of the productive process.

Plant, Process or Unit Operación Example: Input and Output Raw materials Gaseous emissions Plant, Process or Unit Operación Catalyst Products Air/Water By-products Energy Wastewater Recycle Liquid waste Reusable residues in other operation Solid waste

Interpretation Establish standards that will establish a relationship between the material consume and the waste generated. Establish the process efficiency indicators. (for example coffee produced). One dollar invested in prevention is like one thousand dollars in medicine.

The chlor-alkali industry It deals with the production of chlorine (Cl2) and sodium hydroxide (NaOH) by the electrolysis of sodium chloride. Outline some important uses of the products of this process. Discuss the environmental impact of the processes used for the electrolysis of sodium chloride.

Introduction of chloride Chloride is a powerful oxidizing agent with a standard electrode potential of +1.36V. Since it has a very high value of standard electrode potential, very few chemical oxidizing agents can oxidize chloride ions to chlorine (Apart from fluorine). The manufacture of chlorine gas depends on using electrons themselves.

How is chlorine made ? Chlorine gas is formed during the electrolysis of molten sodium chloride. Chlorine is produced by passing an electric current through a solution of brine (common salt dissolved in water). The chemical term for salt is sodium chloride (NaCl). Essential co-products are caustic soda (sodium hydroxide (NaOH) and hydrogen (H2). All three are highly reactive, and technologies have been developed to separate them and keep them apart.

Conti… Caustic soda (sodium hydroxide or NaOH)) is an alkali and widely-used in many industries, including the food industry, textile production, soap and other cleaning agents, water treatment and effluent control. Hydrogen is a combustible gas used in various processes including the production of hydrogen peroxide and ammonia as well as the removal of sulphur fro petroleum derivatives.

Chlorine has been manufactured industrially for more than 100 years. There are three methods to produce Chlorine: The diaphragm cell process The membrane cell process The mercury cell process

The diaphragm cell process In the diaphragm cell process the positive electrode( made of titanium) and negative( made of steel) electrodes are separated by a permeable diaphragm. Chlorine is formed at positive electrode 2Cl-(ag)  Cl2(g) +2e- Hydrogen is formed at negative electrode 2H2O(l) + 2e-  H2(g) +2OH-(ag) Sodium chloride solution can flow between the electrodes. Chlorine and hydrogen gas can’t flow through( preventing the OH- ions flowing towards the positive electrode).

Modern version Another version of the diaphragm cell is known as an ions exchange membrane cell. It uses a partially permeable ion exchange membrane rather than asbestos. The membrane is made of a flourinated polymer and is permeable to positive ions but not negative ions.

The membrane cell process The anode and the cathode are separated by an ion-exchange membrane. Only sodium ions and a little water pass through the membrane. The brine is de-chlorinated and re-circulated. Solid salt is usually needed to re-saturate the brine. After purification by precipitation-filtration, the brine is further purified with an ion exchanger. The chlorine gas contains some oxygen and must often be purified by liquefaction and evaporation.

The mercury cell process In the mercury cell process, negative electrode is made of flowing mercury. Sodium is above hydrogen in the electrochemical series, sodium is preferentially discharged as it forms an alloy (known as an amalgam) with the mercury Na+ (ag) + e- + Hg (l)  Na/Hg (l) The mercury flows out of the electrolysis cell into a separate chamber  Reacts with water to produce hydrogen and sodium hydroxide solution.

The mercury cell process The mercury is recycled back into the electrolytic cell. Na/Hg(l) +H2O(l)  Na+(ag) + OH-(ag) +1/2 H2(g) +Hg(l) The cell is made of PVC-lined steel and the positive electorde where is chlorine is formed is made of graphite. 2Cl-(ag)  Cl2(g) + 2e- The brine is first de-chlorinated and then purified by a precipitation-filtration process. The products are extremely pure. The chlorine, along with a little oxygen, generally can be used without further purification.

The mercury cell process Of the three processes, the mercury process uses the most electricity, but no steam is required to concentrate the caustic solution. The use of mercury demands measures to prevent environmental contamination. Also, mercury must be removed from the hydrogen gas and caustic soda solution. Increasingly, chlorine producers are moving towards membrane technology, which has much less impact on the environment.

Important uses of Chlorine Chlorine is used in a wide range of industrial and consumer products, For example, it is used in making plastics, solvents for dry cleaning and metal degreasing, textiles, agrochemicals and pharmaceuticals, insecticides, dyestuffs, etc. Chlorine is an important chemical for water purification (such as water treatment plants), in disinfectants, and in bleach. Chlorine is usually used (in the form of hypochlorous acid) to kill bacteria and other microbes in drinking water supplies and public swimming pools. Elemental chlorine is an oxidizer (Chemistry).

Environmental impact of the chlor-alkali industry The mercury cell has been replaced with either diaphragm or membrane cells in many countries. The replacement is because of environmental problem since in practice some of the mercury leaks into the environment and can build up in the food chain to toxic levels. The membrane cell is preferably chosen among three methods.

Environmental impact Besides all of the good uses of Chlorine. People are also concerned about over using of chlorinated organic compounds. Several of them have been shown to be carcinogenic (causing cancer). The C-Cl bond can break homolytically (each atom getting one of the two electrons) in the presence of ultraviolet light at higher altitudes to from chlorine radicals which can contribute to ozone depletion.

Industrial Chemistry By Dr. Ghulam Abbas

Laboratory Chemistry vs. Industrial Chemistry There are fundamental differences between the design of a chemical synthesis for industry and that for a research laboratory.

Different Approaches for Different Objectives Laboratory Objectives Synthesize the product in the most convenient manner considering: 1) Chemist’s time 2) Equipment available (usually must use glassware). 3) Conditions achievable (usually close to ambient pressure and temperature. Industrial Objectives Produce the product at minimum total cost on a scale that will generate the maximum economic return. may use: 1) Large range of temperatures and pressures 2) Batch process or continuous operation 3) Reactants in vapor phase or liquid phase

Differences in the Synthetic Approach Example: Formation of ethyl alcohol by hydration of ethylene: Laboratory Scale Bubble ethylene into 98% H2SO4 Dilute and warm the reaction mixture to hydrolyze the resultant sulfate ester Industrial Scale A stream of ethylene is mixed with steam at 325 °C and 1000 psi and passed over a solid catalyst consisting of phosphoric acid absorbed on diatomaceous earth; the process is run continuously, and unreacted ethylene is recovered and recycled to the feed stream.

Cont….

Evaluation of a Reaction If a chemist has an idea for an industrial scale process, what are the steps that must be taken before the process can be implemented? 1) Evaluation of the reaction: Before any serious literature search or laboratory work is started, various possible strategies are proposed. 2) Economic feasibility 3) Technical feasibility 4) Other considerations: environmental issues, etc.

Evaluation of a Reaction The chemist must consider not only the well-known, obvious approaches, but also unknown or untested approaches. e.g., the manufacture of ethylamine.

Economic Feasibility Estimate the difference between the market value of the products and the reactants. First approximation, assume: 1) 100% yield 2) no costs of solvents or catalysts 3) no value for co-products These assumptions must be reassessed further on in the development stage.

Technical Feasibility Generally, the first approach is to consider the thermodynamics of the reaction. This may be done by evaluating the change in Gibbs free energy (ΔG). DGR = DHR - T DSR A spontaneous reaction has a decrease in Gibbs energy of the system. To calculate the Gibbs energy of reaction, use standard Gibbs energies of formation DGf°. DGreaction = DDGf°products - DDGf°reactants

Technical Feasibility There are two basic questions that a chemist or chemical engineer must ask concerning a given chemical reaction: 1. How far does it go, if it is allowed to proceed to equilibrium? (Does it go in the direction of interest at all?) 2. How fast does it progress? Question 1 is concerned with thermodynamics and amounts to evaluating the equilibrium constant (K). Question 2 is a matter of kinetics and reduces to the need to know the rate equation and rate constants (k).

Example: Dissociation of ethyl chloride At 298 K: DHrxn = +17.14 kcal/mole DGrxn = +7.86 kcal/mole DSrxn = +31.15 cal/mole At 1000 K: DHrxn = +17.08 kcal/mole DGrxn = -14.43 kcal/mole DSrxn = +31.52 cal/mole -G at 1000 K only! Change in Free Energy Indication -DG promising small +DG worth further investigation large +DG possible only under unusual conditions

DGreaction = DDGf°products - DDGf°reactants DGf° kcal/mol 298 K 1000 K CH2=CH2 16.28 28.25 CH3-CH3 -7.87 26.13 CH3CH2NH2 8.91 60.96 CH3CH2OH -40.22 1.98 NH3 -3.86 14.85 H2O -54.64 -46.04 H2 0 0 N2 0 0 DGreaction = DDGf°products - DDGf°reactants DG298 = + 16.78 kcal/mol DG1000= + 34.83 kcal/mol DG298 = -1.65 kcal/mol DG1000= -1.91 kcal/mol

Other Considerations Evaluate: Number of possible side-products (and separation difficulties) Air or moisture sensitivity of reactants and intermediates Commercial value of side-products Environmental impact of side-products Health and safety issues