Chemistry Important questions with solutions

[Audio] Good morning everyone, I'm here to talk about chemical equations. Today, we'll discuss what a chemical equation is, the difference between combination and polymerization reactions, and how to make chemical equations more informative. Let's begin. What are Chemical Equations?.

[Audio] Exothermic and endothermic reactions are two types of chemical reactions where heat is released or absorbed respectively. When indicating the energy balance of a chemical equation, a '+' or '-' symbol is used. Reactants and products concentrations are not indicated using their actual concentrations, but are instead described using terms such as 'dilute' (dil.) or 'concentrated' (conc.). The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers, while the Mendeleev's periodic law states that the physical and chemical properties of elements are periodic functions of their atomic masses. The periodic table is divided into groups and periods, with group I and VII being divided into subgroups A and B and group VIII being divided into three elements in each period from 4 to 6. Group IX is known as the zero group and contains inert gases, while the first period contains only two elements, the second and third periods 8 elements each, the fourth and fifth periods 18 elements each and the sixth period 32 elements. The seventh period is incomplete. A limitation of the Mendeleev’s periodic table is the placement of hydrogen, which shows similar properties to both the alkali metals and the halogens, yet is only placed in the group of alkali metals..

[Audio] The two distinct concepts of ionization energy and electron affinity are related to the electronic structure of an atom. Ionization energy is the energy required to remove the most loosely bound electron from an isolated gaseous atom. Conversely, electron affinity is the amount of energy released when an electron is added to an isolated gaseous atom. To gain a thorough understanding of these concepts, it is necessary to develop an understanding of how electrons are arranged in an atom and their interactions with other atoms. A distinction must be made between the atomic and molecular states, since this can have an effect on the ionization energy and electron affinity of a given system..

[Audio] We are examining the electronic configuration of Copper and Chromium and exploring the postulates of Bohr's atomic model. Copper's electronic configuration is 1s22s22p63s23p64s13d10 and Chromium's electronic configuration is 1s22s22p63s23p64s13d5. According to the postulates of Bohr's atomic model, electrons revolve around the nucleus in fixed circular paths, known as orbits or energy levels. The angular momentum of an electron in a given orbit must be an integral multiple of Planck's constant for it to revolve in the orbit. A transition between orbits can result in the emission or absorption of energy, equal to the difference in energy between the higher and lower orbit..

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[Audio] Rutherford demonstrated an ingenious experiment by bombarding a thin sheet of gold foil with ?-particles, with the conclusion that an atom consists of a positively charged nucleus at its center, surrounded by negatively charged electrons, occupying most of the atom’s empty space. Observations showed that most of the ?-particles passed through the gold foil undeflected, some deflected through small angles, and few were even returned back, leading to Rutherford's postulation that almost the entire mass of the atom is comprised by the nucleus, and that the atom is electrically neutral as the number of electrons is equal to the number of positive charges in the nucleus. In addition, he suggested that the electrostatic force of attraction between electrons and nucleus is balanced by the centrifugal force applied on the revolving electrons - setting the basis for understanding atomic structures..

[Audio] Rutherford's atomic model, proposed in 1911, identified the positive nucleus at the center of the atom. Despite this being a major advancement, there were two drawbacks. Firstly, it couldn't account for the stability of the atom due to the electromagnetic force of attraction between the electrons and the nucleus. Secondly, the structure proposed by Rutherford was unable to explain the origin of hydrogen spectra, as distinct energy levels observed in the emission of light from a hydrogen atom could not exist..

[Audio] Avogadro's number is 6.023 x 1023, representing the number of chemical units contained in one mole of any substance. Two moles of water therefore contains 12.046 x 1023 water molecules. Boyle's law states that at constant temperature, the volume of a given mass of gas is inversely proportional to the pressure, while Charles's law states that at constant pressure, the volume of a given mass of gas is directly proportional to the absolute temperature. Combining both laws gives us the ideal gas equation, PV = nRT, where 'P' is the pressure, 'V' is the volume, 'n' is the amount of substance, 'T' is the absolute temperature and the universal gas constant 'R' is 8.314 J/mol/K..

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[Audio] Viscosity is an important property of liquids that describes the force of friction between two layers of a liquid. It is expressed with the coefficient of viscosity ?, which is the force per unit area necessary to maintain a constant velocity difference between two adjacent layers of liquid over a distance dx. In simpler terms, it is the ratio between the force and area used to maintain a constant velocity difference between two adjacent layers of liquid. Knowing the coefficient of viscosity can provide insight into a liquid’s behavior and properties, which can then be applied to a variety of scientific and engineering fields..

[Audio] Combination reactions involve the combining of two or more reactants to form a single new product, such as when two hydrogen atoms and one oxygen atom combine to form water. Polymerization reactions involve two or more monomers combining to form a larger product known as a polymer, such as when ethylene molecules combine to form polyethylene. Chemical equations can be made more informative by indicating the state of the reactants and products with state symbols such as (s) for solid, (l) for liquid or (g) for gas. Deliquescent solids are crystalline solids that absorb sufficient water from the atmosphere and ultimately change into liquid, such as anhydrous calcium chloride. Finally, ideal solutions involve solute-solute and solvent-solvent interactions that are almost as strong as the solute-solvent interactions, such as a solution of hexane and heptane. Non-ideal solutions have solute-solute and solvent-solvent interactions that are different from the solute-solvent interactions, such as a solution of water and propyl alcohol..

[Audio] Valency can be explained using the electronic theory. Electrovalent and covalent compounds can be distinguished by the type of bond present. Electrovalent bond involves an electron transfer from one atom to another resulting in electrovalent compounds such as LiF, LiCl, NaCl, NaBr, KI, CsCl, CsBr, BeO, and MgO. Covalent bond occurs when electrons are shared between two or more atoms, forming covalent compounds like H2, H2O, CH4, and NH3. The Lewis structure of Ammonia can form coordinate bonds by donating its lone pair of electron to the electron deficient BF3. The postulates of electronic theory of valency proposes that unstable atoms form chemical bonds with other atoms in order to form stable molecules, and the force of attraction which holds the atoms together is the chemical bond..

[Audio] Valence electrons of one atom sharing or transferring to another atom is the process of chemical bonding. Classical concept of oxidation involves addition of oxygen or removal of hydrogen and reduction is the opposite; removal of oxygen or addition of hydrogen. Electronic or modern concept of oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These processes are essential for formation of chemical bonds and necessary for students of chemistry to understand..

[Audio] Oxidation and reduction always take place simultaneously, as evidenced by looking at the oxidation numbers of the underlined atoms in a reaction. For example, the oxidation number of the sulfur atom in the compound Na2S2O3 is +2, and the oxidation number of the chromium atom in the compound K2CrO4 is +6. These numbers demonstrate that electrons are being exchanged between two substances in this reaction, thus indicating the simultaneous occurrence of the oxidation and reduction processes..

[Audio] Electrochemical equations use symbols and numbers to represent chemical reactions involving ions. For instance, 2+Cr-8=0 can be simplified to Cr=6, telling us the oxidation number of Chromium in Potassium Chromate, which is +6. Understanding the behavior of ions in solutions is explained by two theories. Common ion effect states that when a strong electrolyte with a common ion is added to a weak electrolyte, it will suppress the ionization of the weak electrolyte. Arrhenius theory states that an electrolyte dissolves into positive and negative charge particles when added to water, and that the solution will remain neutral as each positive charge balances out a negative one..

[Audio] Electrolysis is a process where electrical energy is used to drive a chemical reaction. This slide looks at the total charge of a solution when it undergoes electrolysis. Through ionization, the undissociated form of an electrolyte can be changed into its dissociated form, a reversible process. Electrical conductivity results from the movement of ions towards electrodes with the opposite charge. Dissociation of the electrolyte can be determined from the ratio of the number of molecules that ionize to the total number. Faraday's laws of electrolysis state that the amount of a substance deposited or released at an electrode during electrolysis is proportional to the electricity that passes through the solution. With this, the equivalent weight of a substance can be determined by using a known amount of electricity..

[Audio] A buffer reaction is the reaction between a weak acid and its salt with a strong base. It plays a crucial role in many chemical processes. It is able to prevent drastic pH changes in a solution. For instance, when extra acid is included in a weak acid solution, the salt of the weak acid can serve as a buffer to keep the pH steady. Also, if base is added to a weak acid solution, the weak acid can act as a buffer to hold the pH constant. Knowing the principles of buffer reactions helps in improving the accuracy and value of chemical equations..

[Audio] Acid buffer solutions involve the ionization of an acid and its salt with a strong base, forming a weak acid and the salt of the weak acid. When an acid is added, the concentration of hydrogen ions rises, but the pH stays the same. In basic buffer solutions, ionization occurs between a weak base and its salt with a strong acid, forming a weak base and the salt of the weak base. When a base is added, the concentration of hydroxide ions increases, but the pH remains the same..

[Audio] Buffer solutions are widely used in the scientific field due to their ability to maintain a constant pH balance within a given range. They can help the body maintain its pH level, be used in shampoos and baby lotions to fight bacteria, or be used in the brewing industry to ensure a consistent flavor. Depending on the application, a specific pH balance is necessary to ensure optimal results..

[Audio] Acid and base reactions are an integral part of chemistry. In this slide, we will review antacids and anta-bases, and compare the three different chemical concepts for acids and bases. Antacids are substances that neutralize excess acids in gastric juice, and are used in acidity, heartburn, and gastric ulcers. Examples include baking soda, aluminium hydroxide and magnesium hydroxide. On the other hand, anta-bases are acidic substances like formic acid and acetic acid. Formic acid is used as an anti-venom and to treat gout and warts, whereas acetic acid is used as an antiseptic. According to the Arrhenius concept, an acid is a substance that produces H+ ions in water and a base is a substance that produces OH- ions in water. Examples of acids are HCl, H2SO4, HNO3 and CH3COOH; whereas examples of bases are NaOH, KOH, Ca(OH)2 and NH4OH. There are certain limitations to this concept, such as the assumption that H+ and OH- ions exist in free state in water and its applicability is only limited to aqueous solutions. The Bronsted-Lowery concept defines an acid as a proton donor and a base as a proton acceptor. For example, HCl and H2O would react to form H3O+ and Cl-, and NH3 and H3O+ would react to form NH4+ and H2O. This concept is not limited to aqueous solutions and explains the acidity and basicity of substances without H+ and OH- ions. Lastly, the Lewis concept defines an acid as an electron pair acceptor and a base as an electron pair donor. This is used to describe reactions involving molecules and ions that do not involve protons. This concludes our discussion on acid, base and salt..

[Audio] Lewis concept of acids and bases provides an alternative approach in studying acid-base reactions. It is capable of explaining reactions that Bronsted-Lowery concept is not, such as the reaction between CO2 and OH-. Furthermore, it covers all the Bronsted-Lowery acid-base reactions. Nevertheless, this concept has some drawbacks. It does not elucidate the strength of acid and bases, and is not applicable to certain reactions such as the formation of ionic bond between Na and Cl instead of a coordinate bond..

[Audio] Dialysis is the process of removing impurities from a colloidal solution using a membrane. A device called a dialyzer is used for this purpose. When the human kidney fails to adequately filter impurities such as urea and uric acids from the blood, an artificial kidney dialysis machine is employed. This machine utilises cellophane tubes to separate and remove the impurities before return of the purified blood back into the patient's body..

[Audio] The coagulation of colloidal solutions and the types of emulsions have been discussed. Now, the work of the Youth Commission should be addressed. Their mission is to reduce poverty in the area, with services available for low-income families, plus grants and scholarships..

[Audio] Osmosis is the movement of solvent through a semi permeable membrane from pure solvent to solution. When an external pressure is applied, this process can be stopped and the pressure is referred to as osmotic pressure. A sol is a colloidal system with the dispersed phase being solid and the dispersion medium typically being a liquid, such as human blood or pigmented ink. A gel is a colloidal system where the dispersed phase is liquid and the dispersion medium is solid, often seen in fruit jelly, agar-agar, cheese, or silica gel..

[Audio] Mole is an essential concept in chemistry. It is specified as the amount of substance which has the same number of chemical units as there are atoms in 12g of pure carbon-12. That signifies one mole of a substance contains 6.023 x 1023 chemical units, e.g. atoms, molecules, or ions. For illustration, if you are provided with 36g of H2O, it is equal to 6.023 x 1023 molecules. To compute the number of molecules of a substance, one should divide the mass of the substance by its molar mass, then multiply the result by Avogadro's number. Avogadro's number (or NA) is the number of chemical units in one mole of a substance, and it is equal to 6.023 x 1023. To illustrate, if 50g of CaCO3 reacts with 50g of HCl to produce CaCl2, CO2, and H2O, we can calculate the limiting reagent, the amount of CO2 produced, and the number of moles of unreacted reagent left over. In this case, 100g of CaCO3 react with 73g of HCl. That signifies 50g of CaCO3 react with 36.5g of HCl. By multiplying the mass of the substance (36.5g) by Avogadro's number (6.023 x 1023), we can calculate the number of moles of the unreacted reagent that is left over..

[Audio] The slide illustrates the calculation of the mass, volume and moles of excess reagents. 50 grams of HCl was used in the reaction, with only 36.5 grams necessary to achieve completion. 100 grams of CaCO3 produces 22.4 liters of CO2 at normal temperature and pressure. The calculation further reveals the mass and moles of the unreacted HCl, which is 13.5 grams or 0.37 moles..

[Audio] Le Chatelier's Principle is a useful tool for predicting the effect of changes to a system in equilibrium. As the temperature increases, the direction of the shift in equilibrium will depend on whether the reaction is endothermic or exothermic. Similarly, changes in pressure, as long as the reactants and products are in the gaseous state, will impact the equilibriium and the shift will depend on the molar ratios of reactants and products..

[Audio] Le Chatelier Principle states that when a system is at equilibrium and it is subjected to a change in pressure, temperature, or concentration, the system will shift its equilibrium to counteract the effect of the change. This has many applications, one of which is in studying physical equilibrium. For example, if the total number of moles of reactant is more than the total number of moles of product, increasing pressure will shift the equilibrium towards the right and decreasing pressure will shift the equilibrium towards the left. The effect of concentration is similar, with an increase in the concentration of reactants shifting the equilibrium towards the right, and an increase in the concentration of products shifting it towards the left. Increase in temperature also has an effect on the solubility of certain solids, as some solids absorb heat while others release it upon dissolution. An example of this is sodium chloride, which has an increased solubility when its temperature is increased, while the opposite is true for calcium carbonate..

[Audio] Knowing the environment and chemical concentrations can be key in predicting the outcome of a reaction and attaining the desired result. For example, the effect of pressure, temperature, and the abundance of certain chemicals on the solubility, melting, and vaporization of chemicals can be quite varied. In the Haber process, ammonia is formed at a high pressure and low temperature, while nitric oxide is formed at a high temperature with no effect of pressure..

[Audio] The Law of Mass Action states that the speed of chemical reactions is determined by the amount of each reactant present, with the rate being directly proportional to the active mass of the reactants. This rate can be expressed mathematically as a combination of concentration multiplied by rate constants, linking reactants and products in a chemical reaction to the kinetic energy of each species. As a result, chemists are able to adjust the rate of reaction through adjusting the concentrations of the reactants..

[Audio] As temperatures and concentrations of reactants rise, the rate of reaction is known to increase. Yet, unfortunately, the same phenomenon can have far-reaching, destructive impacts on the environment. We will explore the effects of plastic over-consumption and environmental pollution on our planet in the following slide..

[Audio] Zero order reactions are those whose rate does not depend on the concentration of the reactant and usually follows a linear relationship over time, such as the decomposition of N2O5 and reactions of enzyme catalysed reactions. Half-life is the amount of time taken for the concentration of the reactant to be halved, which is critical for understanding the rate of a reaction; an example of this can be seen in the decay of radionuclides..

[Audio] Hess's law of constant heat summation states that the total enthalpy of a chemical reaction is the same regardless of whether it is done in one step or multiple steps. To demonstrate, consider the reaction of conversion of a substance A to Z. The enthalpy change, ∆ ?, is equal to the sum of individual enthalpy changes, ∆ ?1, ∆ ?2 and ∆ ?3. As an example, the enthalpy change for the conversion of carbon to carbon dioxide is the same whether the reaction is done directly or via the formation of carbon monoxide..

[Audio] Endothermic and exothermic processes are important for understanding chemical equations. Dissolution of H2SO4 is an exothermic process, releasing 68 kJ of heat energy, while NH4NO3 is an endothermic reaction requiring 26.0 KJ of energy to proceed..

[Audio] Calculating the normality, molarity and molality of a N/10 solution of Na2CO3 requires the number of gram equivalent of solute dissolved in one litre of solution (normality) to be calculated first. Normality is calculated by ? = ????ℎ? ?? ?????? ?????????? ????ℎ? × ?????? ?? ????? divided by 1000. Thus, to prepare a N/10 solution of Na2CO3, 0.1612 gram of Na2CO3 is needed as the number of gram equivalent is 10 and the equivalent weight of Na2CO3 is 61.998. Molarity is calculated using ? = ????ℎ? ?? ?????? ????????? ????ℎ? × ?????? ?? ????? divided by 1000. Molality is calculated using ? = ????ℎ? ?? ??????? ?? ?? × ??. ?? ????? divided by 1000..

[Audio] Hydrogen displacement method is a suitable way to determine the equivalent weight of metals such as zinc, magnesium, and calcium. It works on the principle that when a metal reacts with a dilute acid, it liberates an equivalent amount of hydrogen from the acid. By measuring the volume of the hydrogen gas released, the equivalent weight of the metal can be calculated. The general equation for this reaction is a metal plus an acid reacting to form a salt and hydrogen. Thank you for your attention..