Ex 2, 3, 5, 6, 7, 8 Chemistry Lab Experiments PDF

# Ex 2 - When an atom or ion absorbs energy, its electrons can make transitions from lower energy levels to higher energy levels. - After then returns from higher energy levels to lower energy levels. - The energy is released predominantly in the form of electromagnetic radiation. - In a spectrum of...

# Ex 2 - When an atom or ion absorbs energy, its electrons can make transitions from lower energy levels to higher energy levels. - After then returns from higher energy levels to lower energy levels. - The energy is released predominantly in the form of electromagnetic radiation. - In a spectrum of an atom, light of only a certain wavelength is emitted or absorbed, rather than a continuous range of wavelengths. - Why is the light emitted by hydrogen atoms red because? Because the four characteristic lines, and the most intense line in its spectrum is in the red portion at 656 nm. - Why do we observe a yellow color in the spectrum of sodium? Because the most intense lines in its spectrum are in the yellow portion of the spectrum at 589nm. - The light sent from an atom is created from transitions from one energy state to another. - The spacing between energy levels in an atom determines the sizes of the transitions that occur. - The hottest portion of a flame is (blue part). ## Color of light absorbed and color of light transmitted | Color of light absorbed | Approx. A ranges/nm | Color of light transmitted | |---|---|---| | Red | 700 - 620 | Green | | Orange | 620 - 580 | Blue | | Yellow | 580 - 560 | Violet | | Green | 560 - 490 | Red | | Blue | 490 - 430 | Orange | | Violet | 430 - 380 | Yellow | # Ex 3 ## Measuring the pH of a solution ### Objectives 1. Determination of the pH values of solutions using a pH meter. 2. Determination of the concentration of strong acid and weak acid. ### Theoretical information 1. pH is the quantitative measure of the acidity or basicity of aqueous or other liquid solutions. 2. The pH is determined by measuring the concentration of the hydrogen ion into numbers between 0 and 14 (and 10-19). 3. Pure water which is neutral corresponds to the [H+] is 1e-7, which corresponds to a pH of 7. 4. In a solution with a pH less than 7 is considered acidic. 5. In a solution with a pH greater than 7 is considered basic or alkaline. 6. pH is calculated as the negative log of the H+ concentration (pH = -log[H+]). ### pH meter A pH meter is an electronic instrument used to measure hydrogen-ion activity (acidity or alkalinity) in a solution. # Ex 5 ## Chemical Equilibrium (Ionic Equilibrium in Solution) ### Objectives 1. Study of shift in equilibrium in the reaction of ferric ions and thiocyanate ions by increasing the concentration of any one of these ions. 2. Study the effect of temperature changes (hot and cold) on the aqueous equilibrium. ### Theoretical information 1. The state of equilibrium in any reaction is recognized by the constancy of an observable property (macroscopic property), like the color intensity. 2. The color intensity of the solution will decrease very much on dilution. It will not be deep blood red color. ### Le Chatelier's principle If a system in equilibrium is changed in concentration, pressure or temperature, the equilibrium of the system will change. ### Factors affecting the position of equilibrium 1. **The concentration:** - Increasing when the color becomes deeper. - Decreasing when the color becomes lighter. 2. **The temperature:** - Decreasing on the hot water, which makes it lighter. - Increasing on the ice water, which makes it deeper. # Ex 6 ## Chemical equilibrium (Determination of the equilibrium constant for formation of important ethyl acetate) ### Objectives 1. To determine the equilibrium constant. ### Equation at equilibrium ``` CH3CH2OH(aq) + CH3COOH(aq) <-->CH3COOCH2CH3(aq)+ H2O (l) ``` The equilibrium constant for the above reaction is defined as: ``` Kc = [CH3COOCH2CH3(aq)] / [CH3CH2OH(aq)] [CH3COOH(aq)] ``` ### Equation at titration ``` NaOH(aq) + CH3COOH(aq) <--> CH3COONa(aq) + H2O (l) ``` ### Why isn't the [H2O] included in the equilibrium expression Kc= [CH3COOCH2CH3(aq)] / [CH3CH2OH(aq)] [CH3COOH(aq)]? 1. As water is the medium and present in large excess. 2. The [H2O] is a constant. # Ex 7 ## Kinetic Study of Sodium Thiosulfate Reaction with Hydrochloric Acid ### Objective To determine the order of reaction (reaction law) with respect to S2O3-2 and H+. ### Theoretical information 1. The oxidation-reduction reaction that occurs between hydrochloric acid and sodium thiosulfate, produces insoluble sulfur as a product. ``` Na2S2O3 + 2HCl -> S(s) + SO2 + H2O(l) ``` 2. The time required for the cloudiness of sulfur to appear is a measure of the reaction rate. 3. We can measure the rate by measuring the time required for a fixed amount of sulfur to cover a piece of paper with printed text. 4. If the rate equation is R = K[A]^n then n = order. Thus if n = 0 a graph of 1/t vs V would give a horizontal straight line and if n = 1 a graph of 1/t vs V would give a linear straight line. # Ex 8 ## Oxidation-reduction reactions (Redox reactions) ### Objective To determine relative oxidizing and reducing strengths of a series of ions. ### Theoretical information - **Activity:** The ability of an element to react with another element. The easier it is for an element to react with another substance, the greater its activity. - **Displacement reactions:** A more reactive metal displaces a less reactive metal from a salt solution. - The displacement reactions involve simultaneous oxidation and reduction, which is why they are called redox reactions. - **Oxidation:** Electron loss. The atom/ion/molecule losing one or more electrons is said to be oxidized. - **Reduction:** Electron gain. The atom/ion/molecule gaining one or more electrons is said to be reduced. - **Oxidizing agent:** The species that is being reduced (gaining electrons) - oxidizing agent. - **Reducing agent:** The species that is being oxidized (losing electrons) - reducing agent. - **Activity series:** A list or table of elements organized by how easily they undergo a reaction. Metals higher on the activity series are more likely to react relative to those lower on the activity series. - The activity series can be used to predict products of a single-displacement reaction and to predict if a reaction will even occur. # Ex 1 ## Percent Composition of Zinc and Copper in Coins - Determine the densities of pure copper and zinc metals and use this information to find out the percentage of copper and zinc in coins. - Evaluate the uncertainty (error) in scientific measurements and understand the causes of the underlying uncertainty. ### Theoretical Information 1. How to determine the density of pure copper and zinc metals and the density of the coin. 2. From this data, you will estimate the percentages of copper (Cu) and zinc (Zn). 3. In an object composed of multiple materials (like British pence) the density is a weighted average of the densities of pure substances that make up the object. ### Formulas ``` d = P*dcu + (100-P) * dzn ``` where: - d is the density of the coin - P is the percentage of copper - dcu is the density of copper - dzn is the density of zinc ### Error and percent error - **Error** is the difference between the measured value and the true value. - **Percent Error:** ``` % Error = (Actul value - Measured value)/ True Value * 100 ``` ### Percent composition = Measured value / True Value * 100 # Ex 2 ## Atomic Emission spectro-Flame tests Experiment ### Objectives To determine the spectra of atomic emission and calculate the energy of its wavelength. ### Theoretical information - **Flame test:** A qualitative test used in chemistry to help determine the metal or metalloid ion found in an ionic compound. - If the compound is placed in the flame of a gas burner, there may be a characteristic color given off that is visible to the naked eye (in the visible region). - Visible light is the most familiar example of electromagnetic radiation. - Differences in the wavelengths of visible light are manifested as different colors. ### Flame Colors of various metals and metalloids | Metal | Flame color | |---|---| | Li | Deep red | | Na | Bright yellow | | Ca | Brick orange | | K | Light violet | | Ba | Yellow/Green | | Cu | Green/blue |