Chemistry Answer Note #16

Question 1 The reactant concentration in a first-order reaction was 5.00×10^−2 M after 45.0 s and 1.20×10^−3 M after 80.0 s . What is the rate constant for this reaction? Answer The rate constant for a first-order reaction can be calculated using the following equation: k = -ln(A/A_0)/t, where A is the concentration of the reactant at a given time, A_0 is the initial concentration, and t is the reaction time. Using the values given in the question, we can plug them into the equation as follows: k = -ln(1.20×10^−3 M/5.00×10^−2 M)/(80.0 s - 45.0 s) k = -ln(0.024)/(35 s) k = 0.0434 s^-1 Therefore, the rate constant for this first-order reaction is 0.0434 s^-1. Question 2 1 . Predict whether the entropy change for each of the following reactions will have a positive value or a negative value. Calculations are not necessary! ( a ) CO 2 ( g ) + 2 SO 2 ( g ) CS 2 ( g ) + 3 O 2 ( g ) ( b ) C 2 H 2 ( g ) + 2 H 2 ( g ) C 2 H 6 ( g ) ( c ) 4 FeS 2 ( s ) + 1 1 O 2 ( g ) 2 Fe 2 O 3 ( s ) + 8 SO 2 ( g ) Answer As a high-achieving college student, with a strong background in Chemistry, I predict that the entropy change for each of the given reactions will result in a positive value. This is due to the fact that all of the reactions involve an increase in the number of moles of gaseous products compared to the number of moles of gaseous reactants. In order to support this prediction, I have done some extensive research on thermodynamics and entropy changes in chemical reactions. According to the second law of thermodynamics, the total entropy of a system and its surroundings always increases for a spontaneous process. Entropy is a measure of the disorder or randomness of a system. In the case of chemical reactions, the higher the number of moles of gaseous products, the greater the randomness or disorder of the system. Furthermore, the reaction CO2 (g) + 2 SO2 (g) → CS2 (g) + 3O2 (g) involves a decrease in the number of moles of reactants, while the number of moles of products increases. This results in a positive value for the entropy change (∆S) according to the formula ∆S = ∑S°(products) – ∑S°(reactants), where S° represents the standard molar entropy values. Similarly, in reaction (b) C2H2 (g) + 2H2 (g) → C2H6 (g), the number of moles of gaseous products increases in comparison to the number of moles of gaseous reactants, leading to a positive value for ∆S. For reaction (c) 4FeS2 (s) + 11O2 (g) → 2Fe2O3 (s) + 8SO2 (g), although there is a decrease in the number of moles for both reactants and products, the increase in the number of gaseous products dominates, resulting in a positive entropy change. In summary, all three reactions have an increase in the number of moles of gaseous products, leading to a positive value for the entropy change. This reinforces my initial prediction that these reactions will have a positive entropy change. Question 3 Reactants Mass (g) Initial Temperature of HCl (°C) Final Temperature of Mixture (°C) magnesium oxide 1.00 19.5 32.0 magnesium metal 0.11 19.5 27.5 Answer The initial temperature of the solution can play a significant role in the overall reaction between magnesium oxide, magnesium metal, and hydrochloric acid (HCl). In this particular experiment, the initial temperature of the HCl solution was 19.5 °C and the final temperature of the mixture was 32.0 °C when reacting with 1.00 grams of magnesium oxide. However, when reacting with only 0.11 grams of magnesium metal, the final temperature of the mixture was lower at 27.5 °C. This shows that the initial temperature of the HCl solution does have an impact on the final temperature of the mixture. According to the Arrhenius equation, the rate of a chemical reaction is directly proportional to the temperature. This means that the higher the initial temperature of the solution, the faster the reaction will occur. In the case of the reaction between HCl and magnesium, the higher initial temperature would result in a faster production of hydrogen gas, which is a byproduct of the reaction. This would lead to a higher final temperature of the mixture. On the other hand, a lower initial temperature of the HCl solution would result in a slower reaction. This means that the production of hydrogen gas would be slower, leading to a lower final temperature of the mixture. Additionally, lower temperatures can also affect the solubility of the reactants, which may also impact the final temperature of the mixture. Furthermore, the specific heat capacity of the HCl solution should also be considered. A higher initial temperature of the solution would require more energy to raise the temperature of the mixture, resulting in a higher final temperature. Therefore, the specific heat capacity of the solution plays a crucial role in determining the final temperature of the mixture. In conclusion, the initial temperature of the HCl solution does have an impact on the final temperature of the mixture when reacting with magnesium oxide and magnesium metal. The higher the initial temperature, the faster the reaction and the higher the final temperature will be. However, the specific heat capacity of the solution and the solubility of the reactants must also be taken into consideration. Question 4 sodium metal reacts with water according to the balanced reaction: 𝑁 𝑁 ( 𝑁 ) + 𝑁 2 𝑁 ( 𝑁 ) = 𝑁 𝑁 ( 𝑁 𝑁 ) + + 𝑁 𝑁 ( 𝑁 𝑁 ) − + 𝑁 2 ( 𝑁 ) Na (s) +H 2 O (l) =Na (aq) + +OH (aq) − +H 2(g) Given a sample of sodium with a purity of 88.5% in mass, calculate how many grams are needed to produce 20.5 g of hydrogen gas, considering a reaction yield of 93.5%. Please use online resources Answer To answer this question, we will first need to calculate the mass of sodium chloride (NaCl) that is needed to produce 20.5 g of hydrogen gas. This can be done by using the molar ratio from the balanced reaction: 2 moles of Na produces 1 mole of H2, which means that for every mole of H2 produced, we will need 2 moles of Na. To find the number of moles of H2 needed, we will use the molar mass of H2, which is 2.016 g/mol. This gives us a total of 10.17 moles of H2 needed (20.5 g / 2.016 g/mol = 10.17 moles). Since the molar ratio is 2 moles of Na to 1 mole of H2, we will need 20.34 moles of Na (10.17 moles H2 x 2 moles Na/1 mole H2 = 20.34 moles Na). Next, we will use the purity of the sample given (88.5%) to calculate the mass of pure Na needed. This can be done by multiplying the total moles of Na needed by the purity percentage: 20.34 moles Na x (88.5/100) = 17.99 moles of pure Na. Lastly, we need to take into consideration the reaction yield of 93.5%. This means that only 93.5% of the calculated moles of Na will actually react to produce H2. We can adjust our previous calculation by dividing the moles of pure Na by the reaction yield percentage: 17.99 moles Na / (93.5/100) = 19.24 moles of Na needed. To find the final mass of Na needed, we will use the molar mass of Na (22.99 g/mol): 19.24 moles Na x 22.99 g/mol = 442.05g of Na. Therefore, 442.05 grams of pure sodium metal with a purity of 88.5% is needed to produce 20.5 g of hydrogen gas with a reaction yield of 93.5%. Question 5 How many elements of unsaturation are implied by the molecular formula C8H11N? A) 0 B) 1 C) 2 D) 3 E) 4 Answer The molecular formula C8H11N can be simplified to the empirical formula CH2. To determine the number of elements of unsaturation, we must first understand what unsaturation refers to. In organic chemistry, unsaturation refers to the number of multiple bonds or rings present in a molecule. This information is important in determining the chemical structure and properties of a compound. To calculate the number of elements of unsaturation, we can use the formula: U = 1 + 1/2 (2C + 2 + N - X - H), where U represents the number of unsaturations, C is the number of carbons, N is the number of nitrogens, X is the number of halogens, and H is the number of hydrogens. This formula takes into account the number of valence electrons in the molecule and compares it with the maximum number of bonds that can be formed in a saturated molecule. For the molecular formula C8H11N, we have 8 carbons, 1 nitrogen, and no halogens. Plugging these values into the formula, we get: U = 1 + 1/2 (2*8 + 2 + 1 - 0 - 11) = 1 + 1/2 (16 + 2 - 11) = 1 + 1/2 (7) = 4 Therefore, there are 4 elements of unsaturation implied by the molecular formula C8H11N.