Chemistry Answer Note #28

Question 1 if 10.0g of hydrogen are produced when the following reaction takes place how mich energy was involves Zn+2HCl --> ZnCl2+ H2 H=-152kj Answer To determine the energy involved in this reaction, we need to use the formula E = mcΔT, where E is energy, m is mass, c is specific heat, and ΔT is change in temperature. However, since this is a chemical reaction, we need to use the formula ΔH = ΔE + PΔV, where ΔH is change in enthalpy, ΔE is change in energy, P is pressure, and ΔV is change in volume. First, let's determine the moles of hydrogen produced. To do this, we need to use the molar mass of hydrogen, which is 1 g/mol. Therefore, 10.0g of hydrogen is equivalent to 10.0 moles. Next, we need to determine the change in enthalpy for the reaction, which is given as 152 kJ. This value represents the energy released during the reaction. Now, we can use the formula ΔH = ΔE + PΔV to determine the change in energy. Since this is a chemical reaction, we can assume constant pressure and therefore, PΔV would be equal to 0. This means that ΔH is equal to ΔE, and we can rewrite the formula as ΔH = mcΔT. We know the values for ΔH and m, but we need to determine c and ΔT. The specific heat of hydrogen is 14.3 J/(g·K), which means that for every 1 gram of hydrogen, it takes 14.3 joules of energy to raise the temperature by 1 Kelvin. We can rearrange the formula to solve for ΔT, which would be ΔH/mc. Plugging in the values, we get (-152 kJ)/(10.0g * 14.3 J/(g·K)) = ΔT. This results in ΔT = -106.3K. This means that the temperature decreases by 106.3 Kelvin during the reaction. Since the change in temperature is negative, it means that energy is being released from the system. This aligns with the initial given value of -152 kJ, meaning our calculations are accurate. Question 2 What is the change in enthalpy (in kJ) under standard conditions when 172.1 g of sodium sulfate dissolves in water? Answer According to the thermodynamics principle, enthalpy is a measure of the heat energy that is either released or absorbed during a chemical reaction or physical change. In this case, we are looking at the change in enthalpy when a given amount of sodium sulfate dissolves in water under standard conditions. To accurately determine the change in enthalpy, we must first understand the concept of standard conditions. Standard conditions refer to a set of specified conditions where the physical state of a substance is at standard pressure (1 atm) and temperature (298 K). This helps to ensure that the values for enthalpy are consistent and comparable. Now, let's look at the chemical equation for the dissolution of sodium sulfate in water: Na2SO4(s) + H2O(l) → 2Na+(aq) + SO4 2-(aq) Based on this equation, we can see that 1 mole of sodium sulfate (142.04 g) produces 2 moles of sodium ions (2 x 23 = 46 g) and 1 mole of sulfate ions (1 x 96 = 96 g) when it dissolves in water. Therefore, 172.1 g of sodium sulfate will produce (172.1/142.04) = 1.212 moles of sodium ions and 0.606 moles of sulfate ions. To calculate the change in enthalpy, we can use the formula ∆H = ∑H(products) ∑H(reactants), where ∑H represents the sum of enthalpies of the products and reactants. ∆H = [2(46 g x H(Na+(aq))] + [0.606 mol x H(SO4 2-(aq))] - [172.1 g x H(Na2SO4 (s))] H(Na2SO4 (s)) = 1.212 mol x H(Na2SO4 (s)) = 1.212 mol x -1382.5 kJ/mol = -1.676 kJ H(Na+(aq)) = 2 x 23 g x H(Na+(aq)) = 2 x 23 g x -406.6 kJ/mol = -18.712 kJ H(SO4 2-(aq)) = 0.606 mol x H(SO4 2-(aq)) = 0.606 mol x -909.3 kJ/mol = -550.78 kJ ∆H = [2(-18.712)] + [-550.78] - [-1.676] = -575.572 kJ Therefore, the change in enthalpy when 172.1 g of sodium sulfate dissolves in water under standard conditions is -575.572 kJ. Question 3 if 20.0 grams of zinc results in the release of 46.4kj of energy, when the following reaction takes place, what is DH for the reaction as written? Zn+2HCl ---> ZnCl2+H2 Answer To answer this question, we must first understand what DH refers to. DH, or enthalpy change, is the difference in energy between the reactants and products in a chemical reaction. It can be calculated by subtracting the energy of the reactants from the energy of the products. In this reaction, zinc (Zn) reacts with hydrochloric acid (HCl) to form zinc chloride (ZnCl2) and hydrogen gas (H2). The given information states that 20.0 grams of zinc results in the release of 46.4kJ of energy. This information allows us to calculate the enthalpy change for the reaction. The first step is to convert the given mass of zinc (20.0 g) to moles. To do this, we divide the mass of zinc by its molar mass, which can be found on the periodic table. The molar mass of zinc is 65.38 g/mol, so 20.0 g of zinc is equal to 0.306 moles. Next, we need to calculate the enthalpy change per mole of zinc. To do this, we divide the given energy (46.4 kJ) by the number of moles of zinc (0.306 mol). This gives us an enthalpy change of -151.63 kJ/mol. The negative sign indicates that the reaction is exothermic, meaning it releases energy. In this case, the energy released is 151.63 kJ per mole of zinc reacted. Now, we can use a balanced equation to determine the enthalpy change for the reaction as written. For every mole of zinc reacted, 2 moles of hydrochloric acid, 1 mole of zinc chloride, and 1 mole of hydrogen gas are produced. This means that the enthalpy change for the reaction as written will be twice the enthalpy change per mole of zinc reacted. Thus, the enthalpy change for the reaction as written is -303.26 kJ. This indicates that for every mole of zinc reacted, 303.26 kJ of energy is released. Question 4 In a chemical reaction, 9.57 g of product forms. The amount of product calculated using the balanced chemical equation is 24.31 g. What is the percentage yield? Answer The percentage yield is a measure of how efficient a chemical reaction is at producing the desired product. It is calculated by taking the actual yield of the product and dividing it by the theoretical yield, then multiplying by 100 to get the percentage. In this scenario, the actual yield is 9.57 g and the theoretical yield is 24.31 g, so the percentage yield can be calculated as (9.57 g / 24.31 g) * 100% = 39.38%. In order to understand the concept of percentage yield and how it is calculated, it is important to have a strong understanding of stoichiometry and the use of balanced chemical equations. This topic is extensively covered in academic chemistry textbooks, but two reputable sources that can be referenced are "Chemistry: The Central Science" by Brown, LeMay, Bursten, Murphy, Woodward, and Stoltzfus and "General, Organic, and Biological Chemistry: Structures of Life" by Karen C. Timberlake. When calculating percentage yield, it is also important to take into account any limiting reagents present in the reaction. A limiting reagent is the reactant that is used up first and determines the amount of product that can be formed. If a reactant is present in excess, it will not contribute to the actual yield and can skew the calculation of percentage yield. In this particular scenario, it is not mentioned if any limiting reagents are present. However, if that information was provided, the theoretical yield could be recalculated using the amount of the limiting reagent as the limiting factor. This would provide a more accurate calculation of the percentage yield. In conclusion, the percentage yield is a crucial concept in chemistry and is used to assess the efficiency of a chemical reaction. It is calculated by dividing the actual yield by the theoretical yield and multiplying by 100%. In order to fully understand this concept and perform accurate calculations, a strong understanding of stoichiometry, balanced chemical equations, and limiting reagents is necessary. Question 5 What is the mole ratio of octane and water vapour in C8H18(g) + O2(g) --> CO2(g) + H2O(g) Answer The mole ratio is a method of expressing the relative amounts of substances in a chemical reaction. It is used to determine the ratio of reactants and products in a balanced chemical equation. In the given chemical equation, C8H18 is the octane molecule and H2O is the water vapour molecule. The mole ratio of these two substances can be calculated by using the coefficients in the balanced chemical equation. Step-by-Step Calculation: 1. Write out the balanced chemical equation: C8H18(g) + O2(g) --> CO2(g) + H2O(g) 2. Identify the coefficients for the octane and water vapour molecules: Octane: 1 Water vapour: 1 3. Write out the mole ratio: 1 mole of C8H18 : 1 mole of H2O Therefore, in the given chemical reaction, the mole ratio of octane and water vapour is 1:1.