• The H f is the enthalpy for an individual atom or molecule and the H rxn is the enthalpy for the whole reaction. Hess’s Law • Hess’s Law states that when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. This means that we can determine the enthalpy of a specific reaction by combining reactions involving the reactants and products of our final reaction. Problem Calculate the standard enthalpy of formation of carbon disulfide from its elements and provide an equation for the reaction: C(graphite) + O 2 (g) → CO 2 (g) ∆ H rxn = -393.5kJ/mol S(rhombic) + O 2 (g) → SO 2 (g) ∆ H rxn = -296.4kJ/mol CS2(l) + 3O 2 (g) → CO 2 (g) + 2SO 2 (g) ∆ H rxn = -1073.6kJ/mol Calorimetry q calorimeter + q water + q reaction = 0 q = msΔT q = CΔT q = nΔH - s is specific heat capacity and is measured in J/g C - C is heat capacity and is measured in J/ C - H is the symbol for enthalpy and is sometimes referred to as a heat of … (ex: heat of combustion) and it is generally given in kJ/mol
- make sure that when calculating for q that everything is in Joules! (not kilojoules) - If no value for calorimeter is given, assume it is negligible - Sometimes the q of water and calorimeter is combined into one term Problems A 300mL sample of .65 M HCl is mixed with 300mL of .325M of Ba(OH) 2 If the water started at 25°C what is the final temperature? Write out the chemical reaction. H + + OH - H 2 O ΔHrxn= -56.2 kJ/mol If you then place a 10.5g unknown metal at 99 C and the temperature of the water raises to 35C what is the heat capacity of the metal? (Assume the metal does not react with the solution) A 7.50 g sample of sucrose C 12 H 22 O 11 is reacted in a bomb calorimeter with 500g of water. The bomb calorimeter has a heat capacity of 370 J/°C. If the temperature of everything rises from 25°C to 75°C, what is the molar heat of combustion? Periodic Trends • Electronegativity increases as you move to the right and as you move up. • Atomic Radius increases as you move to the left and as you move down.
• Ionization energy increases as you move to the right and as you move up. • Electron Affinity generally increases as you move to the right. Problem What concept can be used to explain most of the periodic trends? Why do electronegativity and Ionization energy have the same relationship? How come the when you go down the periodic table atomic radius increases but electronegativity decreases? Quantum Numbers and Electron Configuration Principle Quantum Numbers Quantum Number Name Value What is tells you n Principle quantum number Integer Size l 0 to n-1 s = 0 Shape p = 1 d = 2 f = 3 m l -L to +L m s Spin Quantum Number ± ½ Electron “spin” Electron Configuration
Problems What are the electron configurations for the following compounds/ ions?
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