Thermochemistry PDF - Principles Of Chemistry

Official (Closed) - Non Sensitive Thermochemistry PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 1 Official (Closed) - Non Sensitive Learning Objectives  To describe enthalpy change in terms of exothermic & endothermic changes  Draw energy le...

Official (Closed) - Non Sensitive Thermochemistry PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 1 Official (Closed) - Non Sensitive Learning Objectives  To describe enthalpy change in terms of exothermic & endothermic changes  Draw energy level diagrams  Explain the bond breaking (endothermic) and bond forming (exothermic) processes  Explain overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds  Draw energy profile diagrams to represent exothermic & endothermic reactions and show the activation energies of reactions. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 2 Official (Closed) - Non Sensitive Introduction Energy cannot be created or destroyed – but it can be changed from one form to another. All chemical reactions involve energy changes. Some physical changes (melting, dissolving) also involve energy changes An energy change can sometimes be observed as an increase or decrease in temperature. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 3  Exothermic & Official (Closed) - Non Sensitive endothermic reactions EXOTHERMIC ENDOTHERMIC Heat is given out to the Heat is absorbed from the surroundings surroundings The surrounding temperature rises The surrounding temperature falls E.g. E.g. Combustion of fuels Dissolving of some ionic compounds in water Dissolving of acids Evaporation Condensation Melting Freezing PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 4 Official (Closed) - Non Sensitive Enthalpy change in a reaction The amount of energy involved in a reaction is known as the heat change or enthalpy change of the reaction. Enthalpy change is measured in kilojoules (kJ) and represented by the symbol ΔH. The enthalpy change, ΔH, represents the difference in energy content of the reactants and products. ΔH = total energy of products – total energy of reactants PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 5 Official (Closed) - Non Sensitive Energy level diagrams Energy changes in reactions can be shown by energy level diagrams. EXOTHERMIC REACTION Energy Heat is given out to the level surroundings Total energy of the products is less reactants than that of the reactants ΔH is negative Heat of reaction (ΔH = negative) products PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 6 Official (Closed) - Non Sensitive Energy level diagrams ENDOTHERMIC REACTION Energy Heat is absorbed from the level surroundings The products have more energy products than the reactants ΔH is positive Heat of reaction (ΔH = positive) reactants PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 7 Official (Closed) - Non Sensitive Example When 1 mol of NaOH solution reacts with 1 mol of HCl solution, 57.3 kJ of heat is produced. The equation for the reaction is NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H2O (l) Draw an energy level diagram for this reaction. Solution: Energy level Since heat is produced during the reaction, the reaction is exothermic. Thus, ΔH is NaOH (aq) + HCl (aq) reactants negative. ΔH = -57.3 kJ NaCl (aq) + H2O (l) products PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 8 Official (Closed) - Non Sensitive Try it yourself Ammonia gas reacts with hydrogen chloride gas to form ammonium chloride. NH3(g) + HCl(g) ⟶ NH4Cl(s) ΔH = -176 kJ Is this reaction exothermic or endothermic? Draw an energy level diagram to represent the energy change in this reaction. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 9  Official (Closed) - Non Sensitive Bond breaking & bond making What causes energy changes in chemical reactions? When a chemical reaction occurs, bonds of the reactants are broken and new bonds are made to form products. E.g. 2H2 (g) + O2 (g) ⟶ 2H2O (l) Energy is given out when bonds are Energy is absorbed formed to break bonds. O O H H H H H O O H H H O H H O H H PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 10 Official (Closed) - Non Sensitive Bond breaking & bond making How can we tell if a reaction is exothermic or endothermic? The overall enthalpy change determines whether the total chemical reaction is exothermic or endothermic Total energy Total energy Overall enthalpy change (ΔH) = absorbed for – released for bond breaking bond making If the energy absorbed in bond breaking is less than the energy given out in bond making, the reaction is exothermic. If the energy absorbed in bond breaking is more than the energy given out in bond making, the reaction is endothermic. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 11 Official (Closed) - Non Sensitive Example 2H2 (g) + O2 (g) ⟶ 2H2O (l) Given that the bond energy of: Note: Bond energy is the energy required to break 1 H-H 436 kJ/mol mol of the bonds. O=O 496 kJ/mol O-H 463 kJ/mol PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 12  Official (Closed) - Non Sensitive Example 436 kJ 463 kJ 463 kJ 496 kJ O O H H H H H O O H 463 kJ H H O 463 kJ H H O H 436 kJ H Total energy absorbed Energy released on forming for bond breaking 2 mol of H2O molecules = 872 + 496 = 2 x 926 =1368 kJ =1852 kJ Overall enthalpy change, ΔH = 1368 – 1852 The negative value of the = -484 kJ overall ΔH indicates that the reaction is exothermic PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 13 Official (Closed) - Non Sensitive Bond breaking & bond making Note: The energy required to break a chemical bond is the same as the energy released when the same chemical bond is formed. H-H (g) ⟶ 2H (g) ΔH = +436 kJ 2H (g) ⟶ H-H (g) ΔH = -436 kJ This energy is called the bond strength of a covalent bond. The stronger a bond is, the more energy is required to break that bond, and thus, the higher its bond energy. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 14 Official (Closed) - Non Sensitive Try it yourself The bond energies of some covalent bonds are shown below. Bond Bond energy (kJ/mol) H-H 436 H-Cl 431 Cl-Cl 242 Calculate the overall ΔH for the reaction: H2 (g) + Cl2 (g) ⟶ 2HCl (g) Ans: -184 kJ PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 15  Official (Closed) - Non Sensitive Activation energy Why does a mixture of hydrogen and oxygen explode only when it is heated? The reaction needs energy in order to get started. Activation energy, Ea, is the minimum amount of energy that reactant particles must possess in order for a chemical reaction to occur. The Invisible Mountain. (2013, November 15). Retrieved November 6, 2015, from http://delvewriters.com/2013/11/15/the-invisible-mountain/ PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 16 Official (Closed) - Non Sensitive Activation energy If reactant particles have less energy than the activation energy… Before collision No reaction upon collision Reactants remain unchanged If reactant particles possess energy equal to or greater than the activation energy… Before collision Molecules react upon collision Products formed PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 17 Official (Closed) - Non Sensitive Collision Theory Developed independently by Max Trautz and William Lewis In 1916 and 1918 respectively!! PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 18  Official (Closed) - Non Sensitive Energy profile diagrams Energy profile diagrams are used to show the enthalpy change and activation energy of a reaction. Activation energy acts as an energy barrier that must be overcome by the reactants before they can react to form products. Activation energy and catalysts. (n.d.). Retrieved November 6, 2015, from http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/calculating_energy_changes/ energy_from_reactions/revision/5/ PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 19 Official (Closed) - Non Sensitive Practice questions The energy profile diagram for a chemical reaction is shown. x y Which has higher energy content – the reactants or products? In the reaction exothermic or endothermic? Activation energy = Overall enthalpy change = PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 20 Official (Closed) - Non Sensitive Practice questions When ammonium nitrate is dissolved in water, the water becomes cold. NH4NO3 (s) + (aq) ⟶ NH4NO3 (aq) Is the dissolution of NH4NO3 an exothermic or endothermic reaction? Why does the temperature of the water drop? Draw the energy profile diagram. Label the enthalpy change & activation energy. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 21 Official (Closed) - Non Sensitive Summary  Exothermic reactions give out heat to the surroundings, whereas endothermic reactions absorb heat from the surroundings.  Energy changes in reactions can be shown by energy level diagrams  Energy is absorbed in bond breaking, and energy is given out in bond making  Activation energy is the minimum amount of energy that reactant particles must possess in order for a chemical reaction to occur. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 22 Official (Closed) - Non Sensitive Questions Water exists in different states, solid, liquid and gas depending on the environmental conditions. List the exothermic conversion(s) between the three states of water. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 27 Official (Closed) - Non Sensitive Questions (a) Write the balanced chemical reaction equation for the Haber process. (b) Sketch the energy profile diagram for the process. Indicate the activation energy for both the forward and reverse reactions. (c) Describe the activation energies for both the forward and reverse reactions. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 28 Official (Closed) - Non Sensitive Questions Is the following reaction exothermic or endothermic? Why? 2A2 (g) + B2 (g) 2A2B (g) H = -484kJ PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 29