Chem Energy of Rxn Ws1

Secondary 4 Express Pure Chemistry – Energy of Reaction Wksheet 1 1 | P a g e Name: ____________________( ) Class: ________ Date: ___________ Write your answers In the spaces provided. 1. The graph shows the energy changes during a chemical reaction. a. What is the quantity represented on the y-axis? ______________________________________________________________ b. What is the appropriate unit for the quantity represented on the y-axis? ______________________________________________________________ c. What does X represent? ______________________________________________________________ d. What does Y represent? ______________________________________________________________ e. What does Z represent? ______________________________________________________________ f. is the reaction endothermic or exothermic? ______________________________________________________________

Secondary 4 Express Pure Chemistry – Energy of Reaction Wksheet 1 2 | P a g e 2. A substance X exists as X2 molecules, it reacts with a substance Y which also exists as Y2 molecules. They form a compound XY. X2(g) + Y2(g)→ 2XY(g) Both X — X and Y — Y bonds are weak but X — Y bonds are strong. a. Is the reaction exothermic or endothermic? ______________________________________________________________ b. Draw an energy level diagram for this reaction. 3. Complete the following table on energy conversions. Example Energy Conversion Electrolysis cell Electrical → ____________________ Bicycle dynamo Kinetic → ______________________ Photoelectric cell ____________________→ Electrical Dry battery ____________________→ Electrical Fluorescent tube Electrical → ____________________ Classify these changes as exothermic or endothermic by putting a tick in the appropriate box. Change Exothermic Endothermic Freezing of water Melting of ice Respiration Evaporation Neutralisation

Secondary 4 Express Pure Chemistry – Energy of Reaction Wksheet 1 3 | P a g e 4. Methane, ethane and propane are the first three members of the homologous series of alkanes. The table below shows the amount of heat energy released when one mole of each of these alkanes is completely burned excess oxygen. Name Molecular Formula Energy released in kJ/mol Methane CH 4 890 Ethane C 2H6 1560 Propane C 3H8 2220 a. Calculate the amounts of energy released by 1.0 g of propane and by 1.0 g of methane when they are completely burned. b. Estimate the amount of energy reIeased when one mole of butane is completely burned. Explain how you arrived at your answer. 5. The table below shows some bond energies, measured in kilojoules per mole. Bond energy is the energy (∆H = +ve) required to break the bonds between pairs of atoms. Bond Bond Energy / kJ/mol Bond Bond Energy / kJ/mol H-H 436 Cl –Cl 242 H-Cl 431 C-H 412 C=C 612 C-C 348 N=N 409 N-N 163 a. Which of the bonds listed above is the strongest? ______________________________________________________________

Secondary 4 Express Pure Chemistry – Energy of Reaction Wksheet 1 4 | P a g e b. Is a double bond between two carbon atoms twice as strong as a single bond? Use the information given above to explain your answer ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ c. Use the information given to calculate the total energy required to break one mole of methane into atoms. CH4→ C+ 4H ; ∆H = ? kJ The energy required is ∆H = _____ kJ. d. Bond-making is an exothermic process (∆H, -ve). Complete the statements below to calculate the energy change expected from the reaction of one mole of hydrogen with one mole of chlorine to form two moles of hydrogen chloride: H 2 + Cl2 → 2HCl; ∆H = ? kJ a. Energy change in breaking the bonds in one mole of H2? _____ kJ. b. Energy change in breaking the bonds in one mole of Cl2? _____ kJ. c. Total energy change for bond breaking = _____ kJ. d. Energy change in making the bonds in two moles of HCI? _____ kJ. e. Hence the ∆H of this reaction is = = _____ kJ.