OCR_CD2_Endo and Exo reactions Chemical Applications Assignment 3 Endothermic and exothermic reactions

OCR_CD2_Endo and Exo reactions You need to understand that: Chemical reactions either give out heat (exothermic) or, take in heat (endothermic). Keywords: energy, bonds, Chemical changes produce heat changes

OCR_CD2_Endo and Exo reactions Exothermic reactions increase the temperature. Examples include: –Burning reactions including the combustion of fuels. –Detonation of explosives. –Reaction of acids with metals. Thermit reaction Magnesium reacting with acid

OCR_CD2_Endo and Exo reactions Secondly… Endothermic reactions

OCR_CD2_Endo and Exo reactions Endothermic reactions cause a decrease in temperature.

OCR_CD2_Endo and Exo reactions Activity: Calculate the Activation energy and the overall output energy for the following combustion reactions It’s best to start by drawing a diagram on the molecules reacting Methane + Oxygen  Carbon Dioxide + Water CH 4 + O 2  CO 2 + H 2 O

OCR_CD2_Endo and Exo reactions Investigating exothermic and endothermic reactions The assignment for this unit requires a thorough description of exo- and endo- thermic reactions. Examples Energy diagrams Key phases: Activation energy, Energy of reactants, energy of products etc.

OCR_CD2_Endo and Exo reactions Assignment details… Scenario … We are a company making cold packs for sports injuries. They also make heat packs for expeditions. We have asked a chemical company to send us a list of reactions which produce a heat change to help us decide what to make our hot and cold packs from. We need to investigate which reactions would be best to use to make our hot and cold packs. You need to suggest reactants that would make effective heat / cold packs.

Exo- and Endothermic Reactions ReactantsInitial Temp ° C Final Temp ° C Temp Change ° C 25cm³ Hydrochloric Acid 25cm³ Sodium Hydroxide 25cm³ Hydrochloric Acid + 2cm Magnesium Ribbon 25cm³ Sulphuric Acid + 2cm Magnesium Ribbon 25cm³ Nitric Acid + 2cm Magnesium Ribbon 25cm³ Water + 1g Potassium Chloride 25cm³ Water + 1g Ammonium Chloride 25cm³ Copper Sulphate + 1.4g Iron filings 25cm³ Hydrochloric Acid +0.9g Zinc Granules 25cm³ Citric acid +1.2g Sodium Bicarbonate 25cm³ Sulphuric Acid + 50cm³ Sodium Hydroxide

OCR_CD2_Endo and Exo reactions Distinction theory: Assignment 3 BTEC Chemistry Theory to help you: Energy changes in reactions involve making and breaking bonds. Energy is absorbed by reactants to break their bonds and then released by products when their bonds are made. Each bond between atoms has its own amount of energy needed to make or break the bond. If we know how much energy is needed to break the existing bonds and how much energy is released when making the new bonds then we can work out whether a reaction is exothermic or endothermic

Burning… When a hydrocarbon burns it joins with oxygen… Methane + oxygen → Oxygen Hydrogen Carbon

Burning… When a hydrocarbon burns it joins with oxygen… Methane + oxygen → The heat you supply with a match Splits up the atoms… This bit needs energy, lets say 10 Joules Oxygen Hydrogen Carbon

When a hydrocarbon burns it joins with oxygen… Methane + oxygen → → Carbon dioxide + water The atoms then recombine to make carbon dioxide and water This bit needed 10 Joules… When these atoms recombine, they release 15 Joules Burning… Oxygen Hydrogen Carbon

Task… Calculate the energy released when hydrogen is burnt in air to produce water. Chemical Bond Energy in Bond (Kj/mol) H - H436 O = O498 H - O464 Hydrogen+Oxygen  Water 2H 2 + O 2  2H 2 O Start by drawing a diagram of the atoms reacting...

OCR_CD2_Endo and Exo reactions e.g. 2H 2 + O 2  2H 2 O Chemical BondEnergy in Bond Kj/mol H - H436 O = O498 H - O464 H H H H + O O To break these bonds takes (2 X 436) + (1 X 498) = 1370 KJ/ mol H H H H O O H H H H O O To make these bonds produces (4 X -464) = KJ/Mol More energy is made creating the bonds than is used breaking the bonds so the reaction is exothermic

OCR_CD2_Endo and Exo reactions Task… Calculate the energy released when methane burns in oxygen Bond strengths… Extension – repeat for propane Chemical Bond Energy in Bond Kj/mol C - H435 O = O498 C=O805 H - O464

OCR_CD2_Endo and Exo reactions Bond Energy (kJ/mol) Bond Energy (kJ/mol) H - H436N - N160 C - H413N = O631 N - H393N triple N941 P - H297N - O201 C - C347N - P297 C - O358O - H464 C - N305O - S265 C - Cl397O - Cl269 C = C607O - O204 C = O805C - F552 O = O498C - S259

OCR_CD2_Endo and Exo reactions Assignment

LongAQABtec Practical work completed Show energy diagrams for exothermic and endothermic reactions. Appropriate terms are used (input energy / output energy / activation energy). Combustion of methane could be used to explain this. Results from practical recorded on worksheet. Company name and logo on top of worksheet. Best reactions for heat and cold are identified. The most exothermic and the most endothermic reactions have both been identified Use the most exothermic reaction to help you explain what an exothermic reaction is. Why is this the best reaction for a hot pack? Use the most endothermic reaction to help you explain what an endothermic reaction is. Why is this the best reaction for a cold pack? Explain how a chemical reaction involves breaking old bonds and making new bonds. Explain how these 2 stages of a reaction involve energy changes. Calculate energy changes for a named exo or endothermic reaction Write about why some reactions are exothermic and others are endothermic by talking about the energy changes involved when old bonds are broken and new bonds are made