Energy in Reactions L.O: To know what happens in an endothermic and exothermic reaction

Endothermic or exothermic 1. Magnesium and acid 2. Burning methane 3. Ice packs 4. Salt on ice 5. Burning coal

Energy in Reactions Some reactions give out heat, and some reactions take heat in ENDOTHERMIC reactions take energy IN. This is usually in the form of heat so they are felt to get colder EXOTHERMIC reactions, give energy OUT. This is usually in the form of heat energy so they can be felt to get warmer

Why is it so difficult to measure the exact temperature change in a chemical reaction? Using the following equipment try to get an accurate measurement of temperature change when hydrochloric acid is reacted with sodium hydroxide Glass beaker Glass beaker Metal beaker Metal beaker Stirring rod Stirring rod Polystyrene cup Polystyrene cup Cotton wool Cotton wool Card Card Thermometers Thermometers Ice cubes Ice cubes Foil Foil Bubblewrap Bubblewrap

Why are some reactions endothermic and some exothermic? Energy is needed to break bonds Energy is released when bonds are formed e.g. N ₂ + H 2  NH 3 N-N bonds and H-H bonds must be broken and N-H bonds are formed In an exothermic reaction, more energy is released from the forming bonds than is needed to break the bonds During a chemical reactions bonds in the reactants are broken and different bonds are reformed to make the products In an endothermic reaction, more energy is needed to break the bonds than is released from the forming of the new bonds

Bond Energies Different bonds need different amounts of energy to form and release different energies when they are formed Are these endothermic or exothermic reactions CH 4 + 2O 2  CO 2 + 2H 2 0 H 2 + Cl 2  2HCl 2H 2 + O 2  2H 2 O H 2 + Cl 2  2HCl Balance this equation and work out if it is endo or exothermic N 2 + H 2  NH 3 BondAverage bond energy (kJ/mol) H - H436 C – H412 C – C348 Cl – Cl242 O = O496 O – H463 H – Cl431 C = O743 N ≡ N 944 H – Br366 Br – Br193

Energy level diagram

Energy Level Diagrams This means the extra energy is released into the atmosphere, it is exothermic This means that products need extra energy which is gained from the surroundings. This is an endothermic reaction This diagram shows that the reactants hold MORE energy than the products This diagram shows that the reactants hold LESS energy than the products The ‘bump’ shows the energy needed to start the reaction (activation energy). This is lowered by a catalyst

26/06/2016 Exothermic vs endothermic: EXOTHERMIC – ∆H is negative. Products have lower energy so energy is given out. ENDOTHERMIC – ∆H is positive. Products have more energy so energy is taken in. ∆H is negative ∆H is positive