Enthalpy Starter: BACK OF BOOK: Why does a mixture of propane and oxygen not explode as soon as it is mixed?

Endothermic and Exothermic reactions 4.10 - understand that chemical reactions in which heat energy is given out are described as exothermic and those in which heat energy is taken in are endothermic 4.11 - describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation in which heat energy changes can be calculated from measured temperature changes 4.12 - calculate molar enthalpy change from heat energy change 4.13 - understand the use of ΔH to represent enthalpy change for exothermic and endothermic reactions

Key words Exothermic reaction 발열반응 Rate of reaction 반응 속도 Endothermic reaction 흡열반응 Collision 충돌사고, 부딪침

Activation Energy Activation energy is the energy needed to break the bonds between the atoms in the starting materials so they can react. Breaking bonds takes energy (is endothermic). The lower the activation energy, the easier it is to start the reaction. e.g. It is easier to make nitrogen tri-iodide react than burn wood.

Energy cannot be created or destroyed Thermodynamics: All energy leaves a system is transferred to the surroundings & vice versa The first law of thermodynamics states: Energy cannot be created or destroyed The energy that is transferred to the surroundings is not lost – its is spread through the surroundings

Energy Level Diagrams: Exothermic Reactions: Enthalpy of reaction (Energy Change) is negative The energy content of the system has decreased Some energy has left the system & entered the surroundings as heat Hproducts < Hreactants *There is a net decrease in the potential energy of the system ΔH is negative

Exothermic Reactions Exothermic Reactions overall release heat energy – they lead to rises in temperature. e.g. Burning Wood Reactants Energy Released Energy Products Progress of the reaction

Hproducts > Hreactants Endothermic Reactions: the enthalpy of reaction is positive Energy content of the system has increased as it has gained from the surroundings Hproducts > Hreactants So ΔH is positive Energy is transferred from the surroundings There is a net increase in the potential energy of the system

Endothermic Reactions Endothermic Reactions overall take in heat energy – they lead to a lowering of the temperature. e.g. the reaction used in sports cold packs Products Energy Absorbed Energy Reactants Progress of the reaction

Practice: Mini whiteboards Draw enthalpy level diagrams for: The endothermic reaction: H2O (g) + C (s)  H2 (g) + CO (g) b. The combustion of methane: CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (l)

Endo - Exo Thermic Method 1. There will be 5 stations around the room and you have 5 experiments to complete. 2. You will be using boiling tubes. Wash out after each experiment. 3. Take 20cm3 of the solution and measure the temperature using a thermometer. (One station has 2 solutions measure either one). (One station has 2 solids dissolve the Citric acid in 20cm3 of water). 4. Add the second chemical. Either 20cm3 of solution or 2 spatulas of a solid. (Only 1 piece of Magnesium). 5. Measure the temperature change after and calculate the temperature change. 6. Rinse out your cup and lid and move on to the next station.

Plenary Check point: Classify each process as endothermic or exothermic: The burning of magnesium in air The crystallisation of copper (II) sulfate from a saturated solution The thermal decomposition of magnesium nitrate The fermentation of glucose by yeast The evaporation of sea water The combustion of magnesium Exo Exo Endo Exo Endo Exo