Describe how a chemical reaction happens

Enthalpy Change and Exothermic and Endothermic Reactions OBJECTIVES Demonstrate an understanding of the term enthalpy change, ΔH Construct simple enthalpy level diagrams showing the enthalpy change Recall the sign of ΔH for exothermic and endothermic reactions OCR AS Chemistry p78-79

Enthalpy Change in Exothermic and Endothermic Reactions Chemical reactions can either release energy to their surroundings, __________, or energy can be transferred to them from the surroundings, ____________. EXOTHERMIC ENDOTHERMIC Video Clip KEY IDEA Bonds contain energy. You need to put in energy to break bonds apart in the REACTANTS in a chemical reaction. Energy is released when new bonds form in the PRODUCTS in a chemical reaction. When bonds break, energy is absorbed (endothermic). When bonds form, energy is released (exothermic).

Energy Level Profile Diagrams 1 EXOTHERMIC The energy released on bond formation in the products is greater than that absorbed through breaking bonds in the reactants Ooh look – -ve Energy of reactants > energy of products These are the most common type of reaction eg. Combustion, neutralisation Example

Examples of Exothermic Reactions Combustion Neutralisation Freezing Condensation Displacement Crystallising liquid salts

Energy Level Profile Diagrams 2 ENDOTHERMIC The energy needed to be absorbed to break bonds in the reactants is greater than the energy transferred to the surroundings as bonds are made in the products Ooh look - +ve Energy of reactants < energy of products These are less common chemical reactions eg. Photosynthesis Example

Examples of Endothermic Reactions Melting Boiling and Evaporation Photosynthesis Making an anhydrous salt from a hydrate Forming a cation (+ve ion) from an atom in the gas phase

Which of the following is true when chemical bonds are broken? The process is exothermic Energy is given out A physical change is occurring The reaction will have a H that is positive(+).

OCR AS Chemistry p79 Answer Questions 1, 2 10 Minute Exercise

KEY IDEAS Enthalpy is the total energy content of the reacting materials. It is given the symbol, H. Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol ΔH. ΔH = ΔH products - ΔH reactants The units are kilojoules per mole (kJmol-1)

ΔH = ΔH products - ΔH reactants EXOTHERMIC An exothermic enthalpy change is always given a negative value, as energy is lost to the surroundings. ΔH = -kJmol-1 ΔH = ΔH products - ΔH reactants Small number Bigger number

ΔH = ΔH products - ΔH reactants ENDOTHERMIC An endothermic enthalpy change is always given a positive value, as the energy is gained by the system from the surroundings. ΔH = + kJmol-1. ΔH = ΔH products - ΔH reactants Big number Small number

Using this in everyday life Heat Packs Video

Balance each of the equations below Name the compounds or elements present in each one Identify each one as exothermic or endothermic. CH4(s) + O2(g) → CO2(g) + H2O(l) ΔH = -890 kJmol-1 HCl(g) → H2(g) + Cl2(g) ΔH = 185 kJmol-1 NH3(g) + O2(g) → NO(g) + H2O(l) ΔH = -1169 kJmol-1

ANSWERS CH4(s) + 2O2(g) → CO2(g) + 2H2O(l) EXOTHERMIC 2HCl(g) → H2(g) + Cl2(g) 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l) EXOTHERMIC Methane Oxygen Carbon dioxide Water ENDOTHERMIC Hydrogen chloride Hydrogen Chlorine EXOTHERMIC Ammonia Oxygen Nitrogen monoxide Water For each of the reactions above, construct a simple enthalpy level diagram showing the enthalpy change.

CH4(s) + 2O2(g) ΔH =-890 kJmol-1 CO2(g) + 2H2O(l)

H2(g) + Cl2(g) ΔH = +185 kJmol-1 HCl(g)

NH3(g) + O2(g) ΔH =-1169 kJmol-1 NO(g) + H2O(l)