Notes For Chemistry 2

[Archimedes]

Industrial Chemistry      
Heat of reaction:
   During a chemical reaction, energy changes occur as bonds are made or broken. The chemical energy level of a substance is often expressed by a quantity called the heat content, or enthalpy (symbol is H).
   Enthalpy cannot be measured directly, but any change in enthalpy is measured as the heat of reaction (symbol is ΔH).
ΔH= energy level of products – energy level of reactants
   Heat of a reaction is measured in joules (J) or kilojoules (kJ) per mol, and the value of ΔH can be + or -.
   In exothermic reactions the energy of the reactants is greater than the energy of the products: ΔH is -, heat is released and the temperature of the surroundings rises.
   In endothermic reactions the energy of the reactants is less than the energy of the products: ΔH is +, heat energy is absorbed and the temperature of the surroundings drops.
   The energy change for a reaction can be represented by an energy profile diagram. In order for the reaction to proceed, a minimum amount of energy is required (this is the activation energy Ea).
(Refer to uploaded pictures)
   A catalyst lowers the activation energy needed.

   The ΔH value for a reaction must be interpreted with the equation for the reaction. Thus in 2CO + O2 → 2CO2, where ΔH= -556kJ, the equation is saying that 2 mol of CO releases 556kJ of heat. If the equation is reversed, so is the sign of the ΔH value, i.e. +556Kj.