Enthalpy change: amount of heat evolved or absorbed in a reaction carried out at constant pressure. is the symbol for enthalpy change
is the symbol for standard enthalpy change. This means the reaction took place under standard conditions, which are: temperature: 298K, pressure: 100 KPa, concentration: 1moldm-3
Standard state: the state you would expect to find the substance in when under standard conditions. E.g. O2 – gas, H2O- liquid.When a standard enthalpy change is quoted, standard conditions are assumed.
– Standard Enthalpy of Formation: The enthalpy change which occurs when 1 mole of the compound is formed from its constituent elements, with both products and reactants in their standard state under standard conditions: 298K, 100 Kpa. (By definition the standard enthalpy of formation of any element is zero)
– Standard Enthalpy of Combustion: The enthalpy change which occurs when 1 mole of a compound is burned completely in oxygen under standard conditions 298K, 100Kpa and both products and reactants in their standard state
Enthalpy changes 2
- Energy is need to break bonds and energy is given out when new bonds are formed.
- If at the end of the reaction energy is given out the reaction is exothermic. Likewise, if energy has been absorbed, the reaction is endothermic.
- If a reaction is endothermic in one direction, it is is exothermic in the other. In exothermic reactions the products end up with less energy than they started with because heat energy has been lost to their surroundings. That is why DH is negative for exothermic reactions. Some endothermic reactions absorb heat from the water
Temperature is related to the average kinetic energy of the particles.
Specific Heat Capacity, c: The amount of heat required to raise the temperature of 1g of a substance by 1K (units: Jg-1K-1). Enthalphy change = mass of substance (normally water) x specific heat capacity x temperature change. q= m x c x T (Specific Heat capacity of water 4.18)
Ex.1 - 0.1g of methanol was burned and the temperature of 220g of water rose by 4.1K. Find the enthalpy of combustion of the substance.
q= m x c x T : 220 x 4.18 x 4= 3678.4, /0.1 (0.1 moles burned) = 36784J = 36.8 KJ
Endothermic reaction enthalpy diagram
Exothermic Reaction Enthalpy Diagram
Hess's Law states: The enthalpy change which occurs in a chemical reaction is independent from the route taken.
- Arrows going down indicate enthalpy of combustion
- Arrows going up indicate enthalpy of formation
- Arrows go down from the reactants to their oxides
Hess's Law 2 - Bond Enthalpies
Draw the molecule and add the appropriate bond entalpies (This data should be given)