- 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
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