8 - Energetics Ⅰ
Enthalpy change (∆H)
● ***Definition: change in heat energy, measured at constant pressure
● ∆H = enthalpy of products - enthalpy of reactants
●
Energy level diagram Energy profile
Exothermic (∆H = -ve)
(products have lower
enthalpy than reactants;
heat given out from
surroundings)
Endothermic (∆H = +ve)
(products have higher
enthalpy than reactants;
heat taken in from
surroundings)
Measuring enthalpy changes
● ***
−1 −1
𝑄 (𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 [𝐽]) = 𝑚 (𝑚𝑎𝑠𝑠 [𝑔]) × 𝑐 (𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 [𝐽 𝑔 𝐾 ]) × 𝑇 (𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐ℎ𝑎𝑛𝑔𝑒 [𝐾])
○ cwater = 4.18
○ ***For m: use mass of solution (if there is any)/limiting reagent
𝑄 (𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 [𝐽])
● ***∆𝐻 = 𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙 {𝑜𝑓 𝑠𝑜𝑙𝑖𝑑/𝑛𝑜𝑛−𝑙𝑖𝑚𝑖𝑡𝑖𝑛𝑔 𝑟𝑒𝑎𝑔𝑒𝑛𝑡}
● Measure volume of solution using pipette rather than measuring cylinder/beaker
(more accurate)
1
, Using graphical methods
●
2
Enthalpy change (∆H)
● ***Definition: change in heat energy, measured at constant pressure
● ∆H = enthalpy of products - enthalpy of reactants
●
Energy level diagram Energy profile
Exothermic (∆H = -ve)
(products have lower
enthalpy than reactants;
heat given out from
surroundings)
Endothermic (∆H = +ve)
(products have higher
enthalpy than reactants;
heat taken in from
surroundings)
Measuring enthalpy changes
● ***
−1 −1
𝑄 (𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 [𝐽]) = 𝑚 (𝑚𝑎𝑠𝑠 [𝑔]) × 𝑐 (𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 [𝐽 𝑔 𝐾 ]) × 𝑇 (𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐ℎ𝑎𝑛𝑔𝑒 [𝐾])
○ cwater = 4.18
○ ***For m: use mass of solution (if there is any)/limiting reagent
𝑄 (𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 [𝐽])
● ***∆𝐻 = 𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙 {𝑜𝑓 𝑠𝑜𝑙𝑖𝑑/𝑛𝑜𝑛−𝑙𝑖𝑚𝑖𝑡𝑖𝑛𝑔 𝑟𝑒𝑎𝑔𝑒𝑛𝑡}
● Measure volume of solution using pipette rather than measuring cylinder/beaker
(more accurate)
1
, Using graphical methods
●
2
