Heat of Reaction & Enthalpy

General Information All chemical reactions involve energy changes, whether energy is being absorbed or given off The energy absorbed or given off comes from chemical bonds When bonds are broken, energy is absorbed Energy is needed in order to break the bonds When bonds are formed, energy is released Energy is given off when the compound becomes more stable after a bond forms

Temperature ≠ Heat Temperature Heat The measurement of the average kinetic energy of the particles in a sample of matter SI unit is Kelvin (K) Temperature is a number - it is not energy Heat Energy that flows between samples of matter because of a difference in the temperature Flows from hot to cold SI unit is Joules (J) Heat is energy Example: Look at ice melting Temperature stays at 0°C But heat is being absorbed

Flow of Energy Exothermic Endothermic The system releases heat to its surroundings Feels warm or hot because the release of heat increases the kinetic energy of the surroundings When energy is release, an energy term will appear on the product side of the equation Endothermic The system absorbs heat from its surroundings Feels cold or cool because it is absorbing energy from the surroundings. This decreases the kinetic energy of the surroundings Is indicated by writing the energy term on the reactant side of the equation

If a mixture of hydrogen and oxygen is ignited, water will form and energy will be released explosively 2H2(g) + O2(g)  2H2O(g) Because energy is released/produced – the reaction is exothermic This equation does not tell you that energy is evolved as heat during the reaction

2H2(g) + O2(g)  2H2O(g) + 483.6 kJ But through experiments we know: 483.6 kJ of energy is released/produced Adding the amount of energy makes it a thermochemical equation 2H2(g) + O2(g)  2H2O(g) + 483.6 kJ States must be included because they influence the overall amount of energy exchanged

Enthalpy Enthalpy (H°) Is the heat content in a system, OR, the total amount of potential and kinetic energy within a substance Enthalpy cannot be measured Note: The ° symbol indicates a temperature of 25°C and 101kPa, which is standard temperature and pressure in thermodynamics

Enthalpy Change An enthalpy change (ΔH°) is the amount of energy absorbed or lost by a system as heat during a process at constant pressure We can calculate the enthalpy changes during a reaction We also call this change in enthalpy the heat of reaction

Thermochemical equations can also be written by designating the value of ΔH For exothermic reaction, ΔH is always negative (-) because the system loses energy Endothermic reactions, ΔH is always positive (+) because the system gains energy Same as before – exothermic 2H2(g) + O2(g)  2H2O(g) ΔH° = -483.6 kJ Opposite process - endothermic 2H2O(g)  2H2(g) + O2(g) ΔH° = +483.6 kJ

Potential Energy Graphs Graphically, we can represent the potential energy for the reactants and products like: Shows the potential energy of the reactants is more than the potential energy of the products The drop in potential energy is the heat released or the heat of reaction (ΔH) Heat is being released so the value is negative

Endothermic is the complete opposite Shows the potential energy of the reactants is less than the potential energy of the products The increase in potential energy is the heat gained or the heat of reaction (ΔH) Heat is being absorbed so the value is positive

Questions? HW 1.8 (8-10) 1.9 (1-3) Tomorrow: Heat cont’d Review – Let me know if you have specific topics you want to review

Heat of Reaction & Enthalpy Day 2

Review An exothermic reaction releases energy Energy can be seen as a product of the reaction An endothermic reaction needs energy for the reaction to occur Energy can be seen as a reactant Enthalpy (H) is the energy (heat) content of a system at constant pressure You cannot measure the actual energy or enthalpy of a substance You can measure the change in enthalpy ΔH An enthalpy change (ΔH°) is the amount of energy absorbed or lost by a system as heat during a process at constant pressure

Chemistry problems involving enthalpy changes are similar to stoichiometry problems The amount energy that is absorbed or released in a reaction depends on the number of moles of reactants involved

2H2O2(l)  2H2O(l) + O2(g) ΔH= -190kJ Example How much heat will be released if 1.0 g of hydrogen peroxide decomposes 2H2O2(l)  2H2O(l) + O2(g) + 190kJ OR 2H2O2(l)  2H2O(l) + O2(g) ΔH= -190kJ

2 NaHSO4(s) ----> Na2SO4(s) + H2O(g) + SO3(g) Another Example Given the decomposition of sodium hydrogen sulfate reaction: 2 NaHSO4(s) ----> Na2SO4(s) + H2O(g) + SO3(g) ΔH= - 231.3kJ If 3.60 g of NaHSO4(s) reacts, how much heat is released?

Given the following reaction: C(s) + 2S(s) + 89.3kJ  CS2(l) How many atoms (particles) of Carbon can be burned if 520.0 kJ of energy are available?

Time to work on HW 1.9 (4-10) Review Tomorrow: Move on to Unit 2 – Solutions Monday: Unit 1 EXAM