Chapter 5 - Thermochemistry Heat changes in chemical reactions

First Law of Thermodynamics - the energy of the universe is constant. energy change = heat + work heat (q) is positive when heat flows into the system. - endothermic work (w) is positive when work is done on the system. In chemistry, work is expansion or compression. Sign convention link

A system that does work on its surroundings. Caption: (a) An apparatus for studying the reaction of zinc metal with hydrochloric acid at constant pressure. The piston is free to move up and down in its cylinder to maintain a constant pressure equal to atmospheric pressure inside the apparatus. Notice the pellets of zinc in the L-shaped arm on the left. When this arm is rotated, the pellets will fall into the main container and the reaction will begin. (b) When zinc is added to the acid solution, hydrogen gas is evolved. The hydrogen gas does work on the surroundings, raising the piston against atmospheric pressure to maintain constant pressure inside the reaction vessel.

exothermic - heat flows out of the system (gets hot) endothermic - heat flows into the system (gets cold) state function - does not depend on the path to get into the "state", depends only on the present state. State functions are useful because any imaginable path will get you to the current state. Energy is a state function. Endothermic and exothermic demo

enthalpy - at constant pressure, the change in enthalpy for a system is equal to the heat flow. Enthalpy is a state function. ΔH = H(products) - H(reactants) ΔH > 0, endothermic ΔH < 0, exothermic

Calorimetry heat capacity: C = heat absorbed / inc. in temp. specific heat: c = heat capacity / gram molar heat capacity = heat capacity / mole energy released = mcΔT Q = mcΔT

coffee cup calorimeter

ex When a student mixes 50. mL of 1.0 M HCl and 50. mL of 1.0 M NaOH in a coffee cup calorimeter, the temperature of the resultant mixture increases from 21.0 o C to 27.5 o C. Assuming that the calorimeter absorbs only a negligible quantity of heat, that the total volume of the solution is 100. mL that its density is 1.00 g/mL and that its specific heat is 4.18 J/g o C, calculate the enthalpy change for the reaction.

bomb calorimeter

ex Hydrazine, N 2 H 4, and its derivatives are widely used as rocket fuel. N 2 H 4 + O 2 --> N 2 + H 2 O When 1.00 g of hydrazine is burned in a bomb calorimeter, the temp of the calorimeter increases by 3.51 o C. If the calorimeter has a heat capacity of kJ/ o C, what is the quantity of heat evolved? What is the heat evolved per mole of N 2 H 4 ?

Hess's Law takes advantage of the fact that enthalpy is a state function and says that if a reaction is carried out in a series of steps, ΔH for the reaction will be equal to the sum of the enthalpy changes for the individual steps. CH 4(g) + 2 O 2(g) --> CO 2(g) + 2 H 2 O (g) ΔH = -802 kJ 2 H 2 O (g) --> 2 H 2 O (l) ΔH = -88 kJ CH 4(g) + 2 O 2(g) --> CO 2(g) + 2 H 2 O (l) ΔH = -890 kJ Link to Hess’s Law simulation

ex Calculate ΔH for the reaction 2 C (s) + H 2(g) --> C 2 H 2(g) given the following reactions and their enthalpy changes: C 2 H 2(g) + 5/2 O 2(g) --> 2 CO 2(g) + H 2 O (l) ΔH = kJ C (s) + O 2(g) --> CO 2(g) ΔH = kJ H 2(g) + 1/2 O 2(g) --> H 2 O (l) ΔH = kJ

Standard Enthalpy of Formation (ΔH f ) Δ H rxn = ΔH f (products) - ΔH f (reactants) elements in their standard state have ΔH f = 0 Tabulated in appendix C pgs ex Using the standard heats of formation calculate the enthalpy change for the combustion of one mole of ethanol.

Notes quiz dform?formkey=dHhabEZqSGZXS0tNSFFBb1g3Q mJGVnc6MQ