Presentation on theme: "CAUSES OF CHANGE Order and Spontaneity. Enthalpy and Reactions Some reactions happen easily, but some others do not. Sodium and chlorine readily react."— Presentation transcript:
Enthalpy and Reactions Some reactions happen easily, but some others do not. Sodium and chlorine readily react to form NaCl when brought together. On the other hand, nitrogen and oxygen coexist in the atmosphere we breath without reacting to form the poisonous gas, NO. We can use our enthalpy values to HELP to predict whether reactions will occur. If there is a decrease enthalpy of if ΔH is negative, the reaction is more likely to occur.
A New Thermodynamic Property A few processes are endothermic and still can occur easily… Why is this?? Entropy, denoted S, is a measure of the randomness or disorder of a system, and is a thermodynamic property. Entropy is NOT a form of energy and has the units of joules per Kelvin, (J/K). A process is more likely to occur if it has an increase in entropy (if the __________ is positive).
Factors That Affect Entropy Diffusion increases the disorder of molecules and thus increases entropy. The more random or disordered a sample is, the higher the entropy is. Entropy increases when solutions become more dilute, entropy also increases when pressure is decreased on a gas. Why may this be?? __________________________________ ___________________________________________________ Why may this be?? The less the solvent, the more the solute can move around. The less the pressure is on a system, the larger the volume.
Phase / Chemical Changes Entropies can also increase with temperature, but this effect is not great unless a phase change occurs. Entropy can change during a chemical reaction. The entropy of a system can increase when the total number of moles of product is greater than the total number of mole of reactant. Entropy increases as number of particles increase. Also, increase when a reaction produces more gas particles than the reactants b/c gases are more disordered than liquids or gases.
Formation of Sodium Chloride As sodium reacts with chlorine to produce NaCl, the entropy decreases. This decrease means what? ______________________. Write out this reaction and try to explain this information. The reverse reaction (NaCl Na + + Cl - ) increases in entropy because the order lost when a crystalline solid dissociates to form the ions.
Hess’s Law and Entropy Molar entropy has the same units as molar heat capacity. What were those units? ________________. Molar entropies can actually be calculated using molar heat capacity information. What is Hess’s Law?? We used this law when calculating ΔH of a reaction. We can also apply this to finding the ΔS, or ______________.
Hess’s Law and Entropy Entropies can be also calculated by using Hess’s Law and entropy data (such as in Data Table 3, Page 359) You can algebraically manipulate the chemical equations similar to as you did when calculating ΔH f ° When equations are added or subtracted, entropy changes must be added or subtracted. Sound Familiar? When equations are multiplied by a constant, the entropies must be _____________________________. Atoms and molecules that appear on both sides can be _______________.
Standard Entropy Standard change in enthalpy was denoted ___________. Using this information, what do you think standard entropy is? ____________. The enthalpy of an elemental substance is ___________. However, the entropy of an elemental substance can have a value other than zero. Also, most entropy values are positive, unlike the sign of most enthalpy values.
Entropy Change of a Reaction Just as we stated, entropies can be used in Hess’s Law also. To calculate enthalpies we used the following equation: ΔH reaction = ΔH products – ΔH reactants To calculate entropies we used the following equation: ΔS reaction = ΔS products – ΔS reactants
Sample Problem Hess’s Law and Entropy. Use Table 4 to calculate the entropy change that accompanies the following reaction. ½H 2(g) + ½CO 2(g) ½H 2 O (g) + ½CO (g) Same 4 rules apply as before: 1. Gather information 2. Plan your work 3. Calculate 4. Verify your results
Sample Problem Hess’s Law and Entropy. Use Table 4 to calculate the entropy change that accompanies the following reaction. ½H 2(g) + ½CO 2(g) ½H 2 O (g) + ½CO (g) 1. Gather your information: Products: Reactants: 2. Plan your work: The general rule is ΔS reaction = ΔS products – ΔS reactants
Sample Problem 2. Plan your work: For H 2 O, S 0 = 188.7 J/K*mol For H 2, S 0 = 130.7 J/K*mol For CO, S 0 = 197.6 J/K*mol For CO 2, S 0 = 213.8 J/K*mol 3. Calculate your work: ΔS = [(½ mol)(188.7 J/K*mol) + (½ mol)(197.6 J/K*mol)] – [(½ mol)( 130.7 J/K*mol) + (½ mol)(213.8 J/K*mol)] ΔS = 20.9 J/K
Sample Problem 4. Verify your results The sum of the standard entropies of gaseous water and carbon monoxide is larger than the sum of the standard entropies of gaseous hydrogen and carbon dioxide. So, the ΔS for the reaction should be ______________.
Third and Final Thermodynamic Property Gibbs Free Energy Enthalpy and Entropy help to determine spontaneity in a change HOWEVER, Gibbs Free Energy is the ULTIMATE DETERMINANT!! If ΔG is positive, it is NOT GOING to happen without help If ΔG is negative, it is will be spontaneous, meaning it will just happen!!
Start Studying Now Chapter Test will be on Wednesday. It covers everything we talked about thermo. Review your notes a little each night. Know all vocab and how to use all equations.