A Level Chemistry 3.1.8 Thermodynamics PDF

A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS ENTROPY (S) Entropy (S) = The measure of the DISORDER of a system (J.K-1.mol-1) Think of it as a measure of how much the atoms / molecules in a chemical system a...

A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS ENTROPY (S) Entropy (S) = The measure of the DISORDER of a system (J.K-1.mol-1) Think of it as a measure of how much the atoms / molecules in a chemical system are free to spread out, move around and rearrange. The two major factors behind the entropy of a chemical system are: 1. State of matter gas S Entropy liquid / aq (s) < (l)/(aq) < (g) (J.K-1.mol-1) solid Temperature (K) Notice how: a) Entropy increases with temperature b) There are large jumps in entropy values between changes of state. Gases have greater entropy than liquids which have greater entropy than solids. 2. Number of moles The greater the number of moles of a substance present, the greater the entropy. e.g. If you have two gases at the same temperature, the one with the greatest number of moles present will have the greatest entropy. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS ENTROPY CHANGE (ΔS) As with enthalpy, we cannot measure entropy directly, but we can estimate and calculate the entropy change (ΔS) in a reaction. During a reaction the overall entropy of the chemical system can change. You need to be able to deduce if a reaction has: an increase in entropy +ΔS a decrease in entropy -ΔS by looking at the changes in state or the number of moles present on either side of the equation. e.g. Equation ΔS Reason Change of state from (g) to (l) 2H2(g) + O2(g) → 2H2O(l) Decrease Liquids have lower entropy than (g) Change of state from (aq) to (s) AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) Decrease Solids have lower entropy than (l) or (aq) Change of state from (aq) to (g) 2HCl(aq) + Mg(s) → MgCl2(aq) + H2(g) Increase Gases have greater entropy than (l) or (aq) All states are the same N2(g) + 3H2(g) → 2NH3(g) Decrease There are fewer moles of (g) products than (g) reactants All states are the same N2O4(g) → 2NO2(g) Increases There are greater moles of (g) products than (g) reactants Entropy change (ΔS) is an indicator of how likely a reaction is to take place. AKA, the “feasibility” of a reaction. +ΔS: Reaction likely to take place -ΔS: Reaction not likely to take place However, it is not the only factor we need to consider. More on this with ΔG. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS CALCULATING ENTROPY CHANGE (ΔS) When given entropy data for the reactants and products, we can calculate a value for the ΔS of a chemical reaction. ΔSsystem = ∑ΔSproducts - ∑ΔSreactants Entropy change (ΔS) = the sum of the entropy values of the products minus the sum of the entropy values of the reactants e.g. given the following entropy data, calculate the ΔS for the reaction. Bond CH4 O2 CO2 H2O Entropy (S) (J.K-1.mol-1) 186 205 214 189 CH4 + 2 O2 CO2 + 2 H2O ΔS = (214 + 2(189)) - (186 + 2(205)) = -4 J.K-1.mol-1 You must take the number of moles of How To Identify & Calculate each substance present in the equation Entropy Changes into account. Be sure to double check the values that you input into the calculation. Remember, a reaction that has a positive ΔS is more likely to be feasible! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS GIBBS FREE-ENERGY CHANGE (ΔG) We’ve previously learned that both ΔH and ΔS can have an impact on how “feasible” a reaction is. In other words, “is the reaction likely to occur or not?” Gibbs Free-energy change (ΔG) brings ΔH, ΔS and temperature together as an overall measure of the feasibility of a reaction. - Reactions with -ΔH values are more likely to be feasible than those with a +ΔH - Reactions with +ΔS values are more likely to be feasible than those with a -ΔS - Temperature can also affect the feasibility of some reactions. Some reactions only occur above a certain temperature and some only occur below a certain temperature. More on this later! HOW TO CALCULATE ΔG △G = △H - T△S kJ.mol-1 kJ.mol-1 K J.K-1.mol-1 In any given question, you will be asked to calculate ΔG to find out if a reaction is feasible. You may have to calculate ΔH and/or ΔS to do so and then use those values in the above equation. The value of ΔG tells you if a reactions feasible at a given temperature … If ΔG is zero, or a negative value, the the reaction IS feasible If ΔG is a positive value, the the reaction IS NOT feasible How To Calculate Gibbs Free-Energy AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS HINTS | TIPS | HACKS Be wary of the units here! ΔH is in kilojoules per mole and ΔS is in joules per mole per kelvin. You must remember to to divide ΔS by 1000 before using it in the calculation of ΔG. If asked to find at what T(K) = ΔH (kJ.mol ) -1 temperature a reaction becomes feasible, use: ΔS (kJ.K.mol ) -1 -1 If a reaction should theoretically be feasible, but does not happen in practise, it will be because it has a HIGH Eact. It acts as a barrier to the reaction that isn’t accounted for here. ΔH and ΔS can be positive or negative. Temperature (T) must always be positive (as it is measured in Kelvin). We can make assumptions based on this: ΔG ΔH ΔS must be Therefore, the reaction… ➖ ➕ ➖ is ALWAYS feasible at all temperatures ➕ ➖ ➕ is NEVER feasible at any temperature ? is only feasible BELOW a certain ➖ ➖ depends on T temperature ? is only feasible ABOVE a certain ➕ ➕ depends on T temperature More on these reactions on the next page… AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.8 THERMODYNAMICS GIBBS FREE-ENERGY - THE GRAPH! Since the feasibility of some reactions depends on temperature, we can manipulate the equation for ΔG to plot ΔG against temperature. ΔG = ΔH - TΔS can be rearranged to give… m 𝑥 + c (a straight line graph) ΔG = -ΔST +ΔH y = △G = the y axis T = the 𝑥 axis -△S = the gradient △H = the y axis intercept Where both △H & △S Where both △H & △S are positive are negative +100 +100 △G 0 △G 0 -1 (kJ.mol ) Temperature -1 (kJ.mol ) Temperature (K) (K) -100 -100 Pay attention to where the line crosses the 𝑥 axis. Reaction becomes feasible Reaction becomes feasible (△G becomes negative) (△G becomes negative) ABOVE a certain temperature. BELOW a certain temperature. The gradient (-△S) can be calculated using: Just watch out for the double negative!! The steeper the gradient, the greater the impact a change in temperature has on △G AQA www.chemistrycoach.co.uk © scidekick ltd 2024