Unit 11: Kinetics and Equilibrium

phET simulator website https://phet.colorado.edu/en/simulation/re actions-and-rates

What is kinetics? Collision Theory The study of how fast a chemical reaction is occurring Describes the rate of change in the concentrations of the reactants and products over time in a chemical rxn Collision Theory A reaction is most likely to occur if the reactant particles collide with sufficient energy and proper orientation

Factors that affect the rate of a reaction What conditions will increase the number of effective collisions between particles?

1. Temperature As temperature increases, the rate of a chemical reaction increases At higher temps, particles collide more frequently and with more energy. Lower Temp Higher Temp Glowsticks

2. Surface Area Increasing the exposed surface area of a solid reactant increases the reaction rate (allows for more effective collisions) Potato and hydrogen peroxide

3. Concentration As the concentration of the reactants increases, the rate of the chemical reaction increases. Potato and concentrated hydrogen peroxide

4. Pressure (only affects gases) Increasing the pressure increases the rate of a reaction involving gases. Higher pressure increases the number of effective collisions that occur

5. Presence of a catalyst Catalysts increase the rate of reaction by providing an alternate reaction pathway, which requires less energy than the normal pathway. A catalyst remains unchanged during a chemical reaction; neither a reactant or a product Yeast and hydrogen peroxide

6. Nature of Reactants Ions dissolved in water react the fastest Gases tend to react faster than solids or liquids Ions tend to react faster than molecules

PEreactants and PEproducts remain the same, so 𝛥H remains the same A catalyst provides an alternate reaction pathway, which has a lower activation energy PE of the activated complex is lower when the catalyst is present PEreactants and PEproducts remain the same, so 𝛥H remains the same

Heat of Reaction (ΔH) -ΔH : exothermic rxn (energy is released) ΔH = amount of energy that is lost or gained in a chemical rxn Also called the change in enthalpy -ΔH : exothermic rxn (energy is released) +ΔH : endothermic rxn(energy is absorbed) ∆H = PEproducts - PEreactants

Table I Shows the ΔH for various different reactions (combustion, synthesis, dissolving) To find ΔH for the reverse of ANY reaction, switch the sign of ΔH

Endo/Exo Things to Remember! Endothermic rxn: absorption of heat from the surroundings. - Beaker/glassware will feel cold - Temp. of solution decreases A + B + energy  C Exothermic reaction: release of heat to the surroundings. - Beaker/glassware will feel hot - Temp. of solution increases A + B  C + energy

What is entropy? Entropy is a measure of randomness An entropy change is represented either by a +∆S or a -∆S Entropy increases from solid  liquid  gas

What is more random? NaCl (s) or NaCl (aq)? Entropy is increased when a soluble solid substance is dissolved.

When are rxns spontaneous? Systems in nature tend to undergo changes toward higher entropy and lower energy Meaning…reactions will be spontaneous (happen on their own) when they tend toward greater randomness (+∆S) and lower energy (-∆H)

Practice- Increase or Decrease in Entropy? NaCl (s)  Na+ (aq) + Cl- (aq) Increase H2O (l)  H2O (g) Increase AgCl (l)  AgCl (s) Decrease K+ (aq) + NO3- (aq)  KNO3 (s) Decrease

Entropy and Chemical Rxns Synthesis = increase in entropy Decomposition = decrease in entropy For chemical rxns involving gases, compare moles of gas on the left and right (ignore solids and liquids) More moles of gas = greater entropy ex) C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (l) ex) 2NO (g)  N2 (g) + O2 (g)

Review: What is equilibrium? At equilibrium…. The rate of the forward reaction equals the rate of the reverse reaction The concentrations of the reactants and products remain constant at equilibrium. Escalator , gondola ride

1. Solution Equilibrium Solution equilibrium exists when a solution is saturated. NaCl(s) Na+(aq) + Cl-(aq)

2. Phase Equilibrium

H2O(l) H2O(g) H2O(s) H2O(l) For water, at 100°C the rate of evaporation equals the rate of condensation. The amounts of liquid and gas will remain constant H2O(l) H2O(g) For water, at 0˚C, the rate of melting equals the rate of freezing; amounts of ice and liquid will remain constant. H2O(s) H2O(l)

3. Chemical Equilibrium A + B C + D The concentrations of the reactants and products remain constant at chemical equilibrium.

Le Chatelier’s Principle If a stress is applied to a system at equilibrium, the equilibrium will shift to reduce or alleviate the stress. Stresses that will affect a system at equilibrium: Concentration Temperature Pressure (gases only)

1. Concentration N2(g) + 3 H2(g) 2 NH3(g) + 91.8 kJ 2 NH3(g) + 91.8 kJ If the conc. of N2 or H2 increases, the forward reaction is favored to make more products N2(g) + 3 H2(g) 2 NH3(g) + 91.8 kJ If the conc. of NH3 increases, the reverse reaction is favored to make more reactants

2. Temperature 2 NH3(g) + 91.8 kJ N2(g) + 3 H2(g) N2(g) + 3 H2(g) If the temp increases, the reverse reaction is favored to use up the heat added N2(g) + 3 H2(g) 2 NH3(g) + 91.8 kJ If temp decreases, the forward reaction is favored to produce more heat

3. Pressure N2(g) + 3 H2(g) 2 NH3(g) + 91.8 kJ 2 NH3(g) + 91.8 kJ If pressure increases, equilibrium shifts to the side with the least # of moles of gas N2(g) + 3 H2(g) 2 NH3(g) + 91.8 kJ If pressure decreases, equilibrium shifts to the side with the greater # of moles of gas

Effect of Catalysts on Equilibrium Addition of a catalyst to a system at equilibrium will increase the rate of BOTH the forward and reverse rxns

The Common Ion Effect AgCl(s) Ag+(aq) + Cl-(aq) What happens to the equilibrium when NaCl(s) is added?

Warm-Up How much heat is released from the combustion of 0.260 moles of C3H8? How much heat is released when 42.0 grams of NH3 is synthesized from its elements?