BIOC 2300 Lecture Notes - Lecture 17: Exergonic Reaction, Equilibrium Constant, Chemical Equilibrium

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rubycheetah358

3 May 2018

School

Dalhousie University

Department

Biochem & Molecular Biology

Course

BIOC 2300

Professor

Dr.Barbara Karten

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Thermodynamics, Control of Directionality and Flux

February 24, 2016

Thermodynamics – energy changes in metabolic pathway

• Endergonic vs exergonic

• Energy needed/released

Directionality – reaction proceeds in a specific direction

• How is direction achieved?

Flux – rate of the overall pathway

• Substrate  product

Gibb’s Free Energy:

• H = enthalphy (energy) * favoured if change is negative

• S = entropy (disorder) * favoured if change is positive

• Reaction is spontaneous of Gibb’s Free Energy is decreased

• (change of)G = (change of)H – T(change of)S

The chemical equilibrium is dynamic

• system is at equilibrium when forward and reverse reactions are equal

• does not mean the products and substrates have the same concentration

• Keq = equilibrium constant

o K = [C][D] / [A][B]

• Standard free energy change: G’ = - RT ln Keq *know for exam

o force when all reactants are present at equal concentrations  equilibrium state

o set characteristic of the reaction; driving force

o R – gas constant *do not need to memorize

o T – temperature

G’ reflects the free energy change for a system with standard concentration to react 

equilibrium

• Keq is inherent property

• A reaction with K=3 is as equilibrium when there is 3 times as much product as substrate

• Standard conditions are defined as 1M for all reactants

• G’ described for of a system moving from its standard conditions to equilibrium

G’ is defined as G under standard conditions.

• T = 298K = 25C

• Pressure = 1 atm

• Concentration of reactant = 1 M

• pH = 7.0

• water concentration = 55.5 M

G’ can be calculated from Keq

• G’ = -RT ln K

• Ex 1. What is G’ is K=3

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Related Questions

ree Energy and Chemical Equilibrium Why? The free energy change for a chemical reaction depends on the temperature concentrations of the reactants and the products. You, like chemists in and the research and manufacturing, can use the free energy change to identify suitable temperatures for reactions, to determine the direction a reaction will go to reu equilibrium, and to obtain values for equilibrium constants and equibrium ch concentrations. Learning Objectives Understand the relationship between the free energy change and the reaction quotient expression. Identify the effect of temperature on the free energy change and on equilibrium constants Success Criteria the free energy change for a reaction at various temperatures and various concentrations of reactants and products. Identify the direction a reaction will proceed spontaneously. Calculate one or more of ÎGo ,aHo, aso, and K given the others. Resources Olmsted and Williams Prerequisites Chemistry 3/e, Wiley, 2002) pp. 615 - 625, 721-729. emical equilibrium, spontaneous processes and free energy change, standard free energy change, reaction quotient, equilibrium constant Information ard The free energy change in a chemical reaction, DG, is related to the stand free energy change, AGe, and the concentrations of reactants and products by the following equation. AG, AGT+RT In(Q) 4Gy and Î = free energy change and standard free energy change at temperature T Q reaction quotient R ideal gas constant T temperature in K When reactants and products are in their standard states, their concentrations are 1 in the free energy when concentrations are 1 M is just the standard free energy change M, then Q is 1, In(Q1-1n(1) = 0, and ÎG = ÎGo. The change An increase in the entropy of the universe or a decrease in the free energy of the system is the driving force behind a spontaneous change. Concentrations in a chemical reaction change spontaneously to lower the free energy. Equilibrium is reached in a chemical because the free energy The freee reaction when the free energy can no longer decrease of the reactants equals the free energy of the products. for the reaction, aG, then is 0, Using Q = K at ã1/2005 all rights reserved 57

Which of the following is FALSE with respect to coupledreactions?

	they occur in ALL cells (bacteria, plants, animals, etc.)
	enzymes are typically involved
	endergonic reactions power exergonic reactions
	the coupled reaction has a ?G value that is negative
	none of the above (i.e., all of the above are TRUE)

Which of the following reactions is LEAST likely to bespontaneous? One with a:

	very large increase in entropy and a very large decrease inenthalpy
	very large increase in entropy and a very small increase inenthalpy
	very small decrease in entropy and a very large decrease inenthalpy
	very small increase in entropy and a very small decrease inenthalpy
	very small decrease in entropy and a very small increase inenthalpy

Energy of activation refers to the:

A)	?G of a reaction
B)	difference in energy between the transition state and theproduct
C)	difference in energy between substrate and product
D)	net change in free energy
E)	energy input needed to reach the transition state

Homeostasis refers to maintaining a relatively constantenvironment, and is achieved to a large extent by how enzymes areregulated. For example, when the end product of a pathway regulatesan enzyme that acts early in the same pathway, the effect is mostoften:

	stimulation if an anabolic pathway, and inhibition if acatabolic pathway
	inhibition if an anabolic pathway, and stimulation if acatabolic pathway
	stimulation, whether an anabolic or catabolic pathway
	inhibition, whether an anabolic or catabolic pathway

Enzymes function by:

	decreasing the free energy level of the reactants
	decreasing the free energy level of the transition state
	increasing the free energy level of the reactants
	changing delta G from positive to negative
	changing delta G from negative to positive
	increasing the free energy level of the products
	decreasing the free energy level of the products

6.	When carbohydrates are oxidized, the C

ivorylion772

Q1. Which of the following processes are spontaneous and which are non spontaneous.
a) The combustion of natural gas
b) Extraction of Aluminum metal from its ore
c) A ball rolling down a hill
d) A bike going up a hill

Q2. Predict the entropy change (positive or negative) for the following processes/reactions
a) a lake freezing
b) weeding a garden
c) N2(g) + 2 H2(g) ? N2H4 (g)
d) Mg(s) + 2H2O(l) ? Mg(OH)2 (s) + H2(g)

Q3. The element Cesium (Cs) freezes at 28.4 C and its molar enthalpy of fusion ΔH_fusion = 2.09 kJ/mol.
a) When molten Cs solidifies to Cs (s) at its normal melting point, is the entropy change ΔS positive or negative?
b) Calculate the entropy change S when 15.0 g of Cs(l) solidifies at 28.4 C.

Q4. Using the values for standard molar entropies ΔS_f° from the appendix in your textbook, calculate ΔS° for the following reactions.
a) Be(OH)₂ (s) ? BeO (s) + H2O (g)
b) Mg(OH)2 (s) + 2 HCl (g) ? MgCl2 (s) + 2H2O (l)
c) 2CH4(g) ? C2H6 (g) + H2(g)

Q5. Using the data in the appendix of your textbook, calculate the Gibbs free energy change ΔG° for the following reactions. In each case, indicate whether the reaction is spontaneous or not
a) H2 (g) + Cl2(g) ? 2HCl (g)
b) 2 POCl3 (g) ? 2 PCl3 (g) + O2 (g)

Q6. Consider the reaction 2 NO2 (g)---N2O4 (g)
a) Using the data in the appendix in your text, calculate the standard Gibbs free energy change (ΔG°) for the reaction at 298K.
b) For the above reaction, calculate the value of Keq at 298K
c) For the same reaction, calculate G at 298 K when the partial pressures for NO₂ and N2O4 are 0.40 atm and 0.36 atm, respectively.

Q7. Consider the reaction I2 (g) + Cl2 (g)----2 ICl (g) Kp = 81.9 at 298 K
a) Calculate the standard free energy change (ΔG°)
b) What will be the Free energy change when the above reaction is at equilibrium
c) Calculate G for the reaction at 298 K when PICl = 2.55 atm, PI2=0.325 atm and PCl2 = 0.221 atm

Q8. Consider the decomposition of Lead carbonate PbCO3 (s) ? PbO(s) + CO2 (g)
a) Calculate the Gibbs free energy change (ΔG°) for the reaction at298 K.
b) Calculate G for the reaction at 400 K.
c) Write down the equilibrium expression and calculate the value of Keq at 298 K
d) Calculate the partial pressure of CO2 at 298 K. (Hint: Look at the equilibrium expression and the value of Keq at 298 K)

Q9. For the reaction: 2Al(s) + 3/2 O2(g) ? Al2O3(s), ΔH^o_rxn = -1617 kJ/mol and ΔH^o_rxn = -1577 kJ/mol. Use this data to calculate ΔS^o_rxn reaction at 298 K.

Q10. Calculate the ΔG^o for the following reaction using the data from your text:
2SO2(g) + O2(g) ? 2SO3(g)

Q11. Using the thermodynamic data available from the Appendix(?Goâs), calculate ΔG^o rxn and calculate the equilibrium constant(K) for the reaction at 25 oC:
CaCO3(s) ? CaO(s) + CO2(g)

Q12. What is the minimum temperature required for the spontaneous conversion of CCl4(l) to CCl4(g) when ΔH°(vap) is 57.3 kJ/mol andΔS°(vap) is 164 J/(mol K)? reaction: CCl4(l) ? CCl4(g)

Q13. Given the following
Fe2O3(s) + 3CO(g) . ? 2Fe(s) + 3CO2(g) ΔG° = 29.4 kJ
3Fe2O3(s) + CO(g) . ? 2Fe3O4(s) + CO2(g) ΔG° = 61.6 kJ
Calculate ΔG° for Fe(s) + Fe2O3(s) + CO2(g) ? Fe3O4(s) +CO(g)

Q14. Calculate ΔG° rxn for 2CO(g) + O2(g) ? 2CO2(g) and then calculate ΔG for the reaction at 298K; if the pressure of CO =20. atm,P O2 = 10. atm and P CO2= 0.50 atm. Will increasing the pressure of the reactants make the reaction more or less spontaneous? Explain using your calculations and Le Chatelierâs principle.

Q15. Write the reaction for photosynthesis (production ofC6H12O6(s) and oxygen from carbon dioxide and water as a gas).Calculate ?H, ? S, and ?G for the reaction at standard state. Is the reaction spontaneous?

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BIOC 2300 Lecture Notes - Lecture 17: Exergonic Reaction, Equilibrium Constant, Chemical Equilibrium

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