Flashcards in Chemical Equilibrium Deck (34)
Gibbs energy is a spontaneous reaction?
In a spontaneous reaction at constant temperate and pressure Gibbs energy of a reaction < 0 so the system moves to a lower Gibbs energy, the Gibbs energy of the system decreases as the reaction proceeds
When is the lowest Gibbs energy change reached?
The lowest Gibbs energy is achieved when the reactants are completely converted to products
What happens when a reaction comes to equilibrium?
Starting with the reactants the system can move to lower Gibbs energy by forming products and proceeding in the forward direction, however once the system reaches the equilibrium point the Gibbs energy has reached a minimum. If the reaction continued the Gibbs energy would increase and this process would not be spontaneous, so the composition of the reaction mixture does not change beyond point equilibrium which represent the equilibrium composition of the reaction mixture
When does a system come to equilibrium?
A system comes to equilibrium when it reaches its minimum Gibbs energy
What happens when an equilibrium is reached?
Reactions do not stop when equilibrium is reached although the overall proportion of reactants and products does not change individual molecules may still react but not every molecule that undergoes the forward reaction another molecule will undergo the reverse reaction, there is a dynamic equilibrium
What is the equilibrium constant K?
The composition of a reaction mixture when it comes to equilibrium is described by an equilibrium constant K
What is Kp?
For gas phase reactions an equilibrium constant Kp calculated in terms of the equilibrium partial pressure Peqm of the component is usually used, the units of Kp depend on the stoichometry of the reaction
What is Kp?
For reactions in solution Kc calculated in terms of the molar concentrations is more useful, in dilute solutions the water term is essentially constant and a new equilibrium constant Ka is defined that incorporates Ka, this is called the acidity constant and is a measure of the strength of an acid
What is K?
Defined as the ratio of the thermodynamic activities of the products at equilibrium to those of the reactants
What is the activity of a gas?
The ratio of its partial pressure measured in bar to the standard state pressure
What is the activity of a solution?
The ratio of its molar concentration to the standard
What does the activity of a gas and solution being measured mean?
K has the same numerical value as Kp if partial pressures are measured in bar or as Kc if concentration are measured in moldm-3
Units of K?
It is important to remember that activities have no units so that K has no units and is dimensionless
Summary of Kc?
Equilibrium constant in terms of concentrations units in terms of concentrations moldm-3 used for reactions in solution and sometimes for reactions in the gas phase
Summary of Kp?
Equilibrium in terms of pressure units in terms of pressure used for reactions in the gas phase
Summary of K?
Equilibrium constant in terms of activities no units since activities are dimensionless used for all reactions
What is Ksp?
Thermodynamic equilibrium constant sometimes defined in concentrations rather than activity, useful for calculating the solubility of sparingly soluble compounds, the molar solubility S is measured in moldm-3 and gives the number of moles of solute dissolved in 1dm3 of a saturated solution, K is an equilibrium constant and does not change at a fixed temperature therefore addition of one of these ions will reduce the concentration of the other
What does the reaction quotient Q do?
If a reaction is not at equilibrium the amounts of products and reactants will change until they reach equilibrium composition. The Gibbs energy of the system changes as the reaction proceeds since the proportions of reactant and product change. To investigate how G changes you need a measure of the composition of the reaction mixture as it changes during the reaction, the reaction quotient can do this
What is the value of Q at equilibrium?
When the system reaches equilibrium the value of Q is the same as the value of the equilibrium constant K
When can Q can be measured?
The value of Q can be calculated at any stage during the reaction and changes as the reaction proceeds towards equilibrium, if Q does not equal K the reaction moves so as to bing it to equilibrium so that Q=K
How can the reaction quotient provide a way to predict the direction of a reaction?
If the Gibbs energy of reaction < 0 or Q < K then the forward reaction proceeds
If the Gibbs energy of reaction > 0 or Q > K then the reverse reaction proceeds
If the Gibbs energy of reaction is equal to zero or Q = K the system is at equilibrium
How does the value of K give some information about the composition of the reaction mixture at equilibrium?
If K is large K >> 1 then the equilibrium mixture will contain mainly product, if K is small K << 1 then the reaction does not proceed vert far and little product forms. If K=1 then the equilibrium mixture will contain substantial amount of both products and reactants
Le Chateliers principle?
When a change is made to a system in dynamic equilibrium the system responds to minimise the effect of the change
Why does pressure and temperature have an effect on the reactions?
Because of the way they influence of the Gibbs energy change of reaction and the equilibrium constant and hence the position of equilibrium
Changing the pressure?
Change in Gibbs reaction energy is the value at 1 bar so by definition does not change with changing pressure therefore the equilibrium constant does not vary with pressure at constant temperature, this however does not mean that pressure has no effect on the position of equilibrium and the composition of the equilibrium
What is the partial pressure given by?
PA = XA x Ptotal
where P is the partial pressure of a component A in a mixture XA is the mole fraction of component A and p total is the total pressure of the system
How will the composition change if p total changes?
K is a constant at constant temperature so if p total changes the mole fractions must change to compensate, if it increases the mole fractions must change to keep k constant, however changing the pressure does not always change the position of equilibrium if there are the same number of moles on each side the p total term will therefore cancel out in the expression for K
Changing the amount of a component?
Adding or removing one of the components from a reaction mixture will affect the concentration or pressure of the others in the equilibrium mixture
Changing the temperature?
Le Chateliers principle states that the equilibrium position for an exothermic reaction will move to favour the reactants if the temperature is increase, for an endothermic reaction an increased temperature favours the product