Flashcards

kp

megan trent A level Chemistry English 39 cards

Cards in this deck (39)

1GasesMolecules of a gas are in constant and random motion. The pressure of a gas is caused by the frequency and momentum of collisions of the molecules with the container walls.
2In any mixture of gases every gas molecule contributes to the total pressure. The contribution each gas makes to the total pressure is calledthe partial pressure, p.
3The partial pressure of a gas, pAin a mixture of gases is the pressure that the gas A would exert if it were alone in the container at that particular temperature
4The total pressure of a mixture of ideal gases, Ptotal is the sum of their individual partial pressures.Ptotal = pA + pB + pC + ….
5The partial pressure of, say, oxygen in a mixture of gases is written asp(O2)
6As one mole of any gas occupies the same volume the partial pressure of a gas can also be expressed as:-Partial Pressure, p, = Molar fraction x Total Pressure
pA = XA x Ptotal
7Where mole fraction of A is $Where mole fraction of A is$
8An application of thisScuba divers need to know a great deal about the constituents of the air they are breathing. The water above a diver exerts considerable pressure on the diver's body. At depths below about 30 m, the partial pressure of nitrogen increases to a value that may lead to nitrogen narcosis. This results from the anaesthetic effect of nitrogen under pressure, making a diver behave as if intoxicated by alcohol.
9Kp =Equilibrium constant for reactions involving gases
10For gases, it is convenient to use partial pressures in the equilibrium expression, since the partial pressure of a gas in a mixture of gases is proportional toits molar fraction or concentration.
11Writing expressions for Kp e.g. (1 H2(g) + I2(g) ⇌ 2HI(g)
12Partial pressure =molar fraction x total pressure
13Expressions for Kp do not include values for solid and liquid phases. e.g. CaCO3(s) ⇌ CaO(s) + CO2(g)Kp = pCO2(g)eqm
14Equilibrium constants, Entropy and Gibbs energy.In this topic we have seen how the equilibrium constant K, is related to the direction and extent of a change. The topic of Entropy introduced the concept that the spontaneous direction of change is determined by Δ Sοtotal being positive and therefore Δ G being negative
15The equilibrium constant is related to the total entropy change:
16This can be rearrangedIn this equation K has no units, it is known as the thermodymanic equilibrium constant.
17what two equations Can be combined to study the effect of changing temperature on the equilibrium constant and hence the position of equilibrium
18what does this equation demonstrateThis equation demonstrates how the equilibrium constant changes with temperature. Whether the change is positive or negative depends on the sign of ΔH
19From this we see that the variable in the equation istemperature, all other terms are temp dependant
20Explain the change in this variable on the position of equilibria, consider both an exothermic and endothermic reaction.Exothermic reaction; The sign for ΔH is negative and therefore the term -ΔH/RT is positive, as the temperature increases this term becomes less positive and therefore the value of K decreases hence the position of equilibria shifts to the left (favours the reactants)
Endothermic reaction; The sign for ΔH is positive and therefore the term -ΔH/RT is negative, as the temperature increases this term becomes less negative and therefore the value of K increases hence the position of equilibria shifts to the right (favours the products)
21Effect of pressureUnlike temperature, pressure has no effect on the value of K, it changes the position of equilibria but NOT the value of the constant. This should be evident from the equations on the previous page, pressure is not a term.
22Effect of pressure Consider the equation we started with H2(g) + I2(g) ⇌ 2HI(g) Write the Kc equation for this reaction. Explain the effect on the position of equilibrium of increasing the concentration of hydrogen at constant temperature and volume.Increasing the concentration of hydrogen does not change the value of Kc. The bottom (the denominator) becomes larger. Therefore the equilibria position shifts to favour the products to bring Kc back to its original value.
The main point to remember is that Kc is a constant at a particular temperature.
23Consider the Haber process. What would happen if the total pressure was doubled, keeping temperature and volume the same. 1
24Consider the Haber process. What would happen if the total pressure was doubled, keeping temperature and volume the same. 2Let the equilibria partial pressures of N2 = a, H2 = b, and NH3 = c.
The expression is then
25Consider the Haber process. What would happen if the total pressure was doubled, keeping temperature and volume the same. 3Now, for example, let us double the total pressure. At that instant all the partial pressures are also doubled so that pNH3= 2c, pH2=2b and pN2=2a
Substituting those values into our Kp expression we get
26Consider the Haber process. What would happen if the total pressure was doubled, keeping temperature and volume the same. 4The equilibrium constant ratio is now ¼ the value of Kp, i.e. the system is NOT at equilibrium. Therefore the system will change to restore equilibrium. Some to the hydrogen and nitrogen must react to form more ammonia thus reducing the ab3 value and increasing the c2 until the equilibrium constant ratio once more equals Kp
This means that increasing the pressure will shift the equilibria position to the right hand side producing more ammonia.
27The basis of equilibrium
28Temperature affects the value of KA change in T cause a change in ΔSsurr and henceΔStotal. This causes a change in K which alters the position of equilibrium.
An increase in temperature makes - ΔH/T a smaller number, hence ΔSsurroundings a smaller positive number (if exothermic) or a smaller negative number (if endothermic)
This cause ΔStotal and hence K to get smaller for exothermic reactions and bigger for endothermic reactions.
If K decreases, the position of equilibrium moves to the left and if K increases, the position of equilibrium moves to the right
NOTE position moves, so K alters.
29Pressure has no effect on the value of KIf there are a different number of gas moles on each side, the concentration / partial pressure term will alter.
K no longer equals the concentration / partial pressure term
The position of equilibrium moves until the concentration / partial pressure term once more equals the unchanged K
30A method for the calculations of:- Kc and Kp expressions
31the total pressure is equal to the sum of the partial pressures
32partial pressures can be worked out from mole fractions $partial pressures can be worked out from mole fractions$
33the equilibrium constant Kp is calculated from partial pressures
34Kp can be used to find partial pressures
35Kp for heterogeneous equilibria still only includes gases
36Le Chatelier's principle and equilibrium constants if conditions change the position of equilibrium will move
37Le Chatelier's principle and equilibrium constants temperature changes alter the equilibrium constant
38general rule for what happens to an equilibrium constant when you change the temp of a reaction
39concentration and pressure changes don't affect the equilibrium constant