Topic 1b: Factors Affecting Equilibrium Flashcards
State Le Chatelier’s principle
Le Chatelier’s principle states that when a system in dynamic equilibrium is disturbed, the equilibrium conditions shift in such a way to partially oppose the change.
Define pressure and identify how volume and pressure are related
Pressure is the force per unit area where as volume decreases pressure increases meaning the relationship is inversely proportional
Identify what factors may change the position of equilibrium
- adding or removing reactant or product
- changing the pressure by changing the volume of the container (gases)
- dilution (equilibria in solution)
- changing temperature
Describe collision theory
Collision theory states that for a reaction to occur, the reactant particles must collide with the correct orientation/allignment. The more successful collisions that occur within a specific time period, the faster the reaction rate
Using the reaction below, predict the effect of adding N2 gas to the equilibrium using Le Chatelier’s principle
Le Chatelier’s principle states that when a system in dynamic equilibrium is disturbed, the equilibrium conditions shift in such a way to partially oppose the change.
Therefore, according to Le Chatelier’s principle, the system will shift to decrease the concentration of the added N2, so a net forward reaction will occur
Even though the concentration of N2 decreases with the new established equilibrium, its final concentration is still higher than the original since the change cannot be fully reverse, according to Le Chatelier’s principle.
Explain the effect of adding N2 in the reaction below using collision theory and reaction rates
Collision theory states that for a reaction to occur, the reactant particles must collide with the correct orientation/allignment. Because the concentration of N2 has increased, there are more collisions with N2 molecules meaning the foward reaction occurs more rapidly than the reverse reaction. These particles collide, reacting successfully, and increasing the rate of the forward reaction until the rate of the foward and reverse reaction become equal again. This occurs because the reactant molecules are used up so there are less collisions and more product molecules are created which increases their rate of collisions. Once foward and reverse reactions are equal again, a new equilibrium has formed. However, this doesn’t mean the rates are the same as before or the concentration is the same as before. It would actually be slightly higher because there is more solution since N2 was added.
Describe what the rate-time graph for adding a reactant looks like
Because a substance has suddenly been added there is sudden spike in the reaction rate of that substance and the substances it reacts with. This rate will gradually decrease as this substance is converted into products. Since more products are being gradually added the rate of the reverse reaction gradually increases. This continues until the foward and reverse reaction is the same and reaches a new equilibrium.
Describe what the concentration-time graph for adding a reactant look like
There is a sudden spike in the concentration of the reactant added. To restore equilibrium and use up this reactant, the foward reaction would occur which decreases the concentration of this reactant and other reactants that it reacts with. Meanwhile, since these reactants are being converted into products the concentration of products gradually increases. The amount which the concentration of reactants and products increases or decreases depends on the stoichiometric ratios (the coefficients in the chemical equation). For an equation 2A <-> B: A would decrease by 2 times the amount B would increase.
Describe what the concentration-time graph looks like when more NH3 is added in the equation below
Since, NH3 is a product, the reverse occurs compared to when a reactant is added. The concentration of NH3 spikes and then begins to decrease. Just because it is considered a ‘product’ does not mean its concentration cannot decrease to create more ‘reactants.’ Equilibrium consists of a foward and reverse reaction with either being able to increase or decrease.
Describe what Le Chatelier’s principle tells you about increasing pressure
An equilibrium system will respond to an increase in pressure by adjusting to reduce the pressure (reverts the change).
Therefore, the position of the equilibrium will move in the direction of the side with the fewest gaseous particles.
Explain the effect of an increase in pressure using collision theory
In a greater pressure environment (induced by decreasing volume), the gases molecules are closer to each other and collisions between molecules become more frequent.
The rate of the reaction involving the greater number of molecules becomes greater than the rate of reaction between the smaller number of molecules since there are more particles to collide.
Overtime, the side with the less gaseous molecules has more of its molecules formed so they collide more frequently to create more reverse reactions. Eventually, the rates of reactions of the foward and reverse reactions converge until a new equilibrium is established
Identify which states of matter pressure changes work on
Pressure changes don’t affect the equilibrium position of liquid or solid substances because they are already too tightly packed for pressure changes to affect collisions.
Therefore, they only affect equilibria in gaseous phases.
Draw and explain what the concentration-time graph for an almost instantaneous volume decrease in equilibrium looks like
As volume decreases pressure increases and concentration of all molecules increases since concentration = amount / volume. Therefore, there would be a spike in the concentration of all molecules. After the spike, the system will shift to decrease pressure. Therefore, the rate of reaction will favour the side with the least number of gaseous particles. Molecules on the side with more gaseous molecules will have their concentration gradually increase and molecules on the side with less gaseous molecules will increase in concentration gradually. This is until concentrations plateau indicating a new equilibrium. Notice the new equilibrium does not have the same concentration as before the change.
Explain what changing pressure by adding an inert gas does
The presence of an additional gas does not change the concentration of the reactants and products, and therefore, there is no effect on the position of the equilibrium. This is because concentration is amount (of a specific molecule) / volume. And adding an inert gas does not affect either of these variables.
Using collision theory, any collisions with inert gas molecules will not produce a successful reaction, so no net reaction occurs.
Describe what Le Chatelier’s principle tells you about diluting solutions
Diluting by adding water reduces the number of particles per volume (concentration). Therefore, there is a shift in the position of equilibrium towards the side that produces the greater number of dissolved particles. This therefore, increases the number of dissolved particles per water to oppose the change.
Draw and describe what the concentration-time graph for a dilution look like
This is essentially the opposite of a pressure change for gases. Because it has been diluted, the concentration immediately drops for all substances. Then, substances on the side with the most dissolvable molecules will increase and the other substances will decrease as the system attempts to revert the change by increasing the number of dissolved particles per volume.
The concentrations will be lower after the change despite the equilibrium reacting to oppose the change because the equilibrium can only partially oppose the change.
A catalyst lowers the activation energy of the forward and reverse reactions by the …… amount
same
Draw and describe the concentration-time graph for temperature change
It is not an instantaneous change in concentration; it’s a gradual increase/decrease.
The rate of the endothermic reaction will increase when temperature increases and therefore the products of the endothermic reaction will gradually increase.
The rate of the exothermic reaction will decrease when temperature increases and therefore the products of the endothermic reaction will gradually decrease.
The opposite will occur for a decrease in temperature.
Explain how increasing temperature affects an exothermic equilibrium solution using activation energies
At higher temperatures, the rate of both forward and reverse reactions increase. One is exothermic, and the other is endothermic
Because the activation energy for the endothermic reaction is greater than for the exothermic reaction, the increased energy means there is a greater proportion of molecules with the necessary energy to overcome the activation energy barrier.
Therefore, the rate of this reaction will increase more than the rate of the exothermic reaction. Since the exothermic reaction is the foward reaction, there is a net increase in the reverse reacton.
Note: you may need to mention how the equilibrium constant changes depending on the question (Refer to Topic 1: Equilibrium Constants for relevant flashcards)
Describe what a rate-time graph looks like for a pressure increase
Because, pressure has increased there is more collisions so there is an increase for both the foward and reverse reaction. However, the reaction with the least number of gaseous molecules is favoured to decrease pressure. Therefore, this reaction would have a greater increase in reaction rate. Eventually, reaction rate will be the same for both the foward and reverse reaction meaning equilibrium has been restored. As can be seen on the graph, the rate of both reactions is higher than the previous equilibrium.
Identify the indicators of chemical equilibrium for rate-time and concentration-time graphs
Draw and describe the reaction rate-time graph for a temperature change
For a temperature increase, the rate of both reactions will increases because all molecules have greater energy so they collide more (because they move faster) and are more likely to have sufficient energy to meet the activation energy. This is seen on the rate-time graph as both reactions having an spike in reaction rate. However, the endothermic reaction will increase more and therefore will have a greater increase in rate of reaction. Therefore, the spike in reaction rate will be greater for this reaction on the graph. Then as the endothermic reaction produces more products the rate of the reverse reaction will increase as their is more products to collide and the endothermic reaction will begin to decrease since there are less reactants to collide. Eventually, an equilibrium will be reached where the rate of the foward and reverse reaction is equal.
