What comes to mind when you see the two pictures?
Consider the following scenarios
The rate at which water runs into a bath is the same rate at which the water is leaving, so the water level does not appear to change, Click here to see an image.
At a road junction, the number of vehicles arriving at the same rate vehicles are leaving
Moving up a down escalator at the same speed as the escalator. Click here to see an image.
Here is another example.Imagine you are looking through the window of a popular restaurant during a busy lunch time. You can see that all of the restaurant’s 200 seats are taken. You come back 30 minutes later, and you see that all the seats are still occupied. However, you can see that people are entering the restaurant and other people leaving the restaurant. The same situation continues over the next 2 hours. The population remains constant at 200 people while all the time people are entering and leaving the restaurant. There is a balance between the number leaving and the number arriving. This state of balance can be described as a state of EQUILIBRIUM. If the same 200 people sat at the tables all the time, one would say that the situation was STATIC (unchanging). However, in the restaurant you ARE OBSERVING. There is motion as some customers arrive and others leave. The thing that remains constant is the balance between the number arriving and the number leaving. This is a DYNAMIC (moving) equilibrium. Click here for more.
Unit 1 Module 2: Kinetics & Equilibria
CHEMICAL EQUILIBRIUM
Principles of Chemical equilibriumSources: CAPE Chemitry Unit 1 guide, Chemistry for Cape by Susan Maraj, Understanding Chemistry for Advanced lever by Lister and Renshaw, AS and A level Chemistry by Lewis and Berry
Dynamic Equilibrium:
An equilibrium that exists in reversible reactions where the rate of the forward reaction=the rate of the reverse reaction
The video looks at dynamic equilibrium established between liquid water + gaseous water. This type of dynamic equilibrium involves phase changes which are physical processes so it can be referred to as a phase(dynamic) equilibrium or a physical (dynamic) equilibrium
DYNAMIC EQUILIBRIUM
Write a title here
VIDEO LOREM IPSUM
The videos show and talk about the dynamic equilibrium between Br2 (l) and Br2 (g). Notice in the second video the number of bromine gas molecules (concentration) is constant even though we can see evaporation and condensation happening.
Other examples of dynamic equilibrium with physical changes are (i) Dissolution (dissolving): preparing a saturated solution of copper (II) sulphate crystals or any other salt at room temperature. What does saturated mean? You may think nothing is happening, and you cannot see anything happening on the macroscopic level and although concentration of saturated solution stays the same, the ions of undissolved solid go into solution while the ions in solution go back to form the undissolved solid and (ii) Insoluble solids/sparingly soluble salts like lead iodide. Click here for more
Dynamic equilibrium with chemical changes
The previous examples all demonstrated dynamic equilibrium with physical changes. Examples of dynamic chemical equilibrium are seen in the pictures on the right.Click here to see another example.
DYNAMIC EQUILIBRIUMDYNAMIC: molecules of reactants are continually being converted to products and molecules of products are continually being converted to reactants EQUILIBRIUM: the rate of the above processes are equal. (i.e. the rate of the forward and backward/reverse reactions are equal). Click here for more.
Dynamic equilibrium
FEATURES OF A SYSTEM IN DYNAMIC EQUILIBRIUM
1. Macroscopic properties are constant
2. Microsocopic properties are continuous.
3. Equilibrium can be achieved from either direction.
4. Equilibrium can only be achieved in a closed system and the reaction must be reversible
5. The rate of the forward reaction equals the rate of the reverse reaction.
Graphical representations of equilibrium. Let's go back to kinetics. The graph on the top left shows [Reactant] changing with time. The graph on the top right shows [Product] changing with time. If we merge the two graphs we can get either one of the two images shown in the graphs at the bottom. Click here to see more.
Take a look at this graph. What do we see as being different from the graphs on the previous page?
VIDEO SUMMARIES OF DYNAMIC EQUILIBRIUM
The equilibrium law/expression
Kc the equilibrium constant
Equilibrium Law/Expression
For a system at equilibrium, there is a relationship between [products] to the [reactants] at a constant temperature and pressure. This relationship is called the equilibrium law or the equilibrium expression. Click here to see more.
We can therefore write a definition for the equilibrium constant Kc.Kc= the ratio of the concentration of the products to the concentration of the reactants at equilibrium each raised to the powers of their coefficients in the stoichiometric equation. You can also define Kc via an equation/expression. Click to see more.
Practice time: Basic calculations with Kc
Write down an expression for Kc and work out the units for each of the equations given in the four images below. Ignore the rest of the information. Click here to see the correct answers
Practice time:Calculating values for Kc
Use the above equation to answer the following question. When equilibrium has been established at 764 K, the mixture was found to contain: hydrogen 2.484 X 10-3 mol dm-3, iodine 2.514 x 10-3 mol dm-3, hydrogen iodide 1.695 x 10 -2 mol dm-3. Calculate a value for Kc at this temperature. Remember to first write the expression for Kc and substitute in the values afterwards. Do not forget your units. Click to see if your answer is correct.
Practice time: Calculating equilibrium concentrations given a Kc value
CAPE Unit 1 June 2012 Qu 5 (d) Phosphorous (V) chloride, PCl5 decomposes and forms an equilibrium mixture represented by the equation One equilibrium mixture at this temperature contains PCl5 and PCl3 at concentrations
of 0.20 mol dm–3 and 0.010 mol dm–3 respectively.
Given Kc at 250 °C = 0.19 mol dm–3, calculate the concentration of Cl2 in the
mixture. Click here when you have completed the question. View the video above for guidance.
CALCULATION OF KC USING THE ICE BOX METHOD
Practice time:Calculating values for Kc using the ICE box method
If an ester like ethyl ethanoate reacts with water in the presence of an acid catalyst, the following equilibrium is established
Exactly 1 mole of ethyl ethanoate was mixed with exactly 1 mole of water and allowed to reach equilibrium. The equilibrium mixture was analysed and found to contain 0.300 moles of ethanoic acid. Calculate a value for Kc at the temperature of this reaction?
VIDEO 2 : Calculating Kc using the ICE box method
Let us look at an example in the video where the stoichiometry of the equation is not 1:1. Let us practise with a past paper question June 2006. Click here for more.
Practice time: Calculating equilibrium concentrations given Kc when Kc is small using the ICE box method
CAPE Unit 1 June 2011 Qu 5At a certain temperature, Kc = 4.66 x 10–3 for the reaction N2O4(g) <----> 2NO2(g) ∆Hθ
= +58 kJ mol–1. (i) Calculate the equilibrium concentration of EACH gas, at the same temperature,
if 0.800 moles of N2O4 were injected into a closed container of volume 1 dm3
Homogeneous vs Heterogeneous equilibria
Heterogeneous vs Homogeneous equilibria
The equilbria we have looked at so far are called homogeneous equilibria where all the chemical species were in the SAME PHASE. When all species are not in the same phase, we refer to it as HETEROGENEOUS EQUILIBRIA. Examples are ionic compounds partially dissolved in water and the decomposition of calcium carbonate a.k.a. limestone. Click here
Determining Kc by experiment
Refer to Chapter 8.2 in the Cape Unit 1 guide
General experimental steps to work out Kc
Next, determine the concentrations of the other species in the reaction.Substitute values in the Kc expression and calculate the final value with units!
Set up the experiment using known concentrations of the reactants and catalyst (if any).Cover the expt and allow to reach equilibrium for about a week
After equilibrium has been reached, determine the concentrations of one of the reactants by using various experimental techniques.
Click on each step to read more!
What is the significance of the Kc value after it is worked out. Remember Kc is the ratio of [products] to [reactants] so it gives an indication of the extent of a chemical reaction in other words the position of equilibriumA large Kc value indicates that there is a large proportion of products to reactants. A small Kc value indicates that there are more reactants than products. Click to see more
The equilibrium constant Kp
Equilibrium constant for gases
Kp, the equilibrium constant for gaseous systems
It is more convenient to measure the pressure of gases at equilibrium rather than concentrations so Kp is used as the equilibrium constant for gaseous systems. The subscript p refers to the pressure of gases. Click below to see more
Key reminders to working out Kp
1. If the question gave you the partial pressures of the gases AT EQUILIBRIUM , you write your Kp expression and plug in the values given in the question 2. If the question gave you the initial partial pressures of the gases BEFORE EQUILIBRIUM and the partial pressures of ONE of the gases AT EQUILIBRIUM, you must use the ICE METHOD to work out the partial pressures of the other gases at equilibrium and then plug those values into the Kp expression (See June 2006 Qu 4) 3. If the question gives you the moles of the gases AT EQUILIBRIUM, you must work out the partial pressures of the gases at equilibrium first by using the formula partial pressure= mole fraction x total pressure and THEN PLUG those partial pressure values you calculated in the Kp expression (See June 2009 Qu 5) 4. If the question gives you the moles of the gases at the start of the experiment BEFORE EQUILIBRIUM and the moles of one of the gases at equilibrium, you must first use the ICE Method to calculate ALL THE MOLES AT EQUILIBRIUM, then work out the partial pressures using the formula as indicated in Point #3 above and then plug those values you calculated in the Kp expression (See June 2016 Qu 5)
Title 2
Title 2L
TLitle 2
Title 2
KKe
Title 2
Title 2Let us look
Let us look at some worked examples based on the tips on the previous page. Each question is linked to each tip. After attempting the questions, click to see the answers.
1) When A is heated in a closed system at 400 oC, the following equilibrium is set up A(g) <---> B(g) + C(g) The equilibrium partial pressures of the three gases were found to be A=5.1kPa ; B=95KPa; C=95kPa. Calculate Kp at this temperature. 2) 2.5 atm of nitrogen gas and 3 atm of hydrogen gas were placed in an empty container and allowed to reach equilibrium. It was found that the pressure of the nitrogen gas decreased by 0.5 atm. Estimate the value of Kp for this system. Equation: N2(g) + 3H2 (g) <-----> 2NH3 (g) 3. Go to CAPE Unit 1 guide Chapter 8.4 and look at Worked example 1. 4. When phosphorous ( V )chloride is heated to a sufficiently high temperature it vapourizes and dissociates according to the equation PCl5 (g)<------> PCl3 (g) + Cl2 (g) When 1 mole of PCl5 was heated in a closed container, the equilibrium pressure was found to be 100kPa and the equilibrium mixture contained 0.816 moles of chlorine. Calculate Kp at the temperature of the reaction
VIDEO
This question is a similar question to #4 on the previous page.
Video examples of more Kp calculations
Kp calculations are often very similar to Kc calculations involving the ICE method
Le Chatelier's Principle
Refer to Chatper 8.5 in the Cape Unit 1 guide
Factors affecting the position of equilibrium
Changes in concentration, temperature and pressure all impact on the position of equilibrium which can be explained by Le Chatelier's Principle
VIDEOS ON LE CHATELIER'S PRINCIPLE. Please support with content from the text book
Le Chatelier's Principle states that when a system in equilibrium is disturbed by small changes in conditions such as temperature, pressure or concentration, the position of equilibrium will shift (either to the left or to the right) to nullify/counteract or minimize the effect of the change on the system
The effect of temperature on a system in equilibrium is a bit tricky. The shift in the position of equilibrium depends on whether the reaction is endothermic or exothermic. In addition once temperature is involved the Kc value will change. Click here.
Industrial applications of Le Chatelier's Principle
Though Le Chatelier's Principle can predict theoretically the optimal conditions to produce a high yield of product, it does not take into consideration kinetic and other factors which may affect product yield. Click to see more
Equililbrium summary
This video is a crash course in equilibrium and summarizes the topic. Enjoy!!
THANKS!
Dynamic equilibrium: an equilibrium that occurs in reversible reactions where reactants and products are continuously being converted to each other and the rate of these conversions are equal i.e the rate of the forward reaction=the rate of the reverse reaction
Chemical equilibrium
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Transcript
What comes to mind when you see the two pictures?
Consider the following scenarios
The rate at which water runs into a bath is the same rate at which the water is leaving, so the water level does not appear to change, Click here to see an image.
At a road junction, the number of vehicles arriving at the same rate vehicles are leaving
Moving up a down escalator at the same speed as the escalator. Click here to see an image.
Here is another example.Imagine you are looking through the window of a popular restaurant during a busy lunch time. You can see that all of the restaurant’s 200 seats are taken. You come back 30 minutes later, and you see that all the seats are still occupied. However, you can see that people are entering the restaurant and other people leaving the restaurant. The same situation continues over the next 2 hours. The population remains constant at 200 people while all the time people are entering and leaving the restaurant. There is a balance between the number leaving and the number arriving. This state of balance can be described as a state of EQUILIBRIUM. If the same 200 people sat at the tables all the time, one would say that the situation was STATIC (unchanging). However, in the restaurant you ARE OBSERVING. There is motion as some customers arrive and others leave. The thing that remains constant is the balance between the number arriving and the number leaving. This is a DYNAMIC (moving) equilibrium. Click here for more.
Unit 1 Module 2: Kinetics & Equilibria
CHEMICAL EQUILIBRIUM
Principles of Chemical equilibriumSources: CAPE Chemitry Unit 1 guide, Chemistry for Cape by Susan Maraj, Understanding Chemistry for Advanced lever by Lister and Renshaw, AS and A level Chemistry by Lewis and Berry
Dynamic Equilibrium:
An equilibrium that exists in reversible reactions where the rate of the forward reaction=the rate of the reverse reaction
The video looks at dynamic equilibrium established between liquid water + gaseous water. This type of dynamic equilibrium involves phase changes which are physical processes so it can be referred to as a phase(dynamic) equilibrium or a physical (dynamic) equilibrium
DYNAMIC EQUILIBRIUM
Write a title here
VIDEO LOREM IPSUM
The videos show and talk about the dynamic equilibrium between Br2 (l) and Br2 (g). Notice in the second video the number of bromine gas molecules (concentration) is constant even though we can see evaporation and condensation happening.
Other examples of dynamic equilibrium with physical changes are (i) Dissolution (dissolving): preparing a saturated solution of copper (II) sulphate crystals or any other salt at room temperature. What does saturated mean? You may think nothing is happening, and you cannot see anything happening on the macroscopic level and although concentration of saturated solution stays the same, the ions of undissolved solid go into solution while the ions in solution go back to form the undissolved solid and (ii) Insoluble solids/sparingly soluble salts like lead iodide. Click here for more
Dynamic equilibrium with chemical changes
The previous examples all demonstrated dynamic equilibrium with physical changes. Examples of dynamic chemical equilibrium are seen in the pictures on the right.Click here to see another example.
DYNAMIC EQUILIBRIUMDYNAMIC: molecules of reactants are continually being converted to products and molecules of products are continually being converted to reactants EQUILIBRIUM: the rate of the above processes are equal. (i.e. the rate of the forward and backward/reverse reactions are equal). Click here for more.
Dynamic equilibrium
FEATURES OF A SYSTEM IN DYNAMIC EQUILIBRIUM
1. Macroscopic properties are constant
2. Microsocopic properties are continuous.
3. Equilibrium can be achieved from either direction.
4. Equilibrium can only be achieved in a closed system and the reaction must be reversible
5. The rate of the forward reaction equals the rate of the reverse reaction.
Graphical representations of equilibrium. Let's go back to kinetics. The graph on the top left shows [Reactant] changing with time. The graph on the top right shows [Product] changing with time. If we merge the two graphs we can get either one of the two images shown in the graphs at the bottom. Click here to see more.
Take a look at this graph. What do we see as being different from the graphs on the previous page?
VIDEO SUMMARIES OF DYNAMIC EQUILIBRIUM
The equilibrium law/expression
Kc the equilibrium constant
Equilibrium Law/Expression
For a system at equilibrium, there is a relationship between [products] to the [reactants] at a constant temperature and pressure. This relationship is called the equilibrium law or the equilibrium expression. Click here to see more.
We can therefore write a definition for the equilibrium constant Kc.Kc= the ratio of the concentration of the products to the concentration of the reactants at equilibrium each raised to the powers of their coefficients in the stoichiometric equation. You can also define Kc via an equation/expression. Click to see more.
Practice time: Basic calculations with Kc
Write down an expression for Kc and work out the units for each of the equations given in the four images below. Ignore the rest of the information. Click here to see the correct answers
Practice time:Calculating values for Kc
Use the above equation to answer the following question. When equilibrium has been established at 764 K, the mixture was found to contain: hydrogen 2.484 X 10-3 mol dm-3, iodine 2.514 x 10-3 mol dm-3, hydrogen iodide 1.695 x 10 -2 mol dm-3. Calculate a value for Kc at this temperature. Remember to first write the expression for Kc and substitute in the values afterwards. Do not forget your units. Click to see if your answer is correct.
Practice time: Calculating equilibrium concentrations given a Kc value
CAPE Unit 1 June 2012 Qu 5 (d) Phosphorous (V) chloride, PCl5 decomposes and forms an equilibrium mixture represented by the equation One equilibrium mixture at this temperature contains PCl5 and PCl3 at concentrations of 0.20 mol dm–3 and 0.010 mol dm–3 respectively. Given Kc at 250 °C = 0.19 mol dm–3, calculate the concentration of Cl2 in the mixture. Click here when you have completed the question. View the video above for guidance.
CALCULATION OF KC USING THE ICE BOX METHOD
Practice time:Calculating values for Kc using the ICE box method
If an ester like ethyl ethanoate reacts with water in the presence of an acid catalyst, the following equilibrium is established
Exactly 1 mole of ethyl ethanoate was mixed with exactly 1 mole of water and allowed to reach equilibrium. The equilibrium mixture was analysed and found to contain 0.300 moles of ethanoic acid. Calculate a value for Kc at the temperature of this reaction?
VIDEO 2 : Calculating Kc using the ICE box method
Let us look at an example in the video where the stoichiometry of the equation is not 1:1. Let us practise with a past paper question June 2006. Click here for more.
Practice time: Calculating equilibrium concentrations given Kc when Kc is small using the ICE box method
CAPE Unit 1 June 2011 Qu 5At a certain temperature, Kc = 4.66 x 10–3 for the reaction N2O4(g) <----> 2NO2(g) ∆Hθ = +58 kJ mol–1. (i) Calculate the equilibrium concentration of EACH gas, at the same temperature, if 0.800 moles of N2O4 were injected into a closed container of volume 1 dm3
Homogeneous vs Heterogeneous equilibria
Heterogeneous vs Homogeneous equilibria
The equilbria we have looked at so far are called homogeneous equilibria where all the chemical species were in the SAME PHASE. When all species are not in the same phase, we refer to it as HETEROGENEOUS EQUILIBRIA. Examples are ionic compounds partially dissolved in water and the decomposition of calcium carbonate a.k.a. limestone. Click here
Determining Kc by experiment
Refer to Chapter 8.2 in the Cape Unit 1 guide
General experimental steps to work out Kc
Next, determine the concentrations of the other species in the reaction.Substitute values in the Kc expression and calculate the final value with units!
Set up the experiment using known concentrations of the reactants and catalyst (if any).Cover the expt and allow to reach equilibrium for about a week
After equilibrium has been reached, determine the concentrations of one of the reactants by using various experimental techniques.
Click on each step to read more!
What is the significance of the Kc value after it is worked out. Remember Kc is the ratio of [products] to [reactants] so it gives an indication of the extent of a chemical reaction in other words the position of equilibriumA large Kc value indicates that there is a large proportion of products to reactants. A small Kc value indicates that there are more reactants than products. Click to see more
The equilibrium constant Kp
Equilibrium constant for gases
Kp, the equilibrium constant for gaseous systems
It is more convenient to measure the pressure of gases at equilibrium rather than concentrations so Kp is used as the equilibrium constant for gaseous systems. The subscript p refers to the pressure of gases. Click below to see more
Key reminders to working out Kp
1. If the question gave you the partial pressures of the gases AT EQUILIBRIUM , you write your Kp expression and plug in the values given in the question 2. If the question gave you the initial partial pressures of the gases BEFORE EQUILIBRIUM and the partial pressures of ONE of the gases AT EQUILIBRIUM, you must use the ICE METHOD to work out the partial pressures of the other gases at equilibrium and then plug those values into the Kp expression (See June 2006 Qu 4) 3. If the question gives you the moles of the gases AT EQUILIBRIUM, you must work out the partial pressures of the gases at equilibrium first by using the formula partial pressure= mole fraction x total pressure and THEN PLUG those partial pressure values you calculated in the Kp expression (See June 2009 Qu 5) 4. If the question gives you the moles of the gases at the start of the experiment BEFORE EQUILIBRIUM and the moles of one of the gases at equilibrium, you must first use the ICE Method to calculate ALL THE MOLES AT EQUILIBRIUM, then work out the partial pressures using the formula as indicated in Point #3 above and then plug those values you calculated in the Kp expression (See June 2016 Qu 5)
Title 2
Title 2L
TLitle 2
Title 2
KKe
Title 2
Title 2Let us look
Let us look at some worked examples based on the tips on the previous page. Each question is linked to each tip. After attempting the questions, click to see the answers.
1) When A is heated in a closed system at 400 oC, the following equilibrium is set up A(g) <---> B(g) + C(g) The equilibrium partial pressures of the three gases were found to be A=5.1kPa ; B=95KPa; C=95kPa. Calculate Kp at this temperature. 2) 2.5 atm of nitrogen gas and 3 atm of hydrogen gas were placed in an empty container and allowed to reach equilibrium. It was found that the pressure of the nitrogen gas decreased by 0.5 atm. Estimate the value of Kp for this system. Equation: N2(g) + 3H2 (g) <-----> 2NH3 (g) 3. Go to CAPE Unit 1 guide Chapter 8.4 and look at Worked example 1. 4. When phosphorous ( V )chloride is heated to a sufficiently high temperature it vapourizes and dissociates according to the equation PCl5 (g)<------> PCl3 (g) + Cl2 (g) When 1 mole of PCl5 was heated in a closed container, the equilibrium pressure was found to be 100kPa and the equilibrium mixture contained 0.816 moles of chlorine. Calculate Kp at the temperature of the reaction
VIDEO
This question is a similar question to #4 on the previous page.
Video examples of more Kp calculations
Kp calculations are often very similar to Kc calculations involving the ICE method
Le Chatelier's Principle
Refer to Chatper 8.5 in the Cape Unit 1 guide
Factors affecting the position of equilibrium
Changes in concentration, temperature and pressure all impact on the position of equilibrium which can be explained by Le Chatelier's Principle
VIDEOS ON LE CHATELIER'S PRINCIPLE. Please support with content from the text book
Le Chatelier's Principle states that when a system in equilibrium is disturbed by small changes in conditions such as temperature, pressure or concentration, the position of equilibrium will shift (either to the left or to the right) to nullify/counteract or minimize the effect of the change on the system
The effect of temperature on a system in equilibrium is a bit tricky. The shift in the position of equilibrium depends on whether the reaction is endothermic or exothermic. In addition once temperature is involved the Kc value will change. Click here.
Industrial applications of Le Chatelier's Principle
Though Le Chatelier's Principle can predict theoretically the optimal conditions to produce a high yield of product, it does not take into consideration kinetic and other factors which may affect product yield. Click to see more
Equililbrium summary
This video is a crash course in equilibrium and summarizes the topic. Enjoy!!
THANKS!
Dynamic equilibrium: an equilibrium that occurs in reversible reactions where reactants and products are continuously being converted to each other and the rate of these conversions are equal i.e the rate of the forward reaction=the rate of the reverse reaction