Reversibility & Equilibrium (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards
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Define reversible reaction.
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Cards in this collection (42)
Define reversible reaction.
A reversible reaction is one in which the products can react with each other to re-form the original reactants. The reaction can occur in both directions and is represented using the symbol ⇌.
In a reversible reaction, if the forward reaction is endothermic, what must be true of the reverse reaction?
The reverse reaction must be exothermic.
The energy change in the reverse direction is equal in magnitude to the energy change in the forward direction. Energy is always conserved.
True or False?
Anhydrous copper sulfate is stored in a sealed container because it is toxic.
False.
Anhydrous copper sulfate is stored sealed to prevent it reacting with water vapour (moisture) in the air. If it absorbs moisture, it turns blue and can no longer be used to test for the presence of water.
Toxicity is a common but incorrect answer.
When anhydrous copper sulfate is added to water, it turns from ________ to ________. This reaction is ________ because heat is released.
When anhydrous copper sulfate is added to water, it turns from white to blue. This reaction is exothermic because heat is released.
What are the two gaseous products when ammonium chloride (NH4Cl) is heated, and what type of energy change does this represent?
The two products are ammonia (NH3) and hydrogen chloride (HCl). Only around half of students correctly identify HCl as the second product.
The forward reaction is endothermic (energy is absorbed).
A student writes: 'The equation shows the reaction is reversible because it has a reversible symbol.'
Why does this response gain no marks?
Exam mark schemes require students to either draw the ⇌ symbol or describe its appearance as two opposing half-arrows ot the reversible arrow.
Simply writing 'the reversible symbol' without drawing or clearly describing it gains zero marks.
Define water of crystallisation.
Water of crystallisation is the water chemically included within the structure of a hydrated salt, shown by a dot in the formula (e.g. CuSO4·5H2O). Removing it by heating gives an anhydrous salt; adding water reforms the hydrated salt.
How is anhydrous copper sulfate used as a test for water, and what result confirms water is present?
Add the unknown substance to anhydrous copper sulfate (white powder).
If water is present, the powder turns blue as hydrated copper(II) sulfate pentahydrate (CuSO4·5H2O) forms. This reverse reaction is exothermic.
True or False?
The reverse reaction in a reversible reaction releases a different amount of energy from that absorbed in the forward reaction.
False.
The energy transferred in both directions is exactly equal.
If the forward reaction absorbs 198 kJ/mol, the reverse reaction releases exactly 198 kJ/mol. The energy change is equal in magnitude but opposite in direction.
CuSO4·5H2O is ________ copper sulfate. When heated, it loses its ________ to form ________ copper sulfate, which is white.
CuSO4·5H2O is hydrated copper sulfate. When heated, it loses its water of crystallisation to form anhydrous copper sulfate, which is white.
An exam question states: 'Ammonia is a gas. Select the correct state symbol for ammonia.'
What is the correct state symbol?
The correct state symbol for a gas is (g).
Many students confuse state symbols and (aq) is incorrectly applied to gases.
The answer is actually given in the question, but many students who do not read carefully lose the mark.
When ammonium chloride is heated it decomposes into ammonia and hydrogen chloride gases.
What happens when these gases cool down in this reversible reaction?
The ammonia (NH3) and hydrogen chloride (HCl) gases recombine to re-form solid ammonium chloride (NH4Cl).
This reverse reaction is exothermic, releasing the same amount of energy that was absorbed during the endothermic forward reaction.
Define dynamic equilibrium.
Dynamic equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the reverse reaction. Both reactions continue at equal rates, so the concentrations of reactants and products remain constant (not zero; not equal to each other).
State the two features of a system at dynamic equilibrium.
The rate of the forward reaction equals the rate of the reverse reaction (both reactions are still occurring).
The concentrations of reactants and products remain constant (they are not equal to each other, just unchanging).
True or False?
On an equilibrium graph, equilibrium is reached at the point where the lines for reactants and products cross each other.
False.
The crossing point is where the amounts happen to be equal, which is not the definition of equilibrium.
Equilibrium is reached when the lines become level (horizontal) and stop changing.
Students who identify the crossing point as the equilibrium point are applying the 'amounts equal' misconception.
On an equilibrium graph, equilibrium is reached when the lines ________ out. Writing that the lines '________ straight' is not accepted because this could describe a vertical or sloping line.
On an equilibrium graph, equilibrium is reached when the lines level/flatten out. Writing that the lines 'go straight' is not accepted because this could describe a vertical or sloping line.
A student correctly writes: 'The number of moles of each gas is not changing at equilibrium.' They then add: 'so the reaction is no longer occurring.'
Why does this lose a mark?
Adding 'the reaction is no longer occurring' contradicts dynamic equilibrium: both the forward and reverse reactions are still occurring at equal rates.
This contradiction causes the examiner to revoke the marking point, even though the first part of the sentence was correct.
Why must equilibrium be established in a closed system?
In a closed system, no reactants or products can enter or leave. This allows the reverse reaction to build up until its rate equals the forward reaction rate.
In an open system, products escape and the reverse reaction cannot reach the same rate, so equilibrium cannot be established.
What is Le Chatelier's Principle?
Le Chatelier's Principle states that if a condition of a system at equilibrium is changed, the system will shift to counteract the change and re-establish equilibrium.
True or False?
When explaining the effect of a changing condition on equilibrium, you can explain the effect on the rate of reaction.
False.
Explaining the effect on the rate of reaction (for example, "particles move faster and collide more") does not gain credit.
You must state which way the equilibrium position shifts and how this affects the yield of products.
What three pieces of information must you include when explaining a shift in the equilibrium position?
The direction the equilibrium shifts (left or right)
Why it shifts: to counteract the change
The outcome, e.g. "more product is formed" or "a new equilibrium is established".
When a condition of a system at equilibrium changes, the equilibrium shifts to ________ the change. This produces ________ of the substance that was reduced, or reduces the amount of the substance that was ________, until a new equilibrium is established.
When a condition of a system at equilibrium changes, the equilibrium shifts to counteract the change. This produces more of the substance that was reduced, or reduces the amount of the substance that was added, until a new equilibrium is established.
A student writes: "The equilibrium would do the opposite."
What is missing from this answer?
"The opposite" is too vague. The student should state:
The direction the equilibrium shifts (left or right)
That it shifts to counteract the change (describing the change)
That a new equilibrium position is established
True or False?
The phrase "a new equilibrium would be established" is an optional conclusion that does not affect the marks awarded.
False.
"A new equilibrium would be established" is often an explicit marking point. Examiner reports confirm very few students include it, and its omission can cost a mark.
What happens to the equilibrium position when the concentration of a reactant is increased?
When the concentration of a reactant is increased, the equilibrium shifts to the right (forward direction), producing more product and reducing the excess reactant, until a new equilibrium is established.
True or False?
Increasing the concentration of a reactant increases the yield of product at equilibrium.
True.
Increasing reactant concentration shifts the equilibrium to the right, so more product is formed. The system opposes the change by consuming the added reactant until a new equilibrium is established.
What happens to the equilibrium position if the concentration of a product is increased?
The equilibrium shifts to the left (reverse direction). The system opposes the increase by converting excess product back into reactants, until a new equilibrium is established.
ICl (l) + Cl2 (g) ⇌ ICl3 (s)
Increasing the concentration of Cl2 shifts the equilibrium to the ________, forming more solid ICl3.
Decreasing the concentration of Cl2 shifts the equilibrium to the ________, forming more liquid ICl.
ICl (l) + Cl2 (g) ⇌ ICl3 (s)
Increasing the concentration of Cl2 shifts the equilibrium to the right, forming more solid ICl3.
Decreasing the concentration of Cl2 shifts the equilibrium to the left, forming more liquid ICl.
A student answers: 'The equilibrium shifts to the right to counteract the change.'
Why might this not gain full marks?
"To counteract the change" is too vague on its own. The answer must also specify what is being counteracted.
For example, that the equilibrium shifts to reduce the added reactant or to produce more product. A final conclusion that a new equilibrium is established is also often an explicit mark point.
True or False?
When the concentration of a reactant is increased, it is acceptable to mention the exothermic or endothermic direction of the reaction in your answer.
False.
Exothermic and endothermic language only applies when temperature is changed. Mentioning it in a concentration question does not gain credit and may contradict correct points.
What determines the direction of equilibrium shift when temperature is changed?
You must know whether the forward reaction is exothermic or endothermic.
Increasing temperature shifts equilibrium in the endothermic direction; decreasing temperature shifts equilibrium in the exothermic direction.
True or False?
A reversible reaction states "the forward reaction releases 198 kJ/mol of energy." Increasing the temperature will shift the equilibrium to the right.
False.
"Releases energy" means the forward reaction is exothermic.
Increasing temperature shifts the equilibrium in the endothermic direction, which is to the left (reverse reaction). The yield of products decreases.
ICl (l) + Cl2 (g) ⇌ ICl3 (s)
The forward reaction is exothermic. What happens to the equilibrium position of the mixture when heated?
The equilibrium shifts to the left (endothermic/reverse direction). More ICl is produced and the yield of ICl3 decreases.
For a reversible reaction where the forward reaction is exothermic:
Increasing temperature shifts the equilibrium to the ________, because the system shifts in the ________ direction. As a result, the yield of products ________.
For a reversible reaction where the forward reaction is exothermic:
Increasing temperature shifts the equilibrium to the left, because the system shifts in the endothermic direction. As a result, the yield of products decreases.
Why can't you use collision theory to explain the effect of temperature on equilibrium yield?
Collision theory explains the effect of temperature on rate — more frequent, more energetic collisions.
Equilibrium yield is determined by the position of equilibrium, which shifts according to Le Chatelier's principle, not by collision frequency.
When explaining the effect of increasing temperature on an equilibrium, what must you specify about the direction of the shift?
You must state the endothermic or exothermic direction — e.g. "the equilibrium shifts in the endothermic direction" or "to absorb the extra energy".
Only saying that it "counteracts the change" is too vague without naming which direction that is.
What happens to the equilibrium position when pressure is increased in a gaseous reaction?
When pressure is increased, the equilibrium shifts towards the side with fewer moles of gas, reducing the number of gas molecules and partially counteracting the pressure increase.
If both sides have equal moles of gas, there is no change in equilibrium position.
True or False?
Changing pressure affects the equilibrium position of all reversible reactions.
False.
Pressure changes only affect equilibria that involve gases. If the number of moles of gas is equal on both sides of the equation, changing pressure has no effect on the equilibrium position.
2NO2 (g) ⇌ N2O4 (g)
dark brown ⇌ colourless
What happens to the equilibrium position and the colour of the mixture when pressure is increased?
The equilibrium shifts to the right because the right side has fewer moles of gas (1 mole vs 2 moles).
More colourless N2O4 forms and the mixture becomes less brown.
2NO2 (g) ⇌ N2O4 (g)
The forward reaction produces ________ mole(s) of gas; the reverse reaction produces ________ mole(s).
Increasing pressure shifts the equilibrium to the ________ because it favours the side with ________ moles of gas.
2NO2 (g) ⇌ N2O4 (g)
The forward reaction produces 1 mole(s) of gas; the reverse reaction produces 2 mole(s).
Increasing pressure shifts the equilibrium to the right because it favours the side with fewer moles of gas.
True or False?
2HCl (g) ⇌ H2 (g) + Cl2 (g)
Increasing pressure shifts this equilibrium to the right because the left side has fewer molecules.
False.
The left side has 2 moles of gas (2 × HCl). The right side also has 2 moles (H2 + Cl2). Equal moles on both sides means no change in equilibrium position.
A common error is to think 2HCl represents just one molecule.
Why can't you use collision theory to explain the effect of pressure on equilibrium yield?
Collision theory explains the effect of pressure on rate — particles are closer together so collisions are more frequent.
Equilibrium yield depends on the position of equilibrium, which shifts towards the side with fewer moles of gas (Le Chatelier's principle).
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