1.7 Simple equilibria and acid-base reactions Flashcards
What is meant by a ‘reversible reaction’?
A reversible reaction is a reaction in which the products can react together to form the original reactants.
What is meant by ‘dynamic equilibrium’?
Dynamic equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the backwards reaction.
The concentration of the reactants and products are constant.
What is Le Chatelier’s principle?
If a change is made to the conditions of a reversible reaction, the position of equilibrium will move to counteract the change.
How does increasing the temperature affect the equilibrium position?
Increasing the temperature will favour the endothermic reaction, so the equilibrium will shift towards the products of the endothermic reaction.
How does decreasing the temperature affect the equilibrium position?
Decreasing the temperature will favour the exothermic reaction, so the equilibrium will shift towards the products of the exothermic reaction.
How does increasing the pressure affect the equilibrium position?
Increasing the pressure favours the side of the reaction with fewer moles of gas, so the position of the equilibrium will move to that side.
How does increasing the concentration of the reactants affect the position of equilibrium?
Equilibrium will shift to the right to increase the concentration of the products.
What is Kc?
The equilibrium constant.
The magnitude indicates whether there are more reactants or products in an equilibrium system.
How do you calculate Kc?
aA + bB ⇌ cC + dD
Kc = [C]ᶜ[D]ᵈ/ [A]ª[B]ᵇ
Deduce an expression for Kc for the equation below:
3H₂(g) + N₂(g) ⇌ 2NH₃(g)
Kc =[NH₃]²/[N₂][H₂]³
How do you work out the units for Kc from the expression?
Kc =[NH₃]²/[N₂][H₂]³
Substitute in the concentration units and cancel down:
Kc =[NH₃]²/[N₂][H₂]³
Kc = (mol dm⁻³)² / mol dm⁻³ x (mol dm⁻³)³
= mol² dm⁻⁶/ mol⁴ dm⁻¹² = dm⁶ mol⁻²
What is the effect on Kc if the concentration of the reactants are increased?
Concentration has no effect on Kc
What is the effect on Kc if the pressure of the system is increased?
Pressure has no effect on Kc.
Why does the addition of a catalyst not affect the value of Kc?
A catalyst does not move the position of equilibrium, it only increases the rate at which equilibrium is reached.
If the forward reaction of a reversible reaction is endothermic, what effect will increasing the temperature have on the position of equilibrium?
Increasing the temperature will favour the forward reaction so the position of equilibrium will shift to the right in order to oppose the change.
How does increasing the pressure affect the position of equilibrium of the following reaction?
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
When there is an increase in pressure, the position of equilibrium shifts to the side with fewer moles of gas. Therefore the position of equilibrium will shift to the right.
How does decreasing the pressure affect the position of equilibrium of the following reaction?
H₂(g)+ I₂(g) ⇌ 2HI(g)
There is an equal number of molecules of gas on either side of the equation so the position of equilibrium is not affected by a change in pressure.
If the forward reaction is exothermic, what effect will increasing the temperature have on the value of Kc?
If the temperature is increased then the backwards endothermic reaction will be favoured.
This means the position of equilibrium will move to the left and the concentration of reactants compared to products will increase.
Therefore the value of Kc will decrease as the denominator of the Kc.
expression is increasing.
What is a Brønsted-Lowry acid?
A proton donor.
What is a Brønsted-Lowry base?
A proton acceptor.
What do acid-base reactions involve the transfer of?
Protons (H⁺ ions).
Acids donate H⁺(aq) ions to the bases, which accept the H⁺(aq) ions.
What is the proton donor and proton acceptor in this reaction:
HNO₂(s) + H₂O(l) → H₃O⁺ (aq) + NO₂⁻(aq)
Proton donor: HNO₂(aq)
Proton acceptor: H₂O(l)
What is pH?
A value which expresses the acidity or alkalinity of a substance, on a scale where 7 is neutral.
It is a simple way of measuring hydrogen ion concentration.
How do you calculate the pH of a strong acid?
HA → H⁺ + A⁻
For a strong acid the concentration of
acid = concentration of H⁺ ions.
pH = -log[H⁺]
What expression can be used to calculate the hydrogen ion concentration of a strong acid, given its pH?
[H⁺] = 10⁻ᵖᴴ
List the apparatus required to carry out a titration
- Burette
- Conical flask
- Pipette and pipette filler
- Funnel
- Clamp and stand
- White tile
Describe how to carry out a standard acid/alkali titration to find the concentration of the alkali
- Add acid to a burette. Use the pipette to add 25 cm’ of alkali to a conical flask.
- Add a few drops of indicator to the conical flask.
- Record initial burette reading. Slowly add the acid to the conical flask, swirling constantly.
- When the solution in the conical flask changes colour, stop adding the acid and record the final volume in the burette.
- Calculate the total volume of acid added from the burette.
Why must the conical flask be swirled during a titration?
To ensure all the particles react.
Why is a white tile used when carrying out a titration?
The white tile is placed under the conical flask so that the colour change of the indicator is easier to see.
What is a strong acid?
A strong acid is an acid that completely dissociates in solution to form hydrogen ions:
HCl(aq) → H⁺(aq) + Cl⁻(aq)
What is a strong base?
A strong base is a base that completely dissociates in solution to form hydroxide ions:
NaOH (aq) → Na⁺(aq) + OH⁻(aq)
What is a weak acid?
A weak acid is an acid that only partially dissociates in solution:
CH₃COOH (aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
What is a weak acid?
A weak acid is an acid that only partially dissociates in solution:
CH₃COOH (aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
What is a weak base?
A weak base is a base that only partially dissociates in solution:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
What apparatus is required to prepare a salt by titration?
- 50 cm³ burette
- 25 cm³ pipette and filler
- Conical flasks
- Funnel
- Evaporating basin
What is a suitable indicator to use in a titration between HCI and NaOH?
Phenolphthalein
What is the colour change of phenolphthalein and at what pH does the colour change occur?
- Colourless in acid
- Pink in alkali
Phenolphthalein will change colour in the pH range 8.3-10.
What are the hazards associated with HCI, NaOH and phenolphthalein?
HCl - irritant
NaOH - irritant
Phenolphthalein - flammable
Describe how to prepare a salt by a titration between NaOH and HCl
- Using a pipette, measure 25cm³ of NaOH and pour it into a conical flask.
- Add two drops of phenolphthalein.
- Pour the HCl into a burette and record the initial volume.
- Add the HCl from the burette into the conical flask a little at the time while swirling the flask.
- When the phenolphthalein starts to turn from pink to colourless, add the HCl solution a drop at a time until one drop is sufficient to turn the solution colourless.
- Record the volume of HCl solution added.
- Carry out the titration again using 25cm³ of NaOH solution and exactly the same volume of HCI.
- Do not add the indicator this time.
- Heat the solution from the conical flask in an evaporating basin until its volume decreases by half.
- Let the evaporating basin cool so that crystals form.
What is the chemical equation for the reaction between NaOH and HCI?
HCI + NaOH → NaCl+ H₂O
What type of reaction takes place between HCl and NaOH?
Neutralisation
Why should the burette be filled below eye level?
The burette should be filled below eye level so that if any of the acid spills whilst being poured in, it will not splash into your face.
Why is the titration repeated without the indicator?
This ensures that the salt produced in the repeated titration will not be contaminated with phenolphthalein.
Why are titrations usually carried out on a white tile?
The white tile allows the point of colour change to be easily identified.
How can you calculate how much HCl was required to neutralise the NaOH?
Volume of HCl =
Initial burette reading - final burette reading
Why is the NaOH placed in a conical flask?
The conical flask allows the mixture to be swirled without losing any of the contents.
Why must the reaction mixture be swirled during the titration?
Swirling ensures all the reacting particles collide and react. This helps to give a more accurate end point for the reaction.
Why are burettes and pipettes always used in titrations?
Burettes and pipettes measure the volumes of solutions very precisely.
What is a standard solution?
A solution with a known concentration.
What apparatus is required to make up a standard solution?
- Weighing bottle or boat
- 250 cm³ volumetric flask
- Digital mass balance
- Funnel
- 250 cm³ beaker
- Glass rod
- Pipette
Describe how to make up a standard solution of anhydrous sodium carbonate
- Accurately weigh out approximately 2.75g of anhydrous Na₂CO₃ into a weighing bottle.
Record the mass. - Tip the solid into a 250 cm³ beaker and reweigh the weighing bottle.
- Dissolve the solid in deionised water, stirring with a glass rod. Do not add more than 150 cm³ of distilled water.
- Pour the solution into the 250 cm³ volumetric flask via a funnel.
- Rinse the beaker and glass rod three times and transfer the washings into the flask.
- Make the solution up to the mark with distilled water so that the bottom of the meniscus is level with the graduation mark.
- Add the stopper and shake the mixture thoroughly.
Why must the weighing bottle be reweighed after the contents are transferred to a beaker?
This means that the exact mass of solid transferred can be calculated - as some traces of the solid may have been left behind in the weighing bottle.
When making up a standard solution, why is it important to not add more than 150 cm³ of deionised water when dissolving the solid?
Exactly 250 cm³ of deionised water will be added in total.
Only 150 cm³ should be added to dissolve the solid as this is enough to dissolve it and this leaves enough volume left for the washings, allowing the solution to be carefully made up to the 250 cm³ mark.
Why must the volumetric flask and graduation mark be at eye level when adding the final drops?
This ensures the graduation mark is being viewed at the right angle, allowing the solution to be made up to 250 cm³ more accurately.
What is the meniscus?
The meniscus is the curved surface of the liquid within a tube. When making the solution up to the graduation mark, the bottom of the meniscus should be exactly in line with the graduation mark.
What is the purpose of ‘washing’ the beaker and glass rod into the solution in the volumetric flask?
Washings ensures there is no solute left behind in the beaker or on the glass rod. This is important to ensure the concentration of the standard solution is as accurate as possible.
What apparatus is required to carry out a titration?
- 50 cm³ burette
- 25 cm³ pipette and filler
- Conical flasks
- Funnel
- Stand and clamp
Why can methyl orange be used as an indicator in the titration between HCl and Na₂CO₃?
Methyl orange can be used for titrations between a strong acid (HCI) and a weak base (Na₂CO₃) since the pH range of colour change coincides with the point of neutralisation.
What is the colour change of methyl orange and at what pH does the colour change occur?
- Red in acid
- Yellow in alkali
Methyl orange will change colour in the pH range 3.1-4.4.
Describe how to titrate a standard solution with HCl
- Fill the burette with the HCl solution and record the initial burette reading.
- Accurately pipette 25.0 cm³ of the Na₂CO₃ solution into a conical flask.
- Add 3 drops of methyl orange indicator to the flask.
- The standard solution is titrated with the acid until, on the addition of one drop of acid, the indicator changes colour from yellow to pink/red.
- Record the burette reading and repeat until the results are concordant.
What is the chemical equation for the reaction between Na₂CO₃ and HCl?
2HCI + Na₂CO₃ → 2NaCI + H₂O + CO₂
How could you identify the gas produced in the reaction between Na₂CO₃ and HCI?
Carbon dioxide is produced which can be collected and identified by bubbling through limewater. The limewater will go cloudy if carbon dioxide is present.
What type of reaction takes place between HCl and Na₂СО₃?
Neutralisation
Why should the burette be filled below eye level?
The burette should be filled below eye level so that if any of the acid spills whilst being poured in, it will not splash into your face.
What is a rough titration?
A rough titration is often done first. Its purpose is to ascertain the approximate point of neutralisation.
This means that for further titres, the acid can be added quickly until around the point of neutralisation - where it is then added dropwise.
The rough titre is not accurate enough to be included in calculations.
What are concordant results?
Results are usually said to be
concordant if they are within 0.1 cm³ of one another.
Why are titrations usually carried out on a white tile?
The white tile allows the point of colour change to be easily identified.
How can you calculate how much HCI was required to neutralise the Na₂CO₃?
Volume of HCl =
Initial burette reading - final burette reading
Why is the Na₂CO₃ solution placed in a conical flask?
The conical flask allows the mixture to be swirled without losing any of the contents.
Why must the reaction mixture be swirled during the titration?
Swirling ensures all the reacting particles collide and react. This helps to give a more accurate end point for the reaction.
Why are burettes and pipettes always used in titrations?
Burettes and pipettes measure the amount of HCl and Na₂CO₃ very precisely.
Why is the pipette rinsed with Na₂CO₃ before use and the burette rinsed with HCl before use?
Rinsing the equipment with the solutions removes any water which may be in the equipment.
This is important because the water will affect the concentrations of the solutions.
Therefore, rinsing ensures a more accurate titration experiment.
What chemicals are required to carry out a back titration to determine the percentage of calcium carbonate in limestone?
- Limestone chips
- 0.1 mol dm⁻³ NaOH solution
- 0.5 mol dm⁻³ HCl solution
- Phenolphthalein indicator
How can a back titration be carried out to determine the percentage of CaCO₃ in limestone?
The known mass of limestone is reacted with an excess known volume of HCl.
The excess HCl is then titrated with NaOH until neutralisation. Use this to calculate the volume of excess HCl.
The amount of excess HCl can be subtracted from the volume of HCl added to the CaCO₃ to indicate how much reacted in the first reaction.
Use this value with the reaction equation to calculate how many grams of CaCO₃ reacted in the first reaction.
The mass of the CaCO₃ can then be compared to the original mass of limestone to determine the percentage of CaCO₃ in limestone.
Give the chemical equation for the reaction between CaCO₃ and HCl
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Give the chemical equation for the reaction between HCI and NaOH
HCI + NaOH → NaCl+ H₂O
Originally 25 cm³ of excess 0.5 mol dm⁻³ HCl was added to limestone. 8 cm³ of 0.1 mol dm⁻³ NaOH is required to neutralise the excess HCl. How much CaCO₃ was in the limestone?
0.0008 mol NaOH (0.008 x 0.1) reacts with excess HCl in 1:1 ratio. So there is 0.0016 dm³ of excess HCI (0.008 ÷ 0.5).
Therefore, 0.0234 dm³ HCI reacts with CaCO₃ in the first reaction (0.025 - 0.0016).
This means 0.0117 mol of HCl (0.0234 x 0.5) reacted with CaCO₃ in a 2:1 ratio, so 0.00585 mol of CaCO₃ reacted.
The Mr of CaCO₃ is 100.1 so 0.586 g of CaCO₃ was in the limestone (0.00585 × 100.1).
0.65 g of limestone powder was reacted with HCl in a back titration. 0.42 g of CaCO₃ was found to have reacted with the HCl. What is the percentage of CaCO₃ in the limestone?
(0.42 - 0.65) x 100 = 64.6% of CaCO₃ in the limestone
Describe a back titration experimental procedure to calculate percentage of calcium carbonate in limestone
- Grind the limestone chips into a fine powder.
- Accurately weigh around 0.5 g of limestone powder and transfer to a conical flask.
- Record the mass of limestone added.
- Using a pipette, add 25 cm³ of HCl solution.
- Stir the reaction mixture with a stirring rod whilst heating gently.
- When the limestone has dissolved, cool to room temperature.
- Add a few drops of phenolphthalein and titrate against the NaOH.
- Record the volume of NaOH solution needed to neutralise the excess HCl solution and use this to calculate the percentage CaCO₃ in the limestone.
Explain why the limestone chips are crushed into a fine powder before reacting with HCl
Making the chips into a powder increases the surface area of the limestone reactant. This increases the rate of reaction because there are more exposed reactant particles - so more frequent successful collisions.
Explain why the HCl and limestone powder are heated
Heating the reaction mixture increases the rate of reaction.
This is because more of the reacting particles will have sufficient energy to react (activation energy) and the kinetic energy of the particles will be greater, meaning there will be more frequent collisions. Both of these effects lead to an increase in the number of successful reaction collisions.
Why must the HCl and limestone powder be heated gently?
It is important that the mixture is only heated gently to prevent any of the solution evaporating.
This ensures the measurements used in the calculations are as accurate as possible.
What apparatus is required to carry out a titration?
- 50 cm³ burette
- 25 cm³ pipette and filler
- Conical flasks
- Funnel
What is a suitable indicator to use in a titration between HCI and NaOH?
Phenolphthalein
What is the colour change of phenolphthalein and at what pH does the colour change occur?
- Colourless in acid
- Pink in alkali
Phenolphthalein will change colour in the pH range 8.3-10.
What are the hazards associated with HCI, NaOH and phenolphthalein?
HC - irritant
NaOH - irritant
Phenolphthalein - flammable
What type of reaction takes place between HCl and NaOH?
Neutralisation
Why should the burette be filled below eye level?
The burette should be filled below eye level so that if any of the acid spills whilst being poured in, it will not splash into your face.
Why are titrations usually carried out on a white tile?
The white tile allows the point of colour change to be easily identified.
How can you calculate how much NaOH was required to neutralise the excess HCI?
Volume of NaOH =
Initial burette reading - final burette reading
Why is the HCl and limestone reaction mixture placed in a conical flask?
During the titration, the conical flask allows the mixture to be swirled without losing any of the contents.
Why must the reaction mixture be swirled during the titration?
Swirling ensures all the reacting particles collide and react. This helps to give a more accurate end point for the reaction.
What apparatus is required to carry out a double titration?
- 50 cm³ burette
- Funnel
- Burette clamp and stand
- 25 cm³ volumetric pipette with safety filler
- 2 × 250 cm³ conical flask
- White tile
Why are burettes and pipettes always used in titrations?
Burettes and pipettes measure the volumes of solutions very precisely.
Before use, why might the pipette be rinsed with the NaOH or Na₂CO₃ mixture and the burette be rinsed with HCI?
Rinsing the equipment with the solutions removes any water which may be in the equipment. This is important because the water will affect the concentrations of the solutions. Therefore, rinsing ensures a more accurate titration experiment.
Outline the experimental procedure to determine the concentration and the mass of NaOH and Na₂CO₃ in a mixed solution
- Titrate 25.00 cm³ of the mixed solution against HCl, using phenolphthalein as the indicator.
Do not agitate the flask any more than necessary when mixing the acid. - Record the volume of HCl used at the phenolphthalein end-point.
- Add methyl orange and continue titrating until its end-point.
- Record the total volume of hydrochloric acid added to this end-point.
- Repeat as necessary until the titration values obtained agree within 0.20 cm³ and separately average results for the two values.
- Calculate the concentration and the mass of NaOH and Na₂CO₃ in the mixed solution.
What type of reaction takes place between HCl and NaOH?
Neutralisation
Give the chemical equation for the reaction that takes place between HCI and NaOH
NaOH + HCI → NaCI + H₂O
How does HCI react with Na₂CO₃?
Na₂CO₃ reacts with HCl in two steps:
1. Na₂CO₃ + HCl → NaHCO₃ + NaCl
2. NaHCO₃ + HCl → NaCl + CO₂ + H₂O
How does a double titration work to determine the concentration of NaOH and Na₂CO₃ in a mixed solution?
In the first titration the following two reactions occur:
NaOH + HCl → NaCI + H₂O
Na₂CO₃ + HCl → NaHCO₃ + NaCl
In the second titration the following reaction takes place:
NaHCO₃ + HCl → NaCl+ CO₂ + H₂O
The second titre gives moles of NaHCO₃ which is equal to the moles of Na₂CO₃
Then, moles of NaOH = moles of HCl in first titre - moles of Na₂CO₃.
Then divide the moles of NaOH and Na₂CO₃ by volume used to determine the concentration.
Why are two different indicators used in the double titration?
There are two different end points in the titration.
Different end points occur at different pH and so an indicator must be chosen so that the pH range of colour change coincides with the end point.
In the double titration of the NaOH and Na₂CO₃ mixture, the first end point has a higher pH so phenolphthalein is used, and the second end point has a lower more acidic pH so methyl orange is used.
What is the colour change of phenolphthalein and at what pH does the colour change occur?
- Colourless in acid
- Pink in alkali
Phenolphthalein will change colour in the pH range 8.3-10.
What is the colour change of methyl orange and at what pH does the colour change occur?
- Red in acid
- Yellow in alkali
Methyl orange will change colour in the ph range 3.1-4.4.
What is the meniscus?
The meniscus is the curved surface of the liquid within a tube. When taking a reading from a burette, the reading should be taken from the value exactly in line with the bottom of the meniscus.
What are the hazards associated with HCI, NaOH, phenolphthalein and methyl orange?
HCI - Irritant
NaOH - irritant
Phenolphthalein - flammable
Methyl orange - flammable
Why should the burette be filled below eye level?
The burette should be filled below eye level so that if any of the acid spills whilst being poured in, it will not splash into your face.
Why are titrations usually carried out on a white tile?
The white tile allows the point of colour change to be easily identified.
Why is the NaOH placed in a conical flask?
The conical flask allows the mixture to be gently swirled without losing any of the contents.
Why must the reaction mixture be swirled during the titration?
Swirling ensures all the reacting particles collide and react. This helps to give a more accurate end point for the reaction.
Why is it important in a double titration that the conical flask is not swirled too vigorously during the first titration?
In the first titration, HCl reacts with Na₂CO₃ and NaOH.
Swirling may encourage the HCl to react with the NaHCO₃ formed from the reaction with Na₂CO₃
This reaction should not take place until the second titration, so it is important that the HCl avoids reacting with it.
