Topic 3 except electrolysis

What makes a solution acidic?

The presence of H+ ions

What makes a solution alkali?

the presence of OH- ions

What does pH tell us?

The pH corresponds to the concentration of H+ ions

How does H+ and OH- concentrations differ across pHs?

pH is a logarithmic scale

1 less pH means the H+ ion concentration increases by 10x

1 more pH means that OH- ion concentration increases by 10x

pH is inversely proportional to H+ ion concentration

What is the range of pH and which side is acidic and which is alkali?

0-14, acidic to alkali

What colours do phenolphthalein appear as in acid and in alkali?

acid - colourless

alkali - pink

What colours do methyl orange appear as in acid and in alkali?

acid - red

alkali - yellow

What colours do litmus appear as in acid and in alkali?

acid - red

alkali - blue

Is litmus indicator suitable for titrations and why?

No - doesn’t have a sharp enough colour change as it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint

What is the most common form of litmus indicator?

as a paper

CP - Investigating pH of a fixed volume of HCl on varying amounts of a solid base

1. Use a pipette to measure a fixed volume of dilute HCl into a conical flask

2. Add one spatula of calcium oxide or calcium hydroxide to the flask and swirl

3. When all the base has reacted record the pH of the solution

4. If using U.I. paper use the glass rod to extract a sample from the flask

5. Repeat for different numbers of spatula (1-10) of solid but the same volume of HCl

6. Record your results neatly in table format

examples of strong acids

HCl

H2SO4

examples of weak acids

ethanoic acid

hydrofluoric acid

dissociation

when molecules, usually ionic, split up into smaller particles/ions in aqueous solution. Usually a reversible process and the particles can recombine under different conditions

When weak acids are added to water, what happens between its molecules and its ions?

An equilibrium is established between them.

Dissociation of strong acids vs weak acids

strong acids - completely dissociate in water

weak acids - partially dissociate in water

H+ conc./pH of strong vs weak acids

strong acids - high concentration of H+ ions, very low pH

weak acids - higher pH but under seven(usually 4-6)

dilute vs concentrated acid/base solution

dilute solution - contains a much fewer number of acid/base molecules in a given volume of solution

concentrated solution - contains a higher number of acid/base molecules in a given volume of solution

bases

substances which can neutralise an acid to form a salt and water

alkalis

bases which are water-soluble

bases vs alkalis

some bases may be insoluble in water, so they aren’t alkalis

all alkalis are bases, but not all bases are alkalis

pH range of alkalis

>7

most common types of bases

Usually oxides, hydroxides or carbonates of metals

What happens to red litmus paper in the presence of alkaline conditions?

turns blue

What makes a solution alkali?

The presence of OH- ions

Which metals will react with dilute acids?

Only metals above hydrogen in the reactivity series

MASH

metal + acid = salt + hydrogen

acid + base =

salt + water

CAWCS

carbonate + acid = water + carbon dioxide + salt

How to show neutralisation when given an equation?

HCl + NaOH ⟶ NaCl + H₂O

Separate the substances into its ions

H⁺  +  Cl^- + Na⁺  +  OH^- ⟶ Na⁺  +  Cl^- + H₂O

Cancel out the spectator ions.

H⁺  +  OH^- ⟶ H₂O

Works for all acid-base neutralisation

Test for hydrogen

hold a burning splint at the open end of a test tube of gas

if hydrogen is present it burns with a loud squeaky pop

Test for carbon dioxide

bubble the gas through limewater.

The limewater turns milky/cloudy if CO2 was present

CP: preparing hydrated salt crystals using the insoluble salt method - steps

1. Add 50 cm3 acid into a beaker and warm gently using a Bunsen burner.

2. Add the copper(II) oxide slwly to the hot dilute acid and stir until the base is in excess(until the base stops dissolving and a suspension of the base forms in the acid)

3. Filter the mixture to remove excess base and transfer the solution to an evaporating basin

4. Gently evaporate water from solution using a Bunsen burner to make the solution saturated. Check if the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end

5. remove the evaporating basin from heat and allow the filtrate to dry and crystallise

6. Wash crystals with distilled water

7. Dry on warm gauze or between filter paper

Why is the base added in excess?(CP: preparing hydrated salt crystals using the insoluble salt method)

To use up all of the acid

What are the solubility rules?

They determine what types of salts are water-soluble

Solubility rules - sodium

all sodium salts are soluble

solubility rules - potassium

all potassium salts are soluble

solubility rules - ammonium

all ammonium salts are soluble

solubility rules - nitrates

all nitrate salts are soluble

solubility rules - chlorides

most are soluble, except for silver and lead(II)

solubility rules - sulfates

most are soluble except for barium, calcium and lead(II)

solubility rules - carbonates

carbonates of sodium potassium and ammonium are soluble

most are insoluble

solubility rules - hydroxides

hydroxides of sodium potassium and ammonium are soluble

calcium hydroxide is sparingly soluble

most are insoluble

How to deduce method of salt extraction from an equation

Check the solubility of the salt using the solubility rules.

If it is water-soluble, then you can use titration

If it is water-insoluble, then it can be prepared by precipitation

When must titration be used?

If salts are prepared from an acid and a soluble reactant

Steps for titration

1. Use the graduated glass pipette and the pipette filler to add exactly 25 cm3 of alkali into the conical flask.

2. Add the acid into the burette and note the starting volume

3. Make sure that the tip of the burette is in the flask

4. Add a few drops of suitable indicator to the solution in the conical flask

5. Whilst swirling the flask start slowly adding in the acid. Near the end point, add it in drop by drop.

6. Stop when the indicator changes to appropriate colour

7. Note and record final acid volume and calculate the volume of acid added by placing your eye level with the meniscus

8. Add this same volume to the same volume of alkali WITHOUT the indicator

9. Gently evaporate water from solution using a Bunsen burner to make the solution saturated. Check if the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end

10. remove the evaporating basin from heat and allow the filtrate to dry and crystallise

11. Wash crystals with distilled water

12. Dry on warm gauze or between filter paper.

13. Repeat this rounding to the nearest 0.05 cm3.

14. Repeat until you reach 2 concordant results (that are within 0.1 cm3 of each other) to increase accuracy

formula relating moles concentration and volume

concentration = moles/volume(dm3)

How to do titration calculations

1: Write out the balanced equation for the reaction

2: Calculate the moles of the known solution given the volume and concentration

3: Use the equation to deduce the moles of the unknown solution

4: Use the moles and volume of the unknown solution to calculate the concentration

How to go from mol/dm3 to g/dm3 and vice versa

mol/dm3 to g/dm3 - multiply by relative formula mass

g/dm3 to mol/dm3 - divide by relative formula mass

How to go from cm3 to dm3 and vice versa

cm3 to dm3 - divide by 1000

dm3 to cm3 - multiply by 1000

How to prepare an insoluble salt - steps

1.Add 2 soluble salts to water and mix

2. Filter to remove precipitate from the mixture

3. Wash the precipitate with distilled water to remove traces of solution

4. Dry precipitate on warm gauze or between filter paper