Chemistry - Unit 1-7

Exothermic reaction

a reaction that releases energy in the form of heat

Endothermic reaction

a reaction in which energy is absorbed

Neutralisation: endo- or exo-?

exothermic

Combustion: endo- or exo-?

exothermic

Examples of exothermic reactions [3]:

• neutralisation

• combustion

• respiration

Thermal decomposition: endo- or exo-?

endothermic

calcium carbonate thermal decomposition

calcium carbonate --> calcium oxide + carbon dioxide

Uses of exothermic reactions [2]:

• hand warmers

• self-heating coffee

Uses of endothermic reactions:

• cold packs

Disposable hand warmer [2]

• iron + sodium chloride --> iron (III) oxide

• lasts for hours

Reusable hand warmer [5]

• often sodium ethanoate (CH3COO-Na+)

• supersaturated solution of dissolved salt

• crystallises when metal disc is pressed

• put in boiling water to re-dissolve

• lasts about 30 minutes

Self-heating can reaction:

calcium oxide + water --> calcium hydroxide

Instant cold packs [3]

• ammonium nitrate and water

• when dissolved, takes in energy

• lasts about 20 minutes

Exothermic reaction profile

High bar, curve, low bar

Endothermic reaction profile

Low bar, curve, high bar

Energy change during reaction =

difference between products and reactants

Activation energy

minimum energy needed to get a reaction started

Activation energy =

reactants to peak of curve

Bond breaking: endo- or exo-?

endothermic

Bond making: endo- or exo-?

exothermic

Bonds in exothermic reactions

making bonds > breaking bonds

Bonds in endothermic reactions

making bonds < breaking bonds

Bond energy

the amount of energy that will break a bond between two atoms

To calculate the energy change for a chemical reaction:

• how much energy is needed to break the bonds

• how much energy is released when bonds are formed

If the overall energy change is negative, the reaction is...

...exothermic

To find the overall energy change:

add the energy required to break bonds and the energy released by their formation

Alkaline fuel cell: hydrogen half equation

H₂ + 2OH⁻ → 2H₂O + 2e⁻

Alkaline fuel cell: oxygen half equation

O₂ + 2H₂O + 4e⁻ → 4OH⁻

Acidic fuel cell: hydrogen half equation

H₂ → 2H⁺ + 2e⁻

Acidic fuel cell: oxygen half equation

O₂ + 4H⁺ + 4e⁻ → 2H₂O

Alkaline fuel cell: full equation

2H₂ + O₂ → 2H₂O

Acidic fuel cell: full equation

2H₂ + O₂ → 2H₂O

Advantages of hydrogen fuel [3]:

• no electrical recharging

• no pollutants

• range of sizes

Disadvantages of hydrogen fuel [3]:

• highly flammable

• produced by non-renewable resources

• difficult to store

Electrolysis

the decomposition of a substance by an electric current

Cathode

negative electrode

Anode

positive electrode

Electrolyte

an ionic compound whose aqueous solution/molten form conducts an electric current

Anion

negative ion

Cation

positive ion

Why can't solid ionic compounds conduct electricity?

ions are held in place by strong electrostatic forces of attraction

Which electrode has electrons?

cathode

Substances are ... at the cathode

reduced

Substances are ... at the anode

oxidised

... become ... at each electrode

ions, atoms

Water molecule ionising

H₂O ⇌ H⁺ + OH⁻

Which product of ionised water goes to the cathode?

H⁺

Which product of ionised water goes to the anode?

OH⁻

Hydrogen is produced at the cathode if...

the metal is more reactive than hydrogen

Electrodes should be made from ... materials

inert (unreactive)

In the electrolysis of aqueous solutions, ... is usually produced at the anode (positive electrode)

oxygen, O₂

Half equation: anode: aqueous solutions

4OH⁻ → O₂ + 2H₂O + 4e⁻

Half equation: cathode: (some) aqueous solutions

2H⁺ + 2e⁻ → H₂

Order of discharge at anode

halide ion > hydroxide > all other negatively charged ions

Oxygen is released at the anode unless...

the solution contains a halide ion

Uses of aluminium (and its alloys) [7]:

- pans
- overhead power cables
- aeroplanes
- cooking foil
- drink cans
- window and patio door frames
- bicycle frames and car bodies

What compound is electrolysed to procure aluminium?

aluminium oxide, Al₂O₃

Where is aluminium oxide found?

bauxite ore

Aluminium oxide melting point

2050°C

How do we reduce the melting point of aluminium oxide?

mix it with cryolite

Cryolite-aluminium oxide mixture melting point

850°C

Aluminium oxide electrolysis reaction

2Al₂O₃ → 4Al + 3O₂

Aluminium forms at the ... electrode

negative

Oxygen is produced at the ... electrode

positive

Aluminium electrolysis cell [4]

- lined with carbon negative electrode
- molten aluminium is tapped or siphoned off
- carbon dioxide and oxygen gas emitted from anodes
- steel case

At the cathode (aluminium extraction):

Al³⁺ + 3e⁻ → Al

At the anode (aluminium extraction):

2O²⁻ → O₂ + 4e⁻

Reaction of oxygen with hot carbon anodes:

C + O₂ → CO₂

... have to be replaced regularly

Carbon anodes

Brine

water saturated with salt (sodium chloride)

Products of electrolysis of brine [3]

- chlorine gas
- hydrogen
- sodium hydroxide

At the anode (brine electrolysis):

2Cl⁻ → Cl₂ + 2e⁻

At the cathode (brine electrolysis):

2H⁺ + 2e⁻ → H₂

What is left after brine electrolysis?

sodium hydroxide, NaOH

Alkali

a soluble hydroxide

Base

substance that can neutralise acids

Acid

any compound that forms H⁺ ions in solution

Neutral

neither acidic nor alkaline

Dissolving sodium hydroxide in water

sodium hydroxide (water→) sodium ions(aq) + hydroxide ions(aq)

Indicators [4]:

- litmus paper
- universal indicator
- phenolphthalein
- methyl orange

Litmus paper [3]

acid: red
neutral: no change
basic: blue

Universal indicator

an indicator with a different colour for each pH value.

Phenolphthalein [3]

acid: colourless
neutral: colourless
basic: pink

Methyl orange [3]

acid: red-orange
neutral: yellow
basic: yellow

pH scale

measurement system used to indicate the concentration of hydrogen ions (H+) in solution; ranges from 0 to 14

Neutral solution + universal indicator

green

Acidic solution + universal indicator

red - yellow

Basic solution + universal indicator

blue - purple

pH meter

a device used to measure the pH of a solution

Equivalence point

the point in a titration where the number of moles of hydrogen ions equals the number of moles of hydroxide ions (middle of vertical drop)

Ionise

the reaction of a molecular substance with a solvent to form ions in solution.

Acids are classed as strong or weak depending on how they...

ionise in water

In aqueous solutions, acid molecules...

ionise and release H⁺ ions

Strong acids ... in aqueous solutions

fully ionise

How can we tell if an acid is weak?

it is a reversible reaction

Weak acids ... in aqueous solutions

partially ionise

Carbonic acid

H₂CO₃

Ethanoic acid

CH₃COOH

Citric acid

C₆H₈O₇

As the pH scale decreases by one unit...

the concentration of hydrogen ions increases by ten times