⚡️ • Lesson 2.3 : Stoichiometry and Electrochemistry (Chem) Flashcards
A complicated lesson, covering compound formulas, relative masses of atoms and molecules, moles and the Avogadro constant, as well as electrolysis, and hydrogen-oxygen fuel cells.
What is the molecular formula of a compound?
It shows the number and type of atoms in one molecule.
How do you deduce the formula of a simple molecular compound?
By counting the number of each atom in a model or diagram.
How do you deduce the formula of an ionic compound?
From the charges on the ions or the ratio shown in a model.
What is meant by “balancing” a symbol equation?
Making sure the number of atoms is the same on both sides.
What are the state symbols?
Symbols showing the physical state:
(s) = solid, (l) = liquid, (aq) = dissolved in water.
What is an ionic equation?
It shows only the ions involved in the chemical reaction.
What is relative atomic mass (Ar)?
The average mass of an element’s isotopes compared to 1/12th the mass of a carbon-12 atom.
What is relative molecular mass (Mr)?
The sum of the relative atomic masses in an ionic compound.
How do you calculate relative atomic mass (Ar)?
Multiply each isotope’s mass by its percentage abundance, add the results, then divide by 100.
Example: Chlorine has two isotopes— Cl-35 (75%) and Cl-37 (25%).
Ar = (35 x 75) + (37 x 25) ÷ 100=35.5
How do you calculate relative molecular mass (Mr)?
Add the relative atomic masses (Ar) of all the atoms in the molecule or formula.
Example: Calculate the Mr of H2O.
H = 1, O = 16 → (2 x 1) + 16 =18
In what units can concentration be measured?
Concentration can be measured in g/dm³
What is a mole (mol) in chemistry?
A mole is the unit of amount of substance.
One mole contains 6.02 × 10²³ particles (Avogadro constant).
What is the formula to calculate amount of substance (mol)?
Amount of substance (mol) = Mass (g) ÷ Molar mass (g/mol)
How do you calculate mass (g) in mole calculations?
Mass (g) = Amount of substance (mol) × Molar mass (g/mol)
How do you calculate molar mass (g/mol)?
Molar mass = Mass (g) ÷ Amount of substance (mol)
How do you calculate relative atomic or molecular mass from a mole calculation?
Relative mass = Molar mass (g/mol)
What is the molar gas volume at room temperature and pressure (r.t.p.)?
24 dm³ per mole of gas.
What formula is used to calculate the volume of a gas at r.t.p.?
Volume of gas (dm³) = Amount of substance (mol) × 24
How do you convert between cm³ and dm³?
1 dm³ = 1000 cm³
What are limiting reactants?
The reactant that is used up first and limits the amount of product formed.
How do you calculate stoichiometric reacting masses?
Use the balanced equation to find the mole ratio, then use mol = mass ÷ molar mass to calculate the mass needed or produced.
What is electrolysis?
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
In electrolysis, what is the anode?
The anode is the positive electrode.
In electrolysis, what is the cathode?
The cathode is the negative electrode.
What is the electrolyte in electrolysis?
The electrolyte is the molten or aqueous substance that undergoes electrolysis.
What are the products at the electrodes during the electrolysis of molten lead(II) bromide?
Cathode (–): Lead (Pb)
Anode (+): Bromine (Br₂) gas
Observation: Grey lead forms at cathode, orange bromine gas at anode.
What are the products at the electrodes during the electrolysis of concentrated aqueous sodium chloride?
Cathode (–): Hydrogen gas (H₂)
Anode (+): Chlorine gas (Cl₂)
Observation: Bubbles at both electrodes, chlorine smell at anode.
What are the products at the electrodes during the electrolysis of dilute sulfuric acid?
Cathode (–): Hydrogen gas (H₂)
Anode (+): Oxygen gas (O₂)
Observation: Bubbles at both electrodes, glowing splint relights at anode (oxygen test).
What happens to electrons in the external circuit during electrolysis?
Electrons flow from the anode (positive) to the cathode (negative) in the external circuit.
What happens at the electrodes during electrolysis?
Cathode: Gain of electrons (reduction)
Anode: Loss of electrons (oxidation)
How do ions move in the electrolyte during electrolysis?
Positive ions (cations) move to the cathode.
Negative ions (anions) move to the anode.
What are the products of electrolysis of aqueous copper(II) sulfate using copper electrodes?
Cathode (–): Copper is deposited
Anode (+): Copper dissolves
Observation: Cathode gains mass, anode loses mass.
What is always formed at the cathode during electrolysis?
Metals or hydrogen.
What is always formed at the anode during electrolysis?
Non-metals (other than hydrogen), like oxygen, chlorine, bromine, etc.
What will the products be in electrolysis of a binary molten compound?
Metal at cathode
Non-metal at anode
Example: Molten NaCl → Sodium (cathode) + Chlorine gas (anode)
Write an ionic half-equation for the reaction at the cathode during electrolysis of molten sodium chloride.
Na⁺ + e⁻ → Na
What is a hydrogen–oxygen fuel cell?
A fuel cell that uses hydrogen and oxygen to produce electricity, with water as the only chemical product.
What are the advantages of hydrogen–oxygen fuel cells compared to petrol engines?
• Only produces water (no carbon emissions)
• Renewable fuel (hydrogen)
• Quiet operation
• High efficiency
What are the disadvantages of hydrogen–oxygen fuel cells compared to petrol engines?
• Expensive to produce
• Hydrogen is difficult to store and transport
• Limited infrastructure (few hydrogen refuelling stations)
• Hydrogen production may require fossil fuels
