11A_Introduction to Redox Reactions.pptx

DEFINING REDOX REACTIONS REDOX REACTIONS & EQUATIONS  DEFINITIONS IN REDOX l lCHEMISTRY a Acid-base reactions involve the transfer of hydrogen atoms c between e R. molecules. Redox reactions involve the balance...

DEFINING REDOX REACTIONS REDOX REACTIONS & EQUATIONS  DEFINITIONS IN REDOX l lCHEMISTRY a Acid-base reactions involve the transfer of hydrogen atoms c between e R. molecules. Redox reactions involve the balanced transfer of electrons between molecules. DEFINITIONS IN REDOX CHEMISTRY Oxidation: The process where electrons are lost to another chemical species. Reduction: The process where electrons are received from another chemical species. In a redox reaction, oxidation and reduction must occur together… in a simultaneous reaction. Also helpful to remember that oxidation is the gain of oxygen atoms.  DEFINITIONS IN REDOX CHEMISTRY Oxidising Agent (Oxidant): A chemical species that causes another substance to undergo oxidation. Reducing Agent (Reductant): A chemical species that causes another substance to undergo reduction. l l ca In a redox reaction, oxidation and reduction occur simultaneously. Re. Oxidants undergo reduction, while reductants undergo oxidation. OXIDATION STATES Oxidation States: Indicate the relative charge on an individual atom in a reaction.  Oxidation states are useful in identifying whether a reaction is a redox reaction. Common Rules for Oxidation States: Examples: Free elements are assigned an oxidation state of zero. Elements bound to themselves are assigned an oxidation state of zero. Oxidation states of single atoms are equal to their charge. The sum of oxidation states in a neutral atom is equal to zero. Additional Notes: -TheHydrogen sum of oxidation states in a polyatomic molecule is (H) often exists in a oxidation state. equal to its charge. - Oxygen (O) often exists in a oxidation state. OXIDATION STATES CALCULATING OXIDATION STATES Worked Example 1. Sulfur trioxide, , is a gas that contributes to the greenhouse effect. Determine the oxidation state of. of the sulfur atom in a molecule of SO3. CALCULATING OXIDATION STATES Worked Example 1. Sulfur trioxide, , is a gas that contributes to the greenhouse effect. Determine the oxidation state of. of the sulfur atom in a molecule of SO3. is a neutral molecule, thus will have an overall oxidation state of Oxygen has an oxidation state of  OXIDATION STATES l l c a Oxidation states are useful in identifying whether a reaction is a redox Re. reaction. The chemical species whose oxidation state increases, undergoes oxidation. The chemical species whose oxidation state decreases, undergoes reduction. Example. (g) (l) 𝐻= 0 𝑃𝑏=0  OXIDATION STATES l l c a Oxidation states are useful in identifying whether a reaction is a redox Re. reaction. The chemical species whose oxidation state increases, undergoes oxidation. The chemical species whose oxidation state decreases, undergoes reduction. Example. (g) (l) 𝐻= 0 𝑃𝑏=0 The oxidation state of Hydrogen has increased, so it has been oxidised. The oxidation state of Lead has decreased, so it has been reduced. REDOX HALF-EQUATIONS Example. (g) (l) 𝐻= 0 𝑃𝑏=0 Redox half equations are used to show the movement of electrons.  There is one half-equation for the oxidised species, and one half- equation for the reduced species. Oxidation: Electrons are on the product side of the half equation… Reduction: Electrons are on the reactants side of the half equation… WRITING REDOX HALF-EQUATIONS Worked Example 2. Write the balanced redox half-equation for the conversion of (g) to (aq). WRITING REDOX HALF-EQUATIONS Worked Example 2. Write the balanced redox half-equation for the conversion of (g) to (aq). (aq) Step 1: Balance the equation normally. (aq) Step 2: Assign oxidation numbers. 0 →−4 has oxygen bound to itself, so its oxidation number is 0. Each ion has an oxidation number of , so together the overall oxidation number is. Step 3: Add electrons as required – each electron has a charge of. (aq) BALANCING REDOX EQUATIONS Use the KOHES Method to balance redox equations: Key Elements: - Balance all elements except for Oxygen: - Balance Oxygen by adding Hydrogen: - Balance Hydrogen by adding (acidic conditions, usually add an acid) Electrons: - Consider oxidation states and add to appropriate side of equation States: - Label states of matter, not BAL ANCING REDOX HALF- EQUATIONS Worked Example 3. Hydrogen sulfite, (aq), is oxidised to form the polyatomic sulfate ion, (aq), in an acidic aqueous solution. Write the balanced oxidation half-equation for this reaction. BAL ANCING REDOX HALF- EQUATIONS Worked Example 3. Hydrogen sulfite, (aq), is oxidised to form the polyatomic sulfate ion, (aq), in an acidic aqueous − solution. Write the balanced oxidation half-equation 2− for this𝐻𝑆 𝑂3 → 𝑆 𝑂4 reaction. 1x on each side of equation 𝐻𝑆 𝑂 3− +𝐻 2 𝑂 → 𝑆 𝑂 42− 3x on LHS, 4x on RHS… Need to add 1x to LHS 3x on LHS, 0x on RHS… Need to add 3x to RHS overall charge on LHS, overall charge on RHS… Need to add 2 to RHS (aq) WRITING REDOX FULL-EQUATIONS To write a balanced overall (full) redox equation: 1) Write two balanced redox half-equations, using the KOHES method. 2) Multiply the redox half-equations to ensure the number of electrons is the same. 3) Add the two reactions together, cancelling out the electrons and any excess or. Example. [Oxidation] [Reduction] WRITING REDOX FULL-EQUATIONS To write a balanced overall (full) redox equation: 1) Write two balanced redox half-equations, using the KOHES method. 2) Multiply the redox half-equations to ensure the number of electrons is the same. 3) Add the two reactions together, cancelling out the electrons and any excess or. Example. ( [Oxidation] ) [Reduction] WRITING REDOX FULL-EQUATIONS To write a balanced overall (full) redox equation: 1) Write two balanced redox half-equations, using the KOHES method. 2) Multiply the redox half-equations to ensure the number of electrons is the same. 3) Add the two reactions together, cancelling out the electrons and any excess or. Example. [Oxidation] [Reduction] WRITING REDOX FULL-EQUATIONS To write a balanced overall (full) redox equation: 1) Write two balanced redox half-equations, using the KOHES method. 2) Multiply the redox half-equations to ensure the number of electrons is the same. 3) Add the two reactions together, cancelling out the electrons and any excess or. Example. [Oxidation] [Reduction] WRITING REDOX FULL- EQUATIONS Worked Example 4. A piece of solid cadmium is added to a solution of acidified potassium dichromate, resulting in the following two half-reactions occurring: 1) (l) What is the balanced overall redox equation for the reaction taking place? WRITING REDOX FULL- EQUATIONS Worked Example 4. A piece of solid cadmium is added to a solution of acidified potassium dichromate, resulting in the following two half-reactions occurring: 1) (l) What is the balanced overall redox equation for the reaction taking 1) (l) place? Only need to consider the numbers of electrons… must 1) (l) multiply equation (2) by 3. (l) (l) WRITING REDOX FULL- EQUATIONS Worked Example 5. Using the half-equations provided, give the overall equation for the redox reaction between permanganate ions, (aq) and iron,. Oxidation: Reduction: (l) WRITING REDOX FULL- EQUATIONS Worked Example 5. Using the half-equations provided, give the overall equation for the redox reaction between permanganate ions, (aq) and iron,. Oxidation: Reduction: (l) Oxidation: Reduction: (l) Need to find the lowest common multiple of 2 & 5… - must multiply oxidation equation by 5 - must multiply reduction equation by 2 Oxidation: Reduction: (l) 2+ ¿ ( aq )+ 8𝐻 𝑂 ( l) ¿ − 2 − +¿ ( aq )+10 𝑒 − → 5 𝐹 𝑒 2+ ¿ ( aq) + 10 𝑒 + 2 𝑀𝑛 ¿ 5 𝐹𝑒 ( s )+ 2 𝑀𝑛 𝑂 ( aq )+16 𝐻 ¿ 4 2 +¿ (aq ) +8 𝐻 𝑂 ( l) ¿ 2 − +¿ ( aq )→ 5 𝐹 𝑒 2 +¿ ( aq )+ 2 𝑀 𝑛 ¿ 5 𝐹𝑒 ( s )+ 2 𝑀𝑛 𝑂 4 ( aq )+16 𝐻 ¿  COMMON OXIDISING AGENTS l l ca Oxidising Agent (Oxidant): Re. A chemical species that causes another substance to undergo oxidation. The two most powerful oxidising agents in the high-school laboratory are… Permanganate Dichromate Equati (aq) (aq) on: Oxidati +7 → +2 𝑀𝑛: 𝐶𝑟 +6 : → +3 on Numbe rs: Colour: 𝑃𝑢𝑟𝑝𝑙𝑒 → 𝐶𝑜𝑙𝑜𝑢𝑟𝑙𝑒𝑠𝑠 𝑂𝑟𝑎𝑛𝑔𝑒 → 𝐺𝑟𝑒𝑒𝑛 Both agents are often Potassium Permanganate Potassium Dichromate found bound to Potassium ()…