Topic 5 Flashcards

Exothermic & Endothermic Reactions

Exothermic & Endothermic Reactions: read all 6 questions and answers

1. What is an exothermic reaction?
   A reaction that transfers energy TO the surroundings, usually by heating. The temperature of the surroundings increases.
2. Give three examples of exothermic reactions.
   1. Combustion (burning fuels). 2. Neutralisation (acid + alkali). 3. Oxidation (metals reacting with oxygen). Also: respiration.
3. What is an endothermic reaction?
   A reaction that takes in energy FROM the surroundings, usually by heating. The temperature of the surroundings decreases.
4. Give three examples of endothermic reactions.
   1. Thermal decomposition (e.g. CaCO₃ → CaO + CO₂). 2. Dissolving ammonium nitrate in water. 3. Citric acid + sodium hydrogencarbonate. Also: photosynthesis.
5. Give an everyday use of an exothermic reaction.
   Self-heating cans and hand warmers. The exothermic reaction heats the contents or releases warmth for the user.
6. Give an everyday use of an endothermic reaction.
   Instant cold packs for sports injuries. The endothermic reaction (e.g. Ammonium nitrate dissolving in water) absorbs heat from the surrounding area, cooling it down.

Reaction Profiles

Reaction Profiles: read all 5 questions and answers

1. What is a reaction profile?
   An energy diagram showing the relative energy levels of reactants and products. The x-axis is "Progress of reaction" and the y-axis is "Energy".
2. On an exothermic reaction profile, where are the products compared to reactants?
   Products are at a LOWER energy level than reactants. The energy difference is released to the surroundings.
3. On an endothermic reaction profile, where are the products compared to reactants?
   Products are at a HIGHER energy level than reactants. The energy difference is taken in from the surroundings.
4. What is activation energy (Eₐ)?
   The minimum energy needed for a reaction to start. On the reaction profile, it is shown by the "hump" - the height from the reactant energy level to the peak of the curve.
5. How does a catalyst affect a reaction profile?
   A catalyst provides an alternative reaction pathway with a LOWER activation energy. The hump is smaller, but the overall energy change (ΔH) remains the same. The catalyst is not used up.

Bond Energy Calculations (Higher Tier)

Bond Energy Calculations: read all 6 questions and answers

1. What is bond energy?
   The energy needed to break one mole of a particular covalent bond, measured in kJ/mol. The same amount of energy is released when that bond forms.
2. Is bond breaking exothermic or endothermic?
   ENDOTHERMIC - energy must be supplied to overcome the attractive forces holding the atoms together.
3. Is bond making exothermic or endothermic?
   EXOTHERMIC - energy is released when new bonds form because the atoms reach a more stable, lower energy state.
4. How do you calculate the overall energy change of a reaction using bond energies?
   Overall energy = Energy to break bonds − Energy released making bonds. Negative result = exothermic. Positive result = endothermic.
5. Calculate: H₂ + Cl₂ → 2HCl. (H-H = 436, Cl-Cl = 242, H-Cl = 431 kJ/mol)
   Breaking: 436 + 242 = 678 kJ. Making: 2 × 431 = 862 kJ. Overall: 678 − 862 = −184 kJ/mol (exothermic).
6. Calculate: CH₄ + 2O₂ → CO₂ + 2H₂O. (C-H = 413, O=O = 498, C=O = 805, O-H = 464)
   Breaking: (4×413) + (2×498) = 2648 kJ. Making: (2×805) + (4×464) = 3466 kJ. Overall: 2648 − 3466 = −818 kJ/mol (exothermic).

Required Practical: Temperature Changes

Required Practical: Temperature Changes: read all 5 questions and answers

1. What is investigated in the Required Practical for energy changes?
   The variables that affect temperature changes in reacting solutions, e.g. The effect of concentration on the temperature change during neutralisation.
2. Why is a polystyrene cup used instead of a glass beaker?
   A polystyrene cup is a good thermal insulator. It minimises heat loss to the surroundings, making the temperature change measurement more accurate.
3. How do you determine if a reaction is exothermic from your data?
   If the temperature of the solution INCREASES, the reaction is exothermic - energy has been transferred from the reaction to the surroundings (the solution).
4. How do you determine if a reaction is endothermic from your data?
   If the temperature of the solution DECREASES, the reaction is endothermic - energy has been taken in from the surroundings (the solution) by the reaction.
5. What variables should be controlled in this practical?
   Volume of acid, starting temperature of both solutions, same equipment (polystyrene cup, thermometer), same acid and alkali. Only the concentration of one solution should change (independent variable).

Chemical & Fuel Cells (Chemistry Only)

Chemical & Fuel Cells: read all 5 questions and answers

1. How does a chemical cell (battery) work?
   Two different metals are dipped into an electrolyte and connected by a wire. Electrons flow from the more reactive metal to the less reactive metal through the external circuit, creating a voltage.
2. What determines the voltage of a chemical cell?
   The greater the difference in reactivity between the two metals, the greater the voltage produced.
3. What is a hydrogen fuel cell?
   An electrochemical cell that converts the chemical energy of hydrogen directly into electrical energy. Hydrogen reacts with oxygen to produce water - the only waste product.
4. Give two advantages of hydrogen fuel cells.
   1. Only product is water - no CO₂ emissions at point of use. 2. No need for recharging - runs continuously as long as fuel is supplied. 3. More efficient than combustion engines.
5. Give two disadvantages of hydrogen fuel cells.
   1. Hydrogen is difficult and expensive to store/transport (must be compressed or liquefied). 2. Often produced from natural gas (not truly zero-carbon). 3. Platinum catalysts are expensive. 4. Limited refuelling infrastructure.