Energy changes flashcards

Topic 5: Energy changes

Exothermic vs endothermic reaction

Exothermic: transfers energy to the surroundings, so the temperature of the surroundings increases. Endothermic: takes in energy from the surroundings, so their temperature decreases.

Examples and uses of exothermic reactions

Examples: combustion, many oxidation reactions, neutralisation. Uses: self-heating cans, hand warmers.

Examples and a use of endothermic reactions

Examples: thermal decompositions; citric acid + sodium hydrogencarbonate. Use: some sports injury packs.

Required practical 4: measuring a temperature change in a reacting solution

Polystyrene cup in a beaker; measure a fixed volume of solution; record the start temperature; add the other reactant, stir, fit a lid; record the highest (or lowest) temperature. Change = final − initial.

On a reaction profile, where are the activation energy and the overall energy change?

Activation energy: from the reactant level up to the peak. Overall energy change: from the reactant level to the product level (products lower for exothermic, higher for endothermic).

Reactants 80 kJ, peak 250 kJ, products 150 kJ. Activation energy and overall change?

Activation energy = 250 − 80 = 170 kJ. Overall change = 150 − 80 = +70 kJ, so endothermic.

Bond breaking and bond making: which takes in energy?Higher

Breaking bonds takes in energy (endothermic). Forming bonds releases energy (exothermic). Overall exothermic when more energy is released forming bonds than is needed to break them.

N₂ + 3H₂ → 2NH₃. N≡N 945, H–H 436, N–H 391 kJ/mol. Energy change?Higher

Broken: 945 + 3 × 436 = 2253. Formed: 2 × 3 × 391 = 2346. Change = 2253 − 2346 = −93 kJ/mol (exothermic).

Simple chemical cell: what is it, and what affects the voltage?

Two different metals in contact with an electrolyte. The voltage depends on the electrodes and the electrolyte; the bigger the difference in reactivity of the metals, the bigger the voltage.

Battery; non-rechargeable vs rechargeable

A battery is two or more cells in series (voltages add). Non-rechargeable: reactions stop when a reactant is used up. Rechargeable: reactions are reversed by an external electrical current.

Hydrogen fuel cell: overall reaction, one advantage and one disadvantage

2H₂ + O₂ → 2H₂O. For: the only product is water; no recharging. Against: hydrogen is hard to store and flammable; making it often uses fossil fuels.

Half equations in a hydrogen fuel cell (acidic electrolyte)Higher

Negative electrode: 2H₂ → 4H⁺ + 4e⁻ (oxidation). Positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O (reduction).