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21 Cards in this Set
- Front
- Back
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reaction where products have less energy than reactants.... |
they must have transferred excess energy usually by heating |
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exothermic reactions |
energy ➡ surroundings rise in temp of surr. eg. combustion, neutralisation, oxidation. uses - hand warmers oxidation of iron in air with salt solution catalyst - self heating cans |
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exothermic energy profile |
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endothermic reactions |
takes in energy from surr. fall in temp of surr. much less common eg. thermal decomposition citric acid + hydrogencarbonate uses - ice pack more convenient than ice, more flexible |
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endothermic energy profile |
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REQUIRED PRACTICAL energy transfer of reactions |
lid - stops evaporation polystyrene cup - insulator cotton wool - insulation measure temp change diff variables see whether endo or exo solid + liquid or 2 liquids |
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reaction energy profiles |
▪️overall energy change - diff in height between reactants n products ▪️whether endo or exo ▪️activation energy (Ea) - energy needed to break binds n start reaction. min amnt of energy reactants need to react |
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units of energy and energy transfer |
1000J = 1kJ transfer = kJ/mol (mol of reactants) |
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bond breaking reaction = |
endothermic |
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bond making - |
exothermic |
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bond energy calculations |
bond energies will be given 1. write out all displayed formulas to see all bonds 2. Work out energy used in bond breaking - bond energies of all reactants 3. energy given out in bond making - bond energies of all products 4. use formula for energy change |
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energy change formula |
e.c. = breaking - making energy energy overall is positive = exo ^ negative = endo |
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cells & batteries |
▪️2 electrodes - conduct (Cu+ Zn) ▪️electrolyte - liquid that conducts n contains ions that react w electrodes ▪️reactions set up charge diff. ▪️electrodes connected by wire ▪️charge can flow between ▪️voltmeter |
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electrodes n voltage |
charge difference bigger diff in reactivity of electrodes = bigger voltage use data to make reactivity series if left electrode is less reactive, voltage is positive |
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electrolytes and voltage |
lower conc. of ions in electrolyte = lower voltage as ions react conc. decreases (so will voltage) until zero and cell stops working |
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batteries |
2 or more cells in series voltage of battery = sum of voltages of all cells |
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(non) rechargeable batteries |
irreversible reactions ➡ reacting particles used up ➡ reaction can't happen in a rechargeable cell, the reaction can be reversed |
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fuel cells |
fuel enters cell is oxidised set up p.d. powers device oxygen enters and reacts to form water |
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hydrogen - oxygen fuel cell |
electrolyte often potassium hydroxide electrodes often carbon w catalyst anode - hydrogen - loses electrons ➡ H+ ions ➡ oxidation ➡ ions move to cathode cathode - oxygen - gains electrons ➡ reacts w H+ ions ➡ water ➡ reduced anode ➡ cathode current redox: 2H2 + O2 ➡ 2H2O half: +: H2 ➡ 2H2 + 2e- - : O2 + 4H+ + 4e- ➡ 2H2O |
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hydrogen fuel cells vs batteries ✔️ |
✔️less pollutants - only by products are water n heat - batteries are polluting to dispose of made of toxic metals ✔️no limit to how many times rechargeable ✔️store more energy than batteries so recharged less often
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hydrogen fuel cells vs batteries ❌ |
❌ hydrogen is gas - takes up more space ❌ hydrogen explosive when mixed w air - hard to store ❌ H-fuel either made from hydrocarbons (fossil fuel) or by electrolysis (electricity made by f.f.) |
