What is an exothermic reaction ? (give 3 examples)
A reaction that releases energy to the surroundings, as bonds are MADE. (respiration, combustion and neutralisation are exothermic)
What is an endothermic reaction ? (give 2 examples)
A reaction that takes in heat energy from its surroundings as BREAKING bonds requires energy. (photosynthesis and citric acid + sodium hydrogencarbonate are endothermic reactions)
Sketch an energy profile graph for the progress of exothermic and endothermic reactions. Label the activation energy, overall energy change, energy of reactants and energy of products.
- exothermic graph should have energy of products lower than reactants
- endothermic should have energy of products higher than energy of reactants
- arrows for energy change should be pointed up or down in conjunction to a corresponding rise or fall in energy
Describe a practical to investigate whether a reaction is exothermic or endothermic.
- Place polystyrene cup in a beaker (stability)
- pour some sodium hydrogencarbonate in the cup
- place thermometer inside cup and measure initial temperature + record it
- mix in citric acid (solid) and after reaction has stopped (no more fizzing) note final temperature
- temp falls, so endothermic reaction
How does a reusable chemical hand warmer work ?
- small metal disk suspended in sodium ethanoate salt solution
- when hand warmer is beaten/pressed, small pieces of the metal flake off and seed the crystallisation reaction, which is exothermic
- boiling the hand warmer dissolves the crystals so the hand warmer can be used again
How does a chemical cold pack work ?
- ammonium nitrate and water are kept in separate compartments of the pack
- when pressed or struck, the compartments burst and the two react in an endothermic reaction
What is a bond energy and how is it used ?
the amount of energy stored by a chemical bond, used to calculate overall energy transfers in a chemical reaction.
H–H + Cl–Cl -> 2H–Cl
(H–H = 436 kJ/mol)
(Cl–Cl = 243 kJ/mol)
(H–Cl = 432 kJ/mol)
calculate the overall energy change for the reaction in kJ/mol.
NOTE: the reactants break bonds (endothermic (+)), products make bonds (exothermic (-)).
436 + 243 = +679 kJ/mol
432x2 = -864 kJ/mol
679 - 864 = -185 kJ/mol (reaction is exothermic)
What affects the voltage produced by an electrical cell ?
The differing reactivities of the two metals used.
Describe an experiment to investigate the voltage produced by different metals paired with magnesium.
- set up simple electrical cell with a magnesium ribbon as one electrode, another metal (one of zinc, iron, tin or copper) as the other, both hooked up to a voltmeter and dipped in a salt solution
- measure the voltage produced by the simple cell when each of the 4 metals is tested as a second electrode
Describe how a battery can be recharged.
power supply reverses reactions at each electrode which occur when cell is discharging, regenerating original reactants.
Describe the structure of a fuel cell.
- alkaline electrolyte in the middle (usually potassium hydroxide solution)
- hydrogen gas in
- oxygen gas in
- water out
Name three advantages of a fuel cell in comparison to rechargeable batteries.
- doesn’t have to be recharged
- doesn’t lose efficiency over charging cycles
- only waste product is water
- can be a range of sizes for different uses
- hydrogen is renewable
Name three disadvantages of a fuel cell in comparison to rechargeable batteries.
- produce less voltage so often need a few in conjunction to do the same work as one normal battery
- hydrogen is highly flammable so difficult to store safely
- hydrogen as a fuel may be produced by non-renewable means
Balance: H2 + O2 -> H2O
2H2 + O2 -> 2H2O
