7.2 Using energy transfers from reactions
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Give an example of the reaction that would happen in disposable hand warmer
• oxidation of iron • iron turns into hydrated iron(III) oxide in an exothermic reaction • the reaction is similar to rusting
What catalyst is used in the oxidation of iron
Sodium chloride (common salt)
How do reusable hand warmers work
• they are based on the formation of crystals from a salt • a supersaturated solution is prepared by dissolving as much salt as possible in hot water, the solution is then allowed to cool • a small disc in the plastic pack is used to start the exothermic change
How does the metal disc start the reaction in reusable hand warmer
• when you press a few times, small particles of metal are scraped off • these start off the crystallisation • the crystals spread throughout the solution, transferring energy to the surroundings in an exothermic change
How do you reuse a reusable hand warmer
Put the solid pack into boiling water to redissolve the crystals. Once it has cooled down, the pack is ready to acativate again.
Example of salt used in a reusable hand warmer
Sodium ethanoate (CH3COO-Na+)
What is the chemical reaction usually used to transfer energy to the food or drink in a self heating can
Calcium oxide + water —> calcium hydroxide
How does a self heating can work
• you press a button in the base of the can • this breaks a seal and lets the water and calcium oxide mix • then the exothermic reaction can begin
What are some issues with self heating cans
• development took years and cost millions of pounds • over a third of the can was taken up with the reactants • in some early versions, the temperature did not rise high enough in cold conditions
Advantage and disadvantage of single use hand warmers
• lasts for hours • can only be used once
Advantage and disadvantage of reusable hand warmers
• can be reused many times • only last for 30 minutes
What do cold packs usually contain
Ammonium nitrate and water
How does an ice pack work
• ammonium nitrate and water (sometimes present in a gel) are kept separate in the pack • when struck, the bag inside the water pack breaks, releasing ammonium nitrate • when the ammonium nitrate dissolves, it absorbs energy from its surroundings, making them colder
Advantages and disadvantages of cold packs
• ideal when there is no ice available to treat a knock or strain • cold packs included in first aid kits, decrease in temperature reduces swelling and numbs pain • can only be used once • only last for 20 minutes
How else can the chemical reaction used for cold packs be used
To chill cans of drinks