7.2 Using energy transfers from reactions

Question 1

Give an example of the reaction that would happen in disposable hand warmer

Answer 1

• oxidation of iron
• iron turns into hydrated iron(III) oxide in an exothermic reaction
• the reaction is similar to rusting

Question 2

What catalyst is used in the oxidation of iron

Answer 2

Sodium chloride (common salt)

Question 3

How do reusable hand warmers work

Answer 3

• 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

Question 4

How does the metal disc start the reaction in reusable hand warmer

Answer 4

• 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

Question 5

How do you reuse a reusable hand warmer

Answer 5

Put the solid pack into boiling water to redissolve the crystals. Once it has cooled down, the pack is ready to acativate again.

Question 6

Example of salt used in a reusable hand warmer

Answer 6

Sodium ethanoate (CH3COO-Na+)

Question 7

What is the chemical reaction usually used to transfer energy to the food or drink in a self heating can

Answer 7

Calcium oxide + water —> calcium hydroxide

Question 8

How does a self heating can work

Answer 8

• 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

Question 9

What are some issues with self heating cans

Answer 9

• 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

Question 10

Advantage and disadvantage of single use hand warmers

Answer 10

• lasts for hours
• can only be used once

Question 11

Advantage and disadvantage of reusable hand warmers

Answer 11

• can be reused many times
• only last for 30 minutes

Question 12

What do cold packs usually contain

Answer 12

Ammonium nitrate and water

Question 13

How does an ice pack work

Answer 13

• 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

Question 14

Advantages and disadvantages of cold packs

Answer 14

• 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

Question 15

How else can the chemical reaction used for cold packs be used

Answer 15

To chill cans of drinks