C7 : Energy changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction that TRANSFERS energy into the surroundings.

• makes surroundings WARMER
• products have less chemical energy than reactants (reaction gets colder)

Question 2

Uses of exothermic reactions. (3)

Answer 2

HAND WARMERS- USED ONCE
- oxidation of IRON releases energy. Iron turns to hydrated iron oxide in exothermic reactions.
REUSABLE HANDWARMERS
- when metal disc is pressed, small particles of metal scraped off.
- this starts CRYSTALLISATION and spreads in solution starts an exothermic change
SELF HEATING CANS
- Calcium oxide + water –> calcium hydroxide transfers energy to food/drink
- pressing button on can, breaks a seal and lets calcium oxide + water mix - start exothermic reaction

Question 3

What is an endothermic reaction?

Answer 3

A reaction that transfers energy FROM the surrounding TO the reacting chemicals.

• TEMPERATURE OF SURROUNDINGS ALWAYS GOES DOWN
• products have MORE chemical energy than reactants

Question 4

Uses of endothermic reactions.

Answer 4

COLD PACKS

• contain ammonium nitrate + water
• when ammonium nitrate dissolves in water, it absorbs energy from surroundings, making them colder

Question 5

What is the reaction when more energy is needed to break bonds than make bonds?

Answer 5

Endothermic (energy is needed to break bonds)

Question 6

What is the reaction where more energy is used to form bonds than break them?

Answer 6

Exothermic

Question 7

Explain, using ideas about particles and atoms, why burning magnesium is an exothermic change.

Answer 7

• After the collisions of particles during a chemical reaction, energy is taken in to break the reactant bonds and then when bonds form, energy is released.
• Burning magnesium is an exothermic reaction because more energy is released to the surroundings in the process of bond-making, than absorbed from the surroundings in the process of bond-breaking.

Question 8

Energy change equation

Answer 8

energy change = break - make

Question 9

How can you tell if a reaction is endo or exo?

Answer 9

If the energy of products is HIGHER than of the reactants it is ENDOTHERMIC.
If the energy of products is LOWER than the reactants it is EXOTHERMIC.

Question 10

What measurements would a student make to show that the reaction is exothermic?

Answer 10

The temperature before and after.