Reactions that release or absorb energy
Grade 8 Science Worksheet
Circle the best answer, or write your answer on the line.
A reaction warms 100 g of water from 20 °C to 25 °C. Energy = mass × 4.18 J/(g·°C) × rise. How many joules did the water take in, Ari?
Bo's reaction heats 50 g of water from 22 °C to 30 °C. Energy = mass × 4.18 J/(g·°C) × rise. How many joules did the water take in?
A hand-warmer test warms 200 g of water from 18 °C to 21 °C. Energy = mass × 4.18 J/(g·°C) × rise. How many joules did Esme's water take in?
Energy is kept in this reaction. How much thermal energy do the surroundings have after the reaction, Jun?
A cup of reacting powder keeps its total energy. How much thermal energy is in Luz's surroundings after the reaction?
Milo's card: a reaction needs a match flame to start, then keeps going by itself and the flask gets hot. Exothermic or endothermic?
- Endothermic, since it needed a flame to start
- Endothermic, since the flame gave it the energy it needed
- Exothermic: the flame only gets it started
- Neither, since a flame is not a kind of energy at all
Read this first
Energy for a burner-heated reaction (example data)
energy amount (kJ) absorbed by the reaction 30 released by the reaction 12 A reaction goes only while a burner heats it. Overall, is it exothermic or endothermic, Noah?
- Exothermic, since it released 12 kJ, which is real energy
- Exothermic, since a burner heating it makes things hot
- Endothermic: it absorbs 30 kJ and releases only 12 kJ
- Neither, since the two energy amounts are not equal
Pia's card: a reaction needs a burner to start, then keeps glowing and warming the table with the burner off. Which is it?
- Neither, since it glows
- Endothermic, since it needed a burner
- Exothermic: the burner only started it
- Endothermic, since the burner supplied the energy
What does the dip show about the reaction, and why does the water warm up again later, Tala?
- It released energy first, then it absorbed energy later
- Energy was destroyed in the dip, then made again later
- It released cold into the water, and the cold wore off
- It absorbed energy, then the warmer room heated the water
Reactions A and B each warm the same amount of water from 22 °C. Which released more energy, and how can you tell, Zee?
- A absorbed more energy, since its water got much hotter
- B, since it ended hotter: 26 °C against 25 °C at 10 min
- The same, since both lines go up and then come down again
- A: its water rose 10 °C above the start, B's only 6 °C