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?
Energy is kept in Kira's reaction. How much thermal energy do the surroundings have after the reaction?
A cup of reacting powder keeps its total energy. How much thermal energy is in Luz's surroundings after the reaction?
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
- Exothermic: the burner only started it
- Endothermic, since the burner supplied the energy
- Endothermic, since it needed a burner
What does the dip show about the reaction, and why does the water warm up again later, Tala?
- It absorbed energy, then the warmer room heated the water
- 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
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: its water rose 10 °C above the start, B's only 6 °C
- A absorbed more energy, since its water got much hotter
- The same, since both lines go up and then come down again
- B, since it ended hotter: 26 °C against 25 °C at 10 min