Reactions that release or absorb energy

Answer Key - Grade 8 Science Worksheet - Challenge Me

Circle the best answer, or write your answer on the line.

1. 1.

   2 g of powder reacts and warms 100 g of water from 22 °C to 27 °C. Energy = mass × 4.18 × rise. How many joules were released per gram of powder, Ari?

   1045 J/g

2. Read this first

   Hand-warmer trials starting at 22 °C (test results)

   reaction	highest temperature (°C)
   A	40
   B	58
   C	29

3. 2.

   A hand warmer must reach at least 35 °C and stay under 50 °C. Which reaction fits, Jun?

   • C, since it is the safest
   • A and B, since both pass 35 °C
   • A, since it reaches 35 °C and stays under 50 °C
   • B, since the hottest reaction warms hands the most

4. Cold-pack trials starting at 22 °C (test results)

   reaction	lowest temperature (°C)
   P	3
   Q	15
   R	-8

5. 3.

   A cold pack must drop to 10 °C or lower but stay above 0 °C. Which reaction fits, Kira?

   • Q, since a small drop is the safer choice for a pack
   • R, since the coldest pack cools the skin the most
   • P: it falls under 10 °C but stays above 0 °C
   • P and R, since both drop below 10 °C so both work

6. Use this

   Investigation card (test results). Changed: the reaction and the mass of water. Measured: temperature rise (°C). Kept the same: start 20 °C. Reaction X, 100 g of water, rose 6. Reaction Y, 50 g of water, rose 9.

7. 4.

   Luz says Y released more energy, since Y's water ended hotter. What does mass × rise show?

   • X released more: 100 g × 6 °C is more than 50 g × 9 °C
   • They released the same, since both warmed the water
   • Y released more, since its water ended hotter
   • Neither released energy, since the start was the same
8. 5.

   How much did the reaction warm the water, Milo?

   • 34 °C, the temperature at the peak
   • 3 °C, the change from the start to the end of the graph
   • 10 °C, the fall after the peak
   • 13 °C, the rise from the start to the peak
9. 6.

   What was the biggest drop in temperature, and what caused the water to warm again, Noah?

   • 9 °C, and the warmer room added energy to the water
   • 15 °C, and the reaction released energy to the water
   • 3 °C, from the start to the end of the graph, and the reaction released energy
   • 3 °C, and the cold wore off
10. 7.

    Reactions A and B warm the same water to the same peak, but A rises much faster. Which statement is best supported, Pia?

    • A released energy faster; the peaks do not show totals
    • A released more total energy, since its line is steeper
    • A released less energy, since it cooled sooner
    • Both released exactly the same energy, since the peaks match
11. 8.

    Ren says any reaction that needs a burner must be endothermic. What is the best reply?

    • No: a burner may only start it; the net energy decides
    • Yes: a burner only heats things, so the reaction takes heat
    • Yes: needing energy means the reaction absorbs energy
    • No: only reactions that make light are exothermic
12. 9.

    Tala says burning wood is endothermic, since it needs a match first. Which reply fits the evidence best?

    • Disagree: the wood stays cold the whole time it burns
    • Disagree: burning releases far more energy than the match
    • Agree: the match supplies the energy that the wood absorbs
    • Agree: it needed the match's energy before it burned
13. 10.

    Zee says a cold pack works by giving out cold to your skin. Which reply fits the evidence best?

    • Agree: the pack makes cold, so it must give it out
    • Disagree: the pack releases energy into your skin
    • Agree: cold flows from the pack into the skin
    • Disagree: the pack takes energy from your skin