Energy in Chemical Reactions — Drills · Grade 11 Science

Chapter: Chemical Energy & Thermodynamics

What these drills covers

By the end of Energy in Chemical Reactions you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: enthalpy change (ΔH), exothermic reaction, endothermic reaction, bond energy, activation energy (Ea), specific heat (c).

Problem 1

In an exothermic reaction, energy is…
  1. created from nothing
  2. released to the surroundings
  3. absorbed from the surroundings
  4. destroyed
Answer and explanation

released to the surroundings

Exothermic reactions release energy to their surroundings, so ΔH is negative and the surroundings warm up.

Problem 2

The specific heat of liquid water is…
  1. 0.00 J/g°C
  2. 1.00 J/g°C
  3. 4.18 J/g°C
  4. 100 J/g°C
Answer and explanation

4.18 J/g°C

Water's specific heat is 4.18 J/g°C — the energy needed to raise the temperature of 1 g of water by 1°C.

Problem 3

How much heat (in J) is absorbed when 250. g of water is warmed from 10.0°C to 30.0°C? (c = 4.18 J/g°C)
Answer and explanation

20900 J

ΔT = 30.0 − 10.0 = 20.0°C, so q = mcΔT = 250 × 4.18 × 20 = 20,900 J.

Problem 4

On a potential-energy diagram for an exothermic reaction, the products are…
  1. at the same level as the reactants
  2. always at zero
  3. higher than the reactants
  4. lower than the reactants
Answer and explanation

lower than the reactants

An exothermic reaction releases energy, so the products end lower than the reactants and ΔH is negative.

Problem 5

Explain, in terms of bonds, why breaking bonds absorbs energy while forming bonds releases energy.
Answer and explanation

A bond is a stable, low-energy arrangement, so pulling the atoms apart to break a bond requires an input of energy. When atoms come together to form a new bond, they settle into that lower-energy state and release the extra energy. Whether a reaction is exothermic or endothermic depends on which total is larger.

Problem 6

A reaction releases 8360 J of heat into 200. g of water (c = 4.18 J/g°C). By how many °C does the water's temperature rise?
Answer and explanation

10 °C

Rearranging q = mcΔT gives ΔT = q ÷ (m × c) = 8360 ÷ (200 × 4.18) = 8360 ÷ 836 = 10°C.

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