Edexcel iGCSE Chemistry -3.3  Heat Energy Calculations (Q = mcΔT)- Study Notes- New Syllabus

Edexcel iGCSE Chemistry -3.3  Heat Energy Calculations (Q = mcΔT)- Study Notes- New syllabus

Edexcel iGCSE Chemistry -3.3  Heat Energy Calculations (Q = mcΔT)- Study Notes -Edexcel iGCSE Chemistry – per latest Syllabus.

Key Concepts:

3.3 calculate the heat energy change from a measured temperature change using the expression
Q = mcΔT

Edexcel iGCSE Chemistry -Concise Summary Notes- All Topics

3.3 Calculating Heat Energy Change Using \( Q = mc\Delta T \)

Key Formula:

\( Q = mc\Delta T \)

Where:

  • \( Q \) = heat energy transferred (J)
  • \( m \) = mass of solution (g)
  • \( c \) = specific heat capacity \( (4.2 \, \mathrm{J\,g^{-1}\,°C^{-1}}) \)
  • \( \Delta T \) = temperature change \( (^\circ\mathrm{C}) \)

Understanding Each Term

  • \( m \) is usually the mass of the solution (often assumed equal to volume in cm³).
  • \( \Delta T = \text{final temperature} – \text{initial temperature} \).
  • If temperature increases → reaction is exothermic.
  • If temperature decreases → reaction is endothermic.

Units Reminder

  • Mass must be in grams.
  • Energy will be in joules.
  • Sometimes convert to kJ: \( 1\,\mathrm{kJ} = 1000\,\mathrm{J} \).

Worked Structure (Exam Method)

  1. Write the formula.
  2. Substitute values clearly.
  3. Show units.
  4. State whether reaction is exothermic or endothermic.

Example 1 (Numerical):

100 g of solution increases in temperature by \( 8^\circ\mathrm{C} \). Calculate the heat energy change.

▶️ Answer/Explanation

Formula:

\( Q = mc\Delta T \)

Substitute values:

\( Q = 100 \times 4.2 \times 8 \)

\( Q = 3360 \, \mathrm{J} \)

Temperature increased → exothermic reaction.

Example 2 (Conceptual):

During a reaction the temperature drops by \( 5^\circ\mathrm{C} \). Is the reaction exothermic or endothermic?

▶️ Answer/Explanation

The temperature decreases.

Heat is absorbed from the surroundings.

The reaction is endothermic.

Example 3 (Hard):

50 cm³ of acid is mixed with 50 cm³ of alkali. The temperature increases from \( 21^\circ\mathrm{C} \) to \( 29^\circ\mathrm{C} \). Calculate the heat energy released.

▶️ Answer/Explanation

Step 1: Calculate total mass.

Total volume = 100 cm³ Assume density = \( 1\,\mathrm{g\,cm^{-3}} \) Mass = 100 g

Step 2: Find \( \Delta T \).

\( \Delta T = 29 – 21 = 8^\circ\mathrm{C} \)

Step 3: Use formula.

\( Q = mc\Delta T \)

\( Q = 100 \times 4.2 \times 8 \)

\( Q = 3360 \, \mathrm{J} \)

The temperature increased, so the reaction is exothermic.

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