Heat and calorimetry: calculate heat changes using q = mC(delta-T) for temperature changes and q = mH for phase changes, with constants from Table B and formulas from Table T.

What this topic is asking

The Core Curriculum asks you to calculate the heat absorbed or released when a substance changes temperature or changes phase. The Regents gives two equations on Table T, $q = mC\Delta T$ for temperature changes and $q = mH$ for phase changes, and the water constants on Table B. This is a reliable Part B-2 and Part C calculation, and it pairs directly with the heating-curve page.

Two equations, two situations

The clue is whether the temperature changes. If the problem gives a starting and ending temperature, you are warming or cooling, so use $q = mC\Delta T$. If the problem says "melt", "freeze", "boil" or "condense" at the phase-change point, the temperature is constant, so use $q = mH$.

The constants on Table B

Table B lists these for water: specific heat $4.18\ \text{J/g}\cdot\text{K}$, heat of fusion $334\ \text{J/g}$, and heat of vaporization $2260\ \text{J/g}$. You read them straight off the table rather than memorizing them. Water's high specific heat is why it warms and cools slowly.

Exothermic and endothermic

This links to the kinetics and thermodynamics module, where reaction energy is described the same way. For calorimetry calculations the magnitude of $q$ is found from the equations, and you state whether the process is exothermic or endothermic from its direction.

Try this

Q1. Calculate the heat released when $10.0$ g of water cools by $5.0$ K (specific heat $4.18\ \text{J/g}\cdot\text{K}$). [2 points]

• Cue. $q = (10.0)(4.18)(5.0) = 209$ J released (exothermic).

Q2. State which equation to use to find the heat needed to boil water at $100\ ^\circ\text{C}$. [1 point]

• Cue. $q = mH_v$ (a phase change at constant temperature), using the heat of vaporization from Table B.