Heat, often symbolized as Q, is the transfer of thermal energy between a system and its surroundings due to a temperature difference. This energy always flows from a warmer object to a cooler object until thermal equilibrium is reached. Heat is a form of energy, and its standard unit of measurement in the International System of Units (SI) is the Joule (J).
Another common unit is the calorie (cal). One calorie represents the amount of energy required to raise the temperature of one gram of water by one degree Celsius. Understanding how this energy relates to physical changes, such as temperature shifts or phase changes, is foundational to calculating heat released or absorbed in any process.
Calculating Heat from Temperature Changes
When a substance absorbs or releases heat and its temperature changes without changing its physical state, the process involves sensible heat. This heat transfer is calculated using the fundamental equation: Q = mcΔT.
In this formula, m is the mass of the substance, ΔT is the change in temperature (final minus initial), and c is the specific heat capacity. Specific heat capacity is a unique physical property indicating the energy needed to raise one unit of mass by one degree of temperature.
Specific heat capacity values differ significantly. For instance, liquid water has a high specific heat capacity (approximately 4.18 Joules per gram per degree Celsius). This high value means water can absorb a large amount of heat energy with only a small temperature increase. In contrast, metals have much lower specific heat capacities, meaning they heat up much faster for the same energy input.
This formula applies only as long as the substance remains entirely in a single phase, such as liquid water heating up or a block of ice cooling down. For example, if 100 grams of water is heated from 20°C to 80°C, the temperature change (ΔT) is 60°C. The heat absorbed (Q) would be calculated as 100 g multiplied by 4.18 J/g·C multiplied by 60°C, equaling 25,080 Joules.
Calculating Heat During Phase Transitions
A different calculation is required when a substance undergoes a change in physical state, such as melting or boiling. During these phase transitions, the temperature remains constant, even though heat is continuously transferred. This heat, which causes a change in state rather than a change in temperature, is referred to as latent heat.
Because the temperature change (ΔT) is zero, the sensible heat formula Q = mcΔT cannot be used. Instead, the heat transferred is calculated using the formula Q = mL, where m is the mass of the substance and L is the latent heat of the transition.
The latent heat value (L) is a physical constant for a given substance and type of transition. The latent heat of fusion (L_f) is used for melting, while the latent heat of vaporization (L_v) is used for boiling. The energy added during melting is absorbed by molecules to break the cohesive bonds holding them in a rigid solid structure, allowing them to move freely as a liquid.
For water, the latent heat of fusion is about 334,000 J/kg. This means it takes a substantial amount of energy to melt ice. Latent heat calculations focus only on the energy required to complete the phase change itself, separate from the energy needed to raise the temperature afterward.
Calculating Heat Released in Chemical Reactions
The calculation of heat released or absorbed in a chemical reaction is conceptually distinct from physical heating and cooling processes. Chemical reactions involve the breaking and forming of chemical bonds, and the heat change in this context is known as the enthalpy of reaction (ΔH). This value represents the net energy difference between the energy stored in the reactants and the energy stored in the products.
Energy is always required to break existing chemical bonds, making this step an energy-absorbing (endothermic) process. Conversely, energy is always released when new chemical bonds form, which is an energy-releasing (exothermic) process. The overall enthalpy change (ΔH) is determined by comparing the total energy absorbed to break the reactant bonds with the total energy released when the product bonds form.
To calculate ΔH, you can use tabulated bond energy values, which represent the energy needed to break a specific type of bond. The calculation is done by summing the bond energies of all the bonds broken in the reactants and subtracting the sum of the bond energies of all the bonds formed in the products.
If the energy released during bond formation is greater than the energy absorbed for bond breaking, the reaction is exothermic, and the resulting ΔH value will be negative, indicating a net release of heat energy to the surroundings. If the ΔH is positive, it means more energy was required to break the initial bonds than was released by forming the new ones, classifying the reaction as endothermic, where heat is absorbed from the surroundings.