# 4.3: Exothermic and Endothermic Processes


## Units of Heat

Heat flow is measured in one of two common units: the calorie and the joule. The joule $$\left( \text{J} \right)$$ is the SI unit of energy. The calorie is familiar because it is commonly used when referring to the amount of energy contained within food. A calorie $$\left( \text{cal} \right)$$ is the quantity of heat required to raise the temperature of 1 gram of water by $$1^\text{o} \text{C}$$. For example, raising the temperature of $$100 \: \text{g}$$ of water from $$20^\text{o} \text{C}$$ to $$22^\text{o} \text{C}$$ would require $$100 \times 2 = 200 \: \text{cal}$$.

Calories contained within food are actually kilocalories $$\left( \text{kcal} \right)$$. In other words, if a certain snack contains 85 food calories, it actually contains $$85 \: \text{kcal}$$ or $$85,000 \: \text{cal}$$. In order to make the distinction, the dietary calorie is written with a capital C.

$1 \: \text{kilocalorie} = 1 \: \text{Calorie} = 1000 \: \text{calories}$

To say that the snack "contains" 85 Calories means that $$85 \: \text{kcal}$$ of energy are released when that snack is processed by the human body.

Heat changes in chemical reactions are typically measured in joules rather than calories. The conversion between a joule and a calorie is shown below.

$1 \: \text{J} = 0.2390 \: \text{cal or} \: 1 \: \text{cal} = 4.184 \: \text{J}$

We can calculate the amount of heat released in kilojoules when a 400 Calorie hamburger is digested:

$400. \: \text{Cal} = 400. \: \text{kcal} \times \frac{4.184 \: \text{kJ}}{1 \: \text{kcal}} = 1.67 \times 10^3 \: \text{kJ}$

## Summary

• Common units for heat include Joules (J)  and calories (cal); 1 cal = 1.184 J
• 1 food Calorie (Cal) = 1000 cal