Some chemical reactions release energy by heat. In other words, they transfer heat to their surroundings. These are known as exothermic reactions -- "exo" means releases and "thermic" means heat. Some examples of exothermic reactions include combustion (burning), oxidation reactions like burning and neutralization reactions between acids and alkalis. Many everyday items like hand warmers and self-heating cans for coffee and other hot beverages undergo exothermic reactions.
TL;DR (Too Long; Didn't Read)
To calculate the amount of heat released in a chemical reaction, use the equation Q = mc ΔT, where Q is the heat energy transferred (in joules), m is the mass of the liquid being heated (in grams), c is the specific heat capacity of the liquid (joule per gram degrees Celsius) and ΔT is the change in temperature of the liquid (degrees Celsius).
Difference Between Heat and Temperature
It's important to remember that temperature and heat are not the same thing. Temperature is a measure of how hot something is -- measured in degrees Celsius or degrees Fahrenheit -- while heat is a measure of the thermal energy contained in an object measured in joules. When heat energy transfers to an object, its temperature increase depends on the mass of the object, the substance the object is made from and the amount of energy transferred to the object. The more heat energy transferred to an object, the greater its temperature increase.
Specific Heat Capacity
The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1 degree Celsius. Different substances have different specific heat capacities, for example, liquid has a specific heat capacity of 4181 joules/kg degrees C, oxygen has a specific heat capacity of 918 joules/kg degrees C and lead has a specific heat capacity of 128 joules/kg degrees C.
To calculate the energy required to raise the temperature of a known mass of a substance, you use the equation E = m × c × θ, where E is the energy transferred in joules, m is the mass of the substances in kg, c is the specific heat capacity in J/kg degrees C and θ is the temperature change in degrees C. For example, to work out how much energy must be transferred to raise the temperature of 3 kg of water from 40 degrees C to 30 degrees C, the calculation is E = 3 × 4181 × (40 - 30), which gives the answer 125,430 J (125.43 kJ).
Calculating Heat Released
Imagine 100 cm3 of an acid was mixed with 100 cm3 of an alkali, then the temperature was increased from 24 degrees C to 32 degrees C. To calculate the amount of heat released in joules, the first thing you do is calculate the temperature change, ΔT (32 - 24 = 8). Next, you use Q = mc ∆T, i.e. Q = (100 + 100) x 4.18 x 8. Dividing the specific heat capacity of water, 4181 joules/kg degrees Celsius by 1000 to get the figure for joules/g degrees C. The answer is 6,688, which means 6688 joules of heat is released.