chemistry- energy calculations

When fuels and food reacts with oxygen the reactions are exothermic. 

We can use a calorimeter to measure the amount of energy released when substances burn. 

energy (J) = mass (kg) × specific heat capacity (J/kg/ºC) × temperature change (ºC)

Simple calorimeters do not give accurate results for the energy released because much of it used to heat the surroundings. 

Endothermic reactions-

These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are:

• Electrolysis
• The reaction between ethanoic acid and sodium carbonate
• The reaction between ammonium nitrate and water
• The thermal decomposition of calcium carbonate in a blast furnace

Exothermic reactions- 

These are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:

• Neutralisation reactions between acids and alkalis
• The reaction between water and calcium oxide
• Explosions

Energy changes in a reaction can be shown using energy diagrams. These show the level of energy of the reactants and of the products. It is therefore simple to see whether the reaction is exothermic or endothermic. The bigger the difference between the two, the more energy is given out or taken in.-this is an exothermic reaction-

In an endothermic reaction, the products are at a higher energy than the reactants.

for an exothermic reaction the answer needs to be negative  

 hydrogen + chlorine     hydrogen chloride.
H_2(g)  +     Cl_2(g)                 2HCl_(g)

Bond Bond Energy (in kJ per mole)

H—H 436  Cl—Cl 242  H—Cl 431