# How is bond making exothermic but it has a negative enthalpy change?

If a reaction is exothermic then the products lose energy to the surroundings (so products have less energy than the reactants), and if a reaction is endothermic then the products gain energy from the surroundings, right? But for an exothermic reaction, the enthalpy change is negative. Shouldn't that mean the products have more energy as enthalpy change is bonds broken - bonds made, which means the products have more energy because more bonds are made?
Energy is given out so the energy of the products decreases, bond making is exothermic
Original post by IsabelJoseph54
If a reaction is exothermic then the products lose energy to the surroundings (so products have less energy than the reactants), and if a reaction is endothermic then the products gain energy from the surroundings, right? But for an exothermic reaction, the enthalpy change is negative. Shouldn't that mean the products have more energy as enthalpy change is bonds broken - bonds made, which means the products have more energy because more bonds are made?

A question that has been asked by thousands of chemistry students over the years! You're right that the products have less chemical energy than reactants when the reaction is exothermic. As energy is always conserved, this 'spare' energy is converted into heat. The enthalpy change is enthalpy of products - enthalpy of reactants which is going to result in a negative number.

Enthalpy change can also be described as (sum of bond broken - sum of bonds made). When we do these estimations, we imagine breaking a bond to produce separate atoms. These separate atoms are going to higher than energy than both the products and reactants. The energy given out when a bond is made is the same as the energy needed to break it. From the diagram below, we can see that the height between products and the separate atoms is higher than the height between reactants and separate atoms. The sum of bonds formed will always be bigger than sum of bonds broken for an exothermic reaction. This is why the enathlpy change is always negative.
(edited 1 year ago)
Original post by IsabelJoseph54
If a reaction is exothermic then the products lose energy to the surroundings (so products have less energy than the reactants), and if a reaction is endothermic then the products gain energy from the surroundings, right? But for an exothermic reaction, the enthalpy change is negative. Shouldn't that mean the products have more energy as enthalpy change is bonds broken - bonds made, which means the products have more energy because more bonds are made?

Enthalpy change is defined in terms of the energy added to the system.

When a bond forms, energy is given out. Which is equivalent to a negative amount of energy being added to the system.

Take H2 + I2 -> 2HI.

You start with chemical energy stored in the H-H bond and the I-I bond. You have to add energy to the bonds until they break and form atoms. Those atoms now have loads of energy in them.

Alternatively, you could start with 2x H-I bonds. These bonds have a different amount of chemical energy in them. But when you add energy to break them, you'll form the same atoms, which will have to same amount of energy in as when you formed them from H2 and I2.

The stronger bonds are the bonds that initially contained the least energy and therefore required the most energy to break them to atoms.