Energy changes during chemical reactions: ΔH

1. Understanding Energy in Reactions

• Energy is absorbed when bonds in reactants break to form the activated complex.
• Energy is released when new bonds form in the products.
• The overall energy change in a reaction is called the enthalpy change $(ΔH).$

2. Formula for Enthalpy Change $(ΔH)$

$ΔH = \text{Energy of products} - \text{Energy of reactants}$

$ΔH = E_{\text{products}} - E_{\text{reactants}}$

OR

$ΔH = H_{\text{products}} - H_{\text{reactants}}$

$ΔH = H_{\text{final}} - H_{\text{initial}}$

3. Types of Reactions Based on ΔH

Reaction Type	Description	ΔH Value
Exothermic	Releases energy (heat flows out)	Negative $(-ΔH)$
Endothermic	Absorbs energy (heat flows in)	Positive $(+ΔH)$

4. Key Takeaways

✅ ΔH determines whether a reaction is exothermic or endothermic.

✅ Exothermic reactions release energy (e.g., combustion).

✅ Endothermic reactions absorb energy (e.g., photosynthesis).

✅ The lower the $ΔH$, the more stable the reaction products.