# How do I tell if a reaction is endothermic?

I know that an endothermic reaction occurs when the total energy absorbed in bond breaking is more than that the total energy released in bond forming.

But if energy level diagrams and bond energy etc are not provided, how do I tell if a reaction is endothermic?

1) 2 H --> H2

2) CH4 --> C + 4H

3) 2 C4H10 + 13 O2 --> 8 CO2 + 10 H20

4) C6H12O6 + 6 O2 --> 6 CO2 + 6 H20

Which of these four reactions is an endothermic reaction? Could you provide an explanation as well? Thanks!

Relevance

the web page (below) provides: Exothermic vs. Endothermic and K

Table of Contents1. Exothermic Reactions2. Exothermic Reactions3. Endothermic Reactions4. The Equilibrium Constant K4.1. Exothermic Reactions4.2. Endothermic Reactions5. K Values6. Practice Problems7. Solutions8. Internal Links9. References10. Outside Links11. Contributors

An exothermic reaction occurs when the temperature of an isolated system, or one that interacts with its surroundings, increases due to the evolution of heat. This heat is then released into the surrounding resulting in an overall negative quantity for the heat of reaction (qrxn< 0). Conversely, an endothermic reaction occurs when the temperature of an isolated system decreases while the surroundings of a non-isolated system gains heat. Endothermic reactions result in an overall positive heat of reaction (qrxn> 0).

Exothermic Reactions

Exothermic and endothermic reactions cause energy level differences and thus differences in enthalpy (ΔH), the sum of all potential and kinetic energies. ΔH is determined by the system, not the surrounding environment in a reaction.

•A system that releases heat to the surroundings, an exothermic reaction, creates a negative ΔH because the enthalpy of the products is lower than the enthalpy of the reactants of the system (Figure 1).

•A system that absorbs heat from the surroundings, an endothermic reaction, creates a positive ΔH, because the enthalpy of the products is higher than the enthalpy of the reactants of the system.

Exothermic Reactions

C(s) + O2(g) → CO2(g); ΔH = –393.5 kJ

H2(g) + 1/2 O2(g) → H2O(l); ΔH = –285.8 kJ

Since the enthalpies of these reactions are less than zero, they are exothermic reactions.

Endothermic Reactions

N2(g) + O2(g) → 2NO(g); ΔH = +180.5 kJ > 0

C(s) + 2S(s) → CS2(l); ΔH = +92.0 kJ > 0

Since the enthalpies of these reactions are greater than zero, they are endothermic reactions.

The Equilibrium Constant K

The equilibrium constant, represented by K, defines the relationship amongst the concentrations of chemical substances involved in a reaction at the time of equilibrium. The Le Châtelier's Principle states that if a stress, such as changing temperature, pressure, or concentration, is added on an equilibrium reaction, the reaction will either shift to restore the equilibrium. When using exothermic and endothermic reactions, this added stress is a change in temperature. The equilibrium constant is represented by K and shows how far the reaction will progress at a specific temperature by determining the ratio of products to reactions using equilibrium concentrations.

In an equilibrium expression: aA+bB ⇌cC+dD

K c = [C] c [D] d [A] a [B] b

K=equilibrium constant, [A], [B], [C], [D] are concentrations and a, b, c, d are the stoichiometric coefficients of the balanced equation.

Exothermic Reactions

•If K decreases with an increase in temperature, the reaction moves to the left.

•If K increases with a decreases in temperature, the reaction to moves to the right.

Endothermic Reactions

•If K increases with an increase in temperature, the reaction to moves to the right.

•If K decreases with a decrease in temperature, the reaction to move to the left.

more at web page