How can you determine whether an equation is endothermic or exothermic?

Quick answer:

In a chemical equation, the location of the word "heat" can be used to quickly determine whether the reaction is endothermic or exothermic. If heat is released as a product of the reaction, the reaction is exothermic. If heat is listed on the side of the reactants, the reaction is endothermic. 

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There are a couple of ways to determine if a reaction is exothermic or endothermic. Endothermic meaning that heat is added to the reaction to make the reactants interact and exothermic meaning heat is released during the reaction between the two reactants.

In a chemical equation the word heat or the triange symbol are used to indicate if the reaction is exothermic or endothermic.  Thus, Na + HOH --> NaOH + heat would indicate the reaction is exothermic.    On the other hand, an equation such as 2H2 + O2 +heat --> 2 HOH would indicate that the reaction is endothermic and no reaction will take place unless heat is added.

Another way is to look at the energy diagram for the reaction.  In an energy diagram the energy is on the y-axis and time is on the x-axis.  The reactants are shown first at their energy level, and the products at their respective energy levels. In the middle between the two is the activation energy required to transform the reactants into products. If the energy level of the reactants is higher than the energy level of the products the reaction is exothermic (energy has been released during the reaction). If the energy level of the products is higher than the energy level of the reactants it is an endothermic reaction.

If the energy is not shown in the chemical reaction and there is no energy diagram to look at, a last alternative is to calculate the total bond energy of all the chemical bonds in the reactants and in the products. There are standard bond energies between almost all elements and you just have to look them up and do the math.

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Endothermic reactions need activation energy to proceed. Therefore, heat energy is needed for an endothermic reaction to occur. The prefix endo- means within, therefore energy must be absorbed for this type of reaction. Usually in this type of reaction, heat energy that is absorbed is changed into chemical energy. An example of an endothermic reaction is the melting of ice. Energy is required for this to occur. With chemical reactions, the added heat energy would be seen on the left side of the equation where the reactants are.

An exothermic reaction releases heat energy. The prefix ex- refers to outside and heat energy is released at the end of this type of reaction. Examples of exothermic reactions are when acids and bases are combined in a neutralization reaction--the products formed are salt and water and heat energy gets released as well.  Combustion of fuels is an exothermic reaction. When fuels combust, heat energy is released. An example of this is when fossil fuels are burned in engines of cars, or in oil or gas heaters.

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How can we identify if a chemical equation is exothermic or endothermic?

An exothermic reaction is one in which heat is released to the surroundings and hence can take place spontaneously. In comparison, an endothermic reaction is one which needs energy to take place and hence cannot take place spontaneously.

To determine if a given chemical equation represents an exothermic reaction or an endothermic reaction, we need to know the enthalpy of formation of products and reactants. Enthalpy is the total heat involved in the reaction. If the products have a lower enthalpy than the reactants, the change in enthalpy of the reaction would be less than zero (`DeltaH < 0`). Such a reaction, by convention, is termed an exothermic reaction and heat will be released when such reactions take place. In comparison, when the enthalpy of products is more than that of the reactants, the change in enthalpy associated with the reaction is greater than zero (`DeltaH > 0`) and the reaction is termed endothermic. 

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How does one know if a chemical reaction is exothermic or endothermic from the chemical equation for the reaction?

A chemical equation describes a reaction by showing the quantitative relationships between all of the reactants and products. Additional information is needed to indicate if a reaction is endothermic or exothermic.

Thermochemical equations include the enthalpy (heat) change in the chemical reaction, designating it as  `Delta` H. If the `Delta` H of a reaction is negative the reaction is exothermic (releases heat), and if it's positive the reaction is endothermic (absorbs heat).

Here's why: The enthalpy change, or `Delta` H of a reaction is the sum of all of the energy absorbed in breaking bonds in the reactants minus the sum of all of the energy released when new bonds form in the products. When the energy aborbed exceeds the energy released there is a net absorption of energy and `Delta` H is positive. Conversely, it's negative for a net release of energy.

An equation that includes enthalpy is typically written like this:

`CH_4 + 2 O_2 -> CO_2 + 2 H_2O, DeltaH = 891 (kJ)/(mol)`

If enthalpy isn't included it can be calculated by looking up bond energies of the individual bonds in all of the reactant and product molecules. The enthalpy change is 

 `Delta` H =  `Sigma` (bond energies of reactants)- `Sigma` (bond energies of products)

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How would one know that a chemical reaction, in a chemical equation, is exothermic or endothermic?

In order to understand whether a reaction in a chemical equation is endothermic or exothermic, we must first define these terms. An endothermic reaction is any reaction that has to absorb energy from the outside environment for the reaction to occur. An example of this is melting ice (although this is a state change, not a reaction), which requires heat from an outside source for the reaction to occur. An exothermic reaction is the opposite, and is therefore defined as any reaction that gives off energy to the surrounding environment when it occurs. An example of this would be an explosion which gives off energy to the surrounding environment as both light and heat.    

To answer your question, there is a way to predict if a chemical reaction will be endothermic or exothermic by looking at the chemical equation. The differences between an endothermic and exothermic reaction are considered differences in enthalpy. The net difference of the enthalpy for all the products in a chemical equation determines if the reaction is endothermic or exothermic. These enthalpy values are known as `Delta`H and are calculated using the formula `Delta`H=energy of the system+PV, where P is pressure and V is volume. Net values are calculated for each of the products going into the reaction. If the `Delta`H value is positive, the reaction requires the addition of energy and is endothermic, however if the ``H is negative the opposite is true, and the reaction is exothermic and gives off energy to the surrounding environment. Hope this helps! 

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