Question

When the solids ammonium thiocyanate and barium hydroxide are mixed in a beaker, a solution forms, and the temperature of the solution drops to \(-5^{\circ} \mathrm{C}\). Is this an endothermic or exothermic reaction? Explain.

Short answer

The reaction of ammonium thiocyanate and barium hydroxide causing the temperature to decrease is an endothermic reaction. This is because endothermic reactions absorb heat from their surroundings, which leads to a decrease in temperature.

Step-by-step solution

Understand endothermic and exothermic reactions

Endothermic reactions are those in which heat is absorbed from the surroundings. In these reactions, there is a decrease in temperature. On the other hand, exothermic reactions are those in which heat is released to the surroundings, resulting in an increase in temperature.

Review the problem statement

In the problem, when ammonium thiocyanate and barium hydroxide are mixed in a beaker, the temperature of the solution drops to \(-5^{\circ} \mathrm{C}\). This means that the reaction causes a decrease in temperature.

Determine the type of reaction

Since there is a decrease in temperature, the reaction in question is absorbing heat from its surroundings. Therefore, it is an endothermic reaction.

Key concepts

Thermochemistry

Thermochemistry is the branch of chemistry that studies the energy and heat associated with chemical reactions and physical transformations. It's critical to understand the principles of thermochemistry to grasp concepts like enthalpy, entropy, and the heat of reaction which tie into whether a process is endothermic or exothermic.

In an endothermic reaction, heat is drawn in from the environment, often resulting in a temperature drop of the surroundings, as energy is absorbed into the system to break bonds in the reactants. The total energy of the system increases. On the contrary, an exothermic reaction releases heat to the surroundings, typically raising their temperature, as energy is released from the system when new bonds form in the products, and the total energy of the system decreases. These heat changes are at the core of thermochemical equations, which give us insight into the energetic behavior of reactions.

Chemical Reactions

A chemical reaction involves the transformation of substances, called reactants, into new substances, called products. During this transformation, bonds between atoms in the reactants are broken and new bonds are formed in the products. There are many types of chemical reactions, but they can all be described in terms of energy changes, linked to the bonds involved.

In understanding chemical reactions, it's important to note that bond breaking requires energy, while bond making releases energy. This means that whether a reaction is endotheric or exothermic depends on the balance between the energy consumed to break the reactants' bonds and the energy released when the products' bonds are formed. In the given exercise, the reaction is endothermic as the temperature decrease indicates that the system absorbed more energy to break bonds than was released in forming new ones.

Heat Transfer in Reactions

Heat transfer in chemical reactions is a manifestation of the laws of thermodynamics. It represents the flow of thermal energy from hotter to cooler regions or objects. In the context of chemical reactions, heat transfer occurs when energy is exchanged between the system (where the reaction takes place) and its surroundings.

Endothermic reactions, like the one involving ammonium thiocyanate and barium hydroxide, cause the surroundings to cool down as heat is transferred from the surroundings to the system. Conversely, exothermic reactions warm up the surroundings due to the transfer of heat from the system to its environment. The direction and magnitude of heat transfer not only dictate the thermal outcome but also influence other reaction conditions, like rate and equilibrium, further underlining the intricate relationship between energy and chemical processes.