Question

If energy flows out of a chemical system and into the surroundings, what is the sign of \(\Delta E_{\text {system }} ?\)

Short answer

\(\Delta E_{\text{system}}\) is negative when energy flows out of the system.

Step-by-step solution

Understanding Energy Flow

The change in internal energy of a system, denoted as \(\Delta E_{\text{system}}\), can be positive or negative depending on the direction of energy flow. If energy flows out of the system, the internal energy of the system decreases.

Defining the Sign of \(\Delta E_{\text{system}}\)

By convention, when a system loses energy, the change in internal energy \(\Delta E_{\text{system}}\) is negative. This is because the final energy is less than the initial energy.

Key concepts

Energy Flow in Chemistry

Chemistry is inherently an energetic science. The flow of energy in chemical processes underpins countless phenomena from the smallest cellular reactions to the largest industrial processes. When we consider a chemical reaction, it is crucial to understand that energy cannot be created or destroyed; it can only be converted from one form to another according to the First Law of Thermodynamics.

In the context of a chemical system, energy transfer occurs when reactions release or absorb energy, often in the form of heat or work. Exothermic reactions are ones that release energy to the surroundings; they typically feel warm or hot as a result. Conversely, endothermic reactions absorb energy, making the surrounding area feel cooler. For students to envision this concept, imagine lighting a match (exothermic) whereas dissolving certain salts in water might feel cold to touch (endothermic).

The sign of the change in internal energy (\( \text{ΔE}_{\text{system}} \) ) is a simple yet powerful indicator of this energy flow. If energy is leaving the system, as in the burn of a match, \( \text{ΔE}_{\text{system}} \) is negative because the system’s energy is decreasing.

Thermodynamics

Thermodynamics is the study of heat and energy and how they interact with matter. It is split into several fundamental laws that describe different aspects of energy flow and conversion.

The First Law of Thermodynamics, also known as the Law of Energy Conservation, states that the energy of the universe is constant – energy can be transformed from one form to another but cannot be created or destroyed. This law explains why the internal energy of a chemical system changes as energy is exchanged with its surroundings. When students measure thermal changes in chemical reactions, they are directly observing the principle of energy conservation.

It's this interplay of energy that governs whether a given reaction will proceed spontaneously, which is a topic dealt with by the Second Law of Thermodynamics. This law states that the total entropy, or disorder, of an isolated system can never decrease over time. Understanding these concepts allows students to predict the feasibility and directionality of chemical reactions.

Chemical System

In chemistry, a 'system' typically refers to the part of the universe we are focusing on; this might be a flask containing a reaction mixture, a single cell, or a complex set of industry-grade reactors. A 'surrounding' is everything external to the system, which can include the rest of the laboratory, the atmosphere, or a cooling bath.

A chemical system can be either open, closed, or isolated, depending on its ability to exchange matter and energy with the surroundings. In an open system, both matter and energy can flow in and out. In a closed system, only energy can be exchanged, and in an isolated system, neither energy nor matter is exchanged. These distinctions are important when considering the energy changes that occur during a chemical reaction.

An understanding of these differences helps clarify why \( \text{ΔE}_{\text{system}} \) might differ under varied conditions. For example, in a closed system where energy can escape as heat, an exothermic reaction will lead to a negative \( \text{ΔE}_{\text{system}} \) as this energy is lost to the surroundings. By contrast, the same reaction in an isolated system might lead to an increase in temperature of the system since the energy has nowhere to go.