# Pre-lab Questions

1. Define enthalpy.

Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.  The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements, because it simplifies certain descriptions of energy transfer. This is because a change in enthalpy takes account of energy transferred to the environment through the expansion of the system under study.

• What is the relationship between the enthalpy of a reaction and its classification as endothermic or exothermic?

Endothermic reactions draw heat in from their surroundings.  Exothermic reactions give heat off to their surroundings.

Endothermic:

• Energy is drawn in from the external environment, causing its surroundings to loose heat, or “cool down.”
• The endothermic chemical reaction creates a product that has a higher energy level than the original materials, causing the reactant’s stored energy to decrease. (In scientific terms, the reactants have “less total enthalpy” than the product.)
• The resulting product of the reaction is less stable because, the higher the energy bond, the less strength its molecules possess.
• Most endothermic reactions are not spontaneous.

Exothermic

• Exothermic chemical reactions cause their surroundings to warm up by giving off heat.
• The reactants contain more stored energy than the product because energy from external sources is not required, but given off. This gives the product more stability due to the lower amount of energy needed. (In this case, the reactants have a “greater total enthalpy” than the product.)
• Exothermic reactions are usually spontaneous.
• With instant hot compresses, calcium chloride dissolves in water and the temperature of the mixture increases. Is this an endothermic or exothermic process?

Exothermic Process

# Questions

## 1.       Calculate the overall temperature change for the cold and hot pack substance. HINT: This is the difference in the maximum temperature and minimum temperature of each.

Cold pack ΔT:

Hand warmer ΔT:

• Which pack works by an exothermic process?  Use experimental data to support your answer.

The chemical reaction that releases heat is called an exothermic reaction, so a heat pack. The cold pack is endothermic

• Which pack works by an endothermic process?  Use experimental data to support your answer.

The cold pack is endothermic because Energy is drawn in from the external environment, causing its surroundings to loose heat, or “cool down.”

• Which pack had the greatest change in enthalpy?  How do you know?
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