Chemical Energetics Flashcards

Flashcards about chemical energetics

What is enthalpy (H)?

The heat energy stored in a chemical system.

Can enthalpy be measured experimentally?

No, but an enthalpy change can be measured from the temperature change in a chemical reaction.

What is an enthalpy change (ΔH)?

The heat energy exchange with the surroundings at constant pressure.

What is the first law of thermodynamics?

Energy may be exchanged between a chemical system and the surroundings, but the total energy remains constant.

What happens during an exothermic reaction?

Heat energy is released to the surroundings.

What is the sign of ΔH in an exothermic reaction?

Negative.

What happens during an endothermic reaction?

Heat energy is taken in from the surroundings.

What is the sign of ΔH in an endothermic reaction?

Positive.

What are standard conditions?

A pressure of 100 kPa, a stated temperature (usually 298K or 25°C), and a concentration of 1 mol dm–3 for aqueous solutions.

What is standard enthalpy change of reaction (ΔH—o r)?

The enthalpy change that accompanies a reaction in the molar quantities that are expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

What is standard enthalpy change of combustion (ΔH —o c)?

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.

What is standard enthalpy change of formation (ΔH —o f)?

The enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions.

What is the standard enthalpy change of formation for an element?

Zero.

What is standard enthalpy change of neutralization (ΔH —o neut)?

The energy change that accompanies the neutralization of an acid by a base to form 1 mole of H2O(l), under standard conditions.

How is the heat energy change (Q) in the surroundings calculated?

Q = mcΔT, where m is the mass of the surroundings, c is the specific heat capacity, and ΔT is the temperature change.

What is Hess’s Law?

If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

What is bond enthalpy?

The enthalpy change required to break and separate 1 mole of bonds in the molecules of a gaseous element or compound so that the resulting gaseous species exert no forces upon each other.

Is bond breaking an endothermic or exothermic process?

Endothermic.

Is bond making an endothermic or exothermic process?

Exothermic.

How can the enthalpy change for a reaction be determined using average bond enthalpies?

ΔH = Σ(bond enthalpies of reactants) – Σ(bond enthalpies of products).