Chemistry Chapter on Enthalpy Changes

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What is indicated by a negative value of ΔH in a chemical reaction?

The reaction has high activation energy.

The reaction requires energy input to proceed.

The reaction is endothermic.

The reaction releases energy. (correct)

What happens to the enthalpy change when a reaction occurs exothermically?

ΔH becomes positive.

ΔH remains constant.

ΔH becomes negative. (correct)

ΔH cannot be defined.

Under standard conditions, enthalpy changes are measured at which temperature?

373 Kelvin

0 Kelvin

310 Kelvin

298 Kelvin (correct)

How is the relationship between the activation energies of the forward and reverse reactions expressed in terms of ΔH?

ΔH = ΔEa - ΔEb (C) Signup and view all the answers

In an energy diagram of an exothermic reaction, which part represents the energy level of the products?

The endpoint after the reactants. (A) Signup and view all the answers

What characterizes an endothermic reaction?

It absorbs heat from its surroundings. (B) Signup and view all the answers

In the reaction $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$, what does the negative enthalpy change ($ abla H = -890 , kJmol^{-1}$) indicate?

The reaction is exothermic. (C) Signup and view all the answers

What is represented by $E_a$ on the energy graph of a reaction?

The activation energy required to start the reaction. (C) Signup and view all the answers

Which of the following statements is true regarding the energy changes during an endothermic reaction?

The enthalpy change ($ abla H$) is greater than zero. (D) Signup and view all the answers

What defines the energy content of the reactants and products in a chemical reaction?

The relative energy levels of reactants and products. (C) Signup and view all the answers

Thermochemistry

The branch of chemistry that studies the heat changes associated with chemical reactions and physical changes.

Enthalpy Change (ΔH)

The energy change that occurs during a reaction, measured in kJ/mol. It's positive for energy absorption and negative for energy release.

Exothermic Reaction

A reaction that releases energy into the surroundings, causing the temperature to rise. The enthalpy change (ΔH) is negative.

Activation Energy (Ea)

The minimum amount of energy required for reactant molecules to collide and form products. It's shown as the peak on an energy diagram.

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Enthalpy Change (ΔH) in terms of Activation Energies

The difference between the activation energy of the forward reaction and the activation energy of the reverse reaction. It's negative for exothermic reactions.

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Energy Profile Diagram

A visual representation of the energy changes during a chemical reaction, showing how energy changes over time. It can help us understand whether a reaction is exothermic or endothermic.

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