Higher Chemistry - Unit 3

What is the definition of enthalpy?

The energy content or the chemical potential energy

What is the equation for the change in enthalpy?

Enthalpy content of the products - Enthalpy content of the reactants

Is the enthalpy change in exothermic reactions positive or negative?

Negative

Is the enthalpy change in endothermic reactions positive or negative?

Positive

What is the definition of activation energy?

The minimum energy required for a reaction to occur

What is the equation for the activation energy?

Top of the curve - Reactants

What is the definition of activated complex?

An unstable arrangement of atoms formed at the maximum of the potential energy diagram for a reaction

How does changing the temperature effect the kinetic energy particles shown in the diagram?

Increases the number of particles with an energy equal to or greater than the activation energy (It is only these collisions which result in a reaction)

How do catalysts increase the rate of a reaction?

By providing an alternative route for the reaction which has a lower activation energy (catalysts do not lower the activation energy)

What is the definition of the enthalpy of combustion?

The energy released when one mole of a substance burns completely in oxygen

What are mean molar enthalpies?

Average values which are quoted for bonds which occur in different molecular environments

Bond breaking is an _____ process

Endothermic (+ve)

Making bonds is an _____ process

Exothermic (-ve)

What is the definition of bond dissociation enthalpy?

The energy needed to break one mole of bonds to give separated atoms

What is the equation for the enthalpy of sublimation of carbon?

C(s) —> C(g)

What does Hess’s Law state?

The enthalpy change for a chemical reaction is independent of the route taken (it is the same no matter the route taken)

What are the two rules for Hess’s Law equations?

• If the equation is reversed, the sign of Δ𝐻 is reversed

• If the equation is multiplied or divided, multiply or divide the Δ𝐻 value

In what conditions is the molar volume for all gases the same?

At room temperature and pressure