1.4 Energetics

Question 1

Define Enthalpy Change

Answer 1

The amount of heat energy taken in or given out during any change in a system provided the pressure is constant

Question 2

What happens in an exothermic change?

Answer 2

Energy is transferred from the system to the surroundings. The products have less energy than the reactants.

Question 3

Is the enthalpy change of an exothermic reaction positive or negative?

Answer 3

Negative

Question 4

What are common oxidation exothermic processes?

Answer 4

The combustion of fuels and the oxidation of carbohydrates such as glucose in respiration.

Question 5

Draw and label an exothermic graph.

Answer 5

Question 6

What happens in endothermic change?

Answer 6

Energy is transferred from the surroundings to the system. They require an input of heat energy e.g. thermal decomposition of calcium carbonate. The products have more energy than the reactants.

Question 7

Is the enthalpy change of an endothermic reaction positive or negative?

Answer 7

Positive

Question 8

Draw and label an endothermic graph.

Answer 8

Question 9

Define the standard enthalpy change of combustion

Answer 9

The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. All reactants and products being in their standard states.

Question 9

Define the standard enthalpy change of formation

Answer 9

The standard enthalpy of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions, all reactants and products being in their standard states.

Question 10

What does incomplete combustion lead to?

Answer 10

Soot, carbon monoxide and water. it will be less exothermic than complete combustion.

Question 11

Whats the equation for energy change and units?

Answer 11

Q = mc /\T

Q - energy change (j)
m - mass solution (g)
Cp - heat capacity (J g-1 k-1)
/\T - temp change - (k)

Question 11

Whats the general method for measuring enthalpy change (Calorimetric)?

Answer 11

Question 11

What are the errors in a general method of calculating the enthalpy change of a reaction?

Answer 11

Question 12

Whats the general method for calculating enthalpy change of reaction?

Answer 12

Question 13

Whats the heat capacity of water?

Answer 13

4.18 Jg-1K-1. In any reaction where the reactants are dissolved in water we assume that the heat capacity is the same as pure water.

Question 14

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Question 16

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Question 17

What does Hess’ law state?

Answer 17

Hess’ law states that total energy change for a reaction is independent of the route by which the chemical change takes place

Question 18

Whats the equation used for Enthalpy change when using hess’ law to determine enthalpy change of formation?

Answer 18

Question 19

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Question 20

Draw a Hess’ law cycle for enthalpy change of formation.

Answer 20

Question 21

What’s the equation used for Enthalpy change when using hess’ law to determine enthalpy change of combustion?

Answer 21

Question 22

Draw a Hess’ law cycle for enthalpy change of combustion.

Answer 22

Question 23

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Question 24

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Question 25

Define mean bond energy.

Answer 25

The mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules.

Question 26

In general (if all substances are gases) whats the equation for /\H

Answer 26

Question 27

Why do we use mean bond energies?

Answer 27

We use values of mean bond energies because every single bond in a compound has a slightly different bond energy. Breaking each one will require a different amount of energy. However, we use an average value for the C-H bond foe all hydrocarbons

Question 28

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Question 29

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