Textbook Ch.8: Thermochemistry

Question 1

System

Answer 1

the part of the universe in which attention is focused

Question 2

Surroundings

Answer 2

exchange energy within the system, make up the rest of the universe

Question 3

State of a system

Answer 3

composition, temperature, and pressure

Question 4

State properties

Answer 4

depend ONLY on the state of the system, not on the way the system reached the state

Question 5

q

Answer 5

the variable for heat flow (magnitude of heat flow)

• expressed in joules and kilojoules
• q is positive when heat flows into the system from the surroundings
• q is negative when heat flows out of the system into the surroundings

Question 6

Endothermic reaction

Answer 6

a reaction in which heat flows from the surroundings into the reaction system

• q is positive (q>0)
• Temperature of the surroundings DROPS
• Ex: melting ice

Question 7

Exothermic reaction

Answer 7

a reaction in which heat flows from the reaction system into the surroundings

• q is negative (q<0)
• Temperature of the surroundings RISES
• Ex: combustion of methane
• Reaction evolves heat

Question 8

Calorie

Answer 8

the amount of heat required to raise the temperature of 1g of water 1°C

• 1 cal = 4.194 J
• 1 kcal = 4.184 kJ

Question 9

What equation gives the relationship between magnitude of heat flow (q) and the temperature change (∆t)

Answer 9

q = C * ∆t

q = magnitude of heat flow
C = heat capacity
∆t = tfinal - tinitial

Question 10

Heat Capacity (C)

Answer 10

the amount of heat required to raise the temperature of the system 1°C and has the units J/°C

Question 11

What equation gives the relationship between heat flow (q), temperature change (∆t), and the mass (m)

Answer 11

q = m * c * ∆t

Question 12

Specific Heat (c)

Answer 12

the amount of heat required to raise 1g of the substance 1°C

• It is like density or melting point because it is an intensive property that can be used to identify a substance or determine its purity

Question 13

Calorimeter

Answer 13

a device used to measure the heat flow in a reaction

• If the reaction is exothermic, the heat flow in the calorimeter is positive because heat flows from the reaction to the calorimeter
• If the reaction is endothermic, the heat flow in the calorimeter is negative because the calorimeter gives up heat to the reaction mixture

reaction = - Ccalt

Question 14

Coffee-cup calorimeter

Answer 14

two foam cups partially filled with water and have a thermometer inserted

Heat capacity of the coffee-cup calorimeter = heat capacity of the water

CcalWater * water = water * 4.18 J/g°C
q reaction = -water * 4.18 J/g°C * ∆t

Question 15

Bomb calorimeter

Answer 15

more versatile than a coffee-cup calorimeter because it can be used with gases, and reactions in which the products reach high temperatures

Ccal = 9.33 kJ/°C

• All heat given off by the reaction is absorbed by the bomb calorimeter

q reaction = -calorimeter
reaction = -Ccal * ∆t

Question 16

Enthalpy

Answer 16

a type of chemical energy sometimes referred to as “heat content”

• The heat flow for the reaction system = the difference in enthalpy (H) between the products and reactants
• In exothermic reactions, the enthalpy is negative
• In endothermic reactions, the enthalpy is positive
• the enthalpy of a substance is a state property

Question 17

Thermochemical equation

Answer 17

a chemical equation that shows the enthalpy relation between products and reactions

Question 18

How do you interpret thermochemical equations?

Answer 18

• Sign of ∆H (enthalpy) indicates whether the reaction is endothermic (+∆H), or exothermic (-∆H)
• The coefficients represent the number of moles
• The phases of all species must be specified

Question 19

Rules of Thermochemistry

Answer 19

1. The magnitude of ∆H is directly proportional to the amount of reactant or product
2. ∆H for a reaction is equal in magnitude but opposite sign to ∆H for the reverse reaction (AKA heat evolved = heat absorbed)
3. The value of ∆H for a reaction is the same whether it occurs in one step of in a series of steps (Hess’s Law)

Question 20

Heat of fusion

Answer 20

the heat absorbed when a solid melts (solid –> liquid)

Question 21

Heat of vaporization

Answer 21

the heat absorbed when a liquid vaporizes (liquid –> gas)

Question 22

Standard molar enthalpy of formation ΔHf

Answer 22

enthalpy change when one mole of the compound is formed at a constant pressure of 1 atm and a fixed temperature of 25°C

Question 23

The enthalpy of formation of an element in its stable state at 25°C and 1 atm is taken to be:

Answer 23

zero

Question 24

The standard enthalpy of change for a given thermochemical equation is equal to:

Answer 24

the sum of the standard enthalpies of formation of the compounds minus the sum of the standard enthalpies of formation f the reactant compounds

ΔH = ΣΔH(products) − ΣΔH(reactants)

Question 25

If ΔH is a large negative number:

Answer 25

the reaction gives off a lot of heat

Question 26

If ΔH is positive

Answer 26

heat must be absorbed for the reaction to occur

Question 27

Bond enthalpy

Answer 27

ΔH when one mole of bonds is broken in the gaseous state

ALWAYS POSITIVE

Question 28

A reaction is expected to be endothermic if:

Answer 28

• The bonds in the reactants are stronger than in the products
• There are more bonds in the reactants than in the products

Question 29

Bond enthalpy is ____ for a multiple bond than a single bond

Answer 29

larger

Question 30

Thermodynamics distinguishes between:

Answer 30

two types of energy: Heat (q) and work (w)

Question 31

Work

Answer 31

all forms of energy except heat

Question 32

Law of conservation of energy

Answer 32

energy (E) can neither be created nor destroyed; it can only be transferred between system and surroundings

Question 33

1st Law of Thermodynamics

Answer 33

in any process, the total change in the energy of a system, ΔE, is equal to the sum of the heat (q), and the work (w) transferred between the system and the surroundings

ΔE = q + w

Question 34

q and w are positive when:

Answer 34

heat or work enters the system

Question 35

q and w are negative when

Answer 35

heat or work goes to the surroundings