Chemistry Unit #6: Energetics.............................. Test on March 14, 2017

Question 1

Define Exothermic 3

Answer 1

Is a chemical reaction that releases energy in the form of heat

Temperature of reaction increases 

May be shown as product A + B => C + D + heat

Question 2

Define Calorimeter

Answer 2

A calorimeter is a device used to measure the heat flow of a chemical reaction or physical change.

Question 3

Define Standard Enthalpy Change 7

Answer 3

Measures the amount of heat energy transferred under standard conditions

Enthalpy difference between products and reactants under standard conditions

Pressure is @100.0 kPa

Temperature is @298 K or 25°C

Standard conditions are indicated by °

It can't measure internal energy of substance 

It can only measure change in heat content 

It can measure the heat energy transferred for a chemical reaction

Question 4

Define calorie (Not Calorie)

Answer 4

It is the amount of heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C. 1 calorie = 4.184 joules.

Question 5

Define Activation Energy 2

Answer 5

It is the minimum energy required for reaction to occur

The activation energy is usually represented by the symbol Ea

Question 6

Define Endothermic 3

Answer 6

Is a chemical reaction that absorbs energy from the surroundings

Temperature of reaction decreases 

May be shown as product A + B + heat => C + D

Question 7

State what a calorimeter is and how it works (including how to adjust the temperature for heat loss) 3-4

Answer 7

A calorimeter can as simple as a Styrofoam container.

Using a thermometer, we can measure the hamount of heat involved in a chemical or physical process from the surroundings of the calorimeter.

To adjust the temperature for heat loss, we extrapolate the maximum temperature and get ∆T by subtracting T(extrapolated) by T(initial).

This means that the amount of heat produced or consumed in the reaction equals the amount of heat absorbed or lost by the solution.

Question 8

Define Specific Heat Capacity

Answer 8

Is the specific amount of energy needed to raise the temperature of 1g of substance by 1 K or (1°C)

Question 9

Define Heat 2

Answer 9

Heat is a measure of the total kinetic energy of a substance

The total energy depends on the amount of material present

Question 10

Define Temperature 2

Answer 10

Measures average kinetic energy of the a substance

Number of particles is unimportant

Question 11

When the solids Ba(OH)2 and NH4SCN are mixed, a solution is produced and the temperature drops.

Ba(OH)2(s) + 2NH4SCN(s) → Ba(SCN)2(aq) + 2NH3(g) + 2H2O(l)

Which statement about the energetics of this reaction is correct?

A. The reaction is endothermic and ∆H is negative.
B. The reaction is endothermic and ∆H is positive.
C. The reaction is exothermic and ∆H is negative.
D. The reaction is exothermic and ∆H is positive.

Answer 11

B

Question 12

A sample of a metal is heated. Which of the following are needed to calculate the heat absorbed by the sample?

I. The mass of the sample
II. The density of the sample
III. The specific heat capacity of the sample

A. I and II only
B. I and III only
C. II and III only
D. I, II and III

Answer 12

B

Question 13

Using the equations below:
Cu(s) + O2(g) → CuO(s)∆Hο = –156 kJ
2Cu(s) + O2(g) → Cu2O(s)∆Hο = –170 kJ

What is the value of ∆Hο (in kJ) for the following reaction?

2CuO(s) → Cu2O(s) + O2(g)

A. 142 B. 15
C. –15 D. –142

Answer 13

A

Question 14

34. How much energy, in joules, is required to increase the temperature of 2.0 g of aluminium from 25 to 30°C? (Specific heat of Al = 0.90 J g–1 K–1).

A. 0.36
B. 4.5
C. 9.0
D. 54

Answer 14

C

Question 15

Which statements about exothermic reactions are correct?

I. They have negative ∆H values.
II. The products have a lower enthalpy than the reactants.
III. The products are more energetically stable than the reactants.

A. I and II only
B. I and III only
C. II and III only
D. I, II and III

Answer 15

D

Question 16

The average bond enthalpies for O—O and O==O are 146 and 496 kJ mol–1 respectively.
What is the enthalpy change, in kJ, for the reaction below?

H—O—O—H(g) ===> H—O—H(g) + ½O==O(g)

A. – 102 B. + 102
C. + 350 D. + 394

Answer 16

A

Question 17

What energy changes occur when chemical bonds are formed and broken?

A. Energy is absorbed when bonds are formed and when they are broken.
B. Energy is released when bonds are formed and when they are broken.
C. Energy is absorbed when bonds are formed and released when they are broken.
D. Energy is released when bonds are formed and absorbed when they are broken.

Answer 17

D

Question 18

Which statements are correct for an endothermic reaction?

I. The system absorbs heat.
II. The enthalpy change is positive.
III. The bond enthalpy total for the reactants is greater than for the products.

A. I and II only
B. I and III only
C. II and III only
D. I, II and III

Answer 18

A

Question 19

Using the equations below:

C(s) + O2(g) → CO2(g)	∆H = –390 kJ Mn(s) + O2(g) → MnO2(s) 	∆H = –520 kJ

What is ∆H (in kJ) for the following reaction?
MnO2(s) + C(s) ===> Mn(s) + CO2(g)

A. 910
B. 130
C. –130
D. –910

Answer 19

B

Question 20

When 40 joules of heat are added to a sample of solid H2O at –16.0°C the temperature increases to –8.0°C. What is the mass of the solid H2O sample?

[Specific heat capacity of H2O(s) = 2.0 J g–1K–1]

A. 2.5 g
B. 5.0 g
C. 10 g
D. 160 g

Answer 20

A

Question 21

Which statement about bond enthalpies is correct?

A. Bond enthalpies have positive values for strong bonds and negative values for weak bonds.
B. Bond enthalpy values are greater for ionic bonds than for covalent bonds.
C. Bond breaking is endothermic and bond making is exothermic.
D. The carbon–carbon bond enthalpy values are the same in ethane and ethene.

Answer 21

C

Question 22

What two types of reactions are always exothermic?

Answer 22

Combustion and neutralization

Question 23

Is Ice melting an exothermic reaction?

Answer 23

No

Question 24

Consider the following equations.

Mg(s) +  O2(g) → MgO(s)    ∆Hο = –602 kJ H2(g) +  O2(g) → H2O(g)      ∆Hο = –242 kJ

What is the ∆H° value (in kJ) for the following reaction?

MgO(s) + H2(g) → Mg(s) + H2O(g)

A. –844 B. –360
C. +360 D. +844

Answer 24

C

Question 25

For which of the following is the sign of the enthalpy change different from the other three? A. CaCO3(s) → CaO(s) + CO2(g) B. Na(g) → Na+(g) + e– C. CO2(s) → CO2(g) D. 2Cl(g) → Cl2(g)

Answer 25

D

Question 26

Separate solutions of HCl(aq) and H2SO4(aq) of the same concentration and same volume were completely neutralized by NaOH(aq). X kJ and Y kJ of heat were evolved respectively. Which statement is correct? A. X = Y B. Y = 2X C. X = 2Y D. Y = 3X

Answer 26

B

Question 27

Which statement is correct for an endothermic reaction? A. The products are more stable than the reactants and ∆H is positive. B. The products are less stable than the reactants and ∆H is negative. C. The reactants are more stable than the products and ∆H is positive. D. The reactants are less stable than the products and ∆H is negative.

Answer 27

C

Question 28

The standard enthalpy change of formation values of two oxides of phosphorus are: P4(s) + 3O2(g) ===> P4O6(s) ∆HӨf= –1600 kJ mol–1 P4(s) + 5O2(g) ===> P4O10(s) ∆HӨf= –3000 kJ mol–1 What is the enthalpy change, in kJ mol–1, for the reaction below? P4O6(s) + 2O2(g) ===> P4O10(s) A. +4600 B. +1400 C. –1400 D. –4600

Answer 28

C

Question 29

Which statements are correct for all exothermic reactions? I. The enthalpy of the products is less than the enthalpy of the reactants. II. The sign of ∆H is negative. III. The reaction is rapid at room temperature. A. I and II only B. I and III only C. II and III only D. I, II and III

Answer 29

A

Question 30

``` The mass m (in g) of a substance of specific heat capacity c (in J g–1 K–1 ) increases by t°C. What is the heat change in J? A. mct B. mc(t + 273) C. mct/1000 D. [mc(t + 273)]/1000 ```

Answer 30

A

Question 31

Define the term average bond enthalpy, illustrating your answer with an equation for methane, CH4. 2

Answer 31

Average value of the energy required to break a bond (average of each C-H in Ch4) {1} When molecule and atoms are in gaseous states {1}

Question 32

Using the equations below: C(s) + O2(g) → CO2(g) ∆Hο = –394 kJ mol–1 Mn(s) + O2(g) → MnO2(s) ∆Hο = –520 kJ mol–1 What is ∆H, in kJ, for the following reaction? MnO2(s) + C(s) → Mn(s) + CO2(g) A. 914 B. 126 C. –126 D. –914

Answer 32

B

Question 33

Which combination is correct for a chemical reaction that absorbs heat from the surroundings? Type of reaction ΔH at constant pressure A. Exothermic Positive B. Exothermic Negative C. Endothermic Positive D. Endothermic Negative

Answer 33

C

Question 34

Consider the specific heat capacity of the following metals. ``` Metal Specific heat capacity / J kg–1 K–1 Cu 385 Ag 234 Au 130 Pt 134 ``` Which metal will show the greatest temperature increase if 50 J of heat is supplied to a 0.001 kg sample of each metal at the same initial temperature? A. Cu B. Ag C. Au D. Pt

Answer 34

C

Question 35

The temperature of a 2.0 g sample of aluminium increases from 25°C to 30°C. How many joules of heat energy were added? (Specific heat of Al = 0.90 J g–1K–1) A. 0.36 B. 2.3 C. 9.0 D. 11

Answer 35

C

Question 36

Consider the following reactions. S(s) + O2(g) ===> SO3(g) ∆HӨ = -395 kJ mol1 SO2(s) + O2(g) ===> SO3(g) ∆HӨ = -98 kJ mol1 What is the ∆HӨ value (in kJ mol–1) for the following reaction? S(s) + O2(g) ===> SO2(g) A. –297 B. +297 C. – 493 D. +493

Answer 36

A

Question 37

Which statement is correct for an endothermic reaction? A. Bonds in the products are stronger than the bonds in the reactants. B. Bonds in the reactants are stronger than the bonds in the products. C. The enthalpy of the products is less than that of the reactants. D. The reaction is spontaneous at low temperatures but becomes non-spontaneous at high temperatures.

Answer 37

B

Question 38

Comparing a 100 ml solution with the temperature of 10°C, to a 100 ml solution with the temperature of 20°C, what solution has the highest temperature and has the highest heat? 2

Answer 38

The 100 ml solution with the temperature of 20°C has the highest temperature The 100 ml solution with the temperature of 20°C has the highest heat

Question 39

Describe the changes in bonding that take place during the combustion of C₃H₈. [2]

Answer 39

[C₃H₈ + O₂] bonds are broken, requiring energy as they separate. [H₂O + CO₂] bonds are formed, releasing energy as they form.

Question 40

Describe an experiment to determine the enthalpy change of the reaction between dilute hydrochloric acid and aqueous sodium hydroxide. Show how the value of ΔH would be calculated from the data obtained. [10]

Answer 40

Take a known volume of sodium hydroxide solution Take a known molarity/concentration of sodium hydroxide solution Record the temperature of the sodium hydroxide solution Place sodium hydroxide solution in an isolated vessel Add the solution of HCI Such that an equal/excess number of moles/known quantity is added Stir/mix the solution Record the final tempearature HCI + NaOH ===> NaCl + H₂O ΔH = Total mass (or volume) X temperature change X specific heat capacity Divide ΔH by the # of mole of limiting reactant = kJmol

Question 41

In an enthalpy level diagram, what does the symbol R signifies?

Answer 41

Initial Enthalpy of Reactants

Question 42

Comparing a 100 ml solution with the temperature of 10°C, to a 500 ml solution with the temperature of 10°C, what solution has the highest temperature and has the highest heat? 2

Answer 42

The temperature for both solutions are the same The 500 ml solution with the temperature of 10°C has the highest heat

Question 43

A negative ΔH is an _______ reaction.

Answer 43

Exothermic Heat was absorbed by the sorroundings

Question 44

What is the symbol for Activation energy?

Answer 44

Ea

Question 45

In an enthalpy level diagram, what does the symbol P signifies?

Answer 45

Final Enthalpy of Products

Question 46

What reaction is it where Products are more stable than the reactants?

Answer 46

Exothermic reaction

Question 47

A positive ΔH is an _______ reaction.

Answer 47

Endothermic Heat was absorbed from the sorroundings

Question 48

In an enthalpy level diagram, what does the symbol ΔH signifies?

Answer 48

Change in Enthalpy

Question 49

What are conclusions from Enthalpy Level Diagrams? 6

Answer 49

Stronger Bonds release more energy when formed and are more difficult to break In an exothermic reaction, Bonds in products are stronger than in reactants Products are more stable than the reactants In an endothermic reaction, Bonds in products are less strong than in reactants Reactants are more stable than products Generally need extra energy to proceed

Question 50

What are the limitations of Bond Enthalpies? 2

Answer 50

➫ Values from table are only suitable for substances in gaseous states Producing a liquid or solid would release extra energy Recall energy changes involved in altering states of matter Breaking bonds in solids or liquids require more energy than stated First must become gaseous ➫ The values are only average bond enthalpies Average obtained by considering a number of similar compounds Neighboring atoms will affect the exact energy value of a bond

Question 51

How do you calculate ΔT in a graph?

Answer 51

ΔT = T(extrapolated) - T(initial)

Question 52

What are we assuming in when graphing a Calorimeter Data?

Answer 52

The rate of heat loss after reaction is constant

Question 53

What reaction is it where Reactants are more stable than the Products?

Answer 53

Endothermic reaction

Question 54

State Hess' Law and one reason why it could be used. 2-3

Answer 54

Hess' Law states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. Hess's Law lets us find the enthalpy change of a reaction we cannot measure using other reactions that lead to the one we can't measure because we know their values. Official definition: The enthalpy change accompanying a chemical reaction is independent of the pathway between the initial and final steps.

Question 55

What is an assumption we make about the heat of the reaction in a calorimeter? 2-3

Answer 55

We assume that all heat of the reaction was transferred to/from the water. It cannot actually occur We assume that the rate of energy change that occurs is the same for the entire time

Question 56

What are we assuming for Graphing Calorimeter Data?

Answer 56

We assume that the rate of heat loss for the calorimeter is constant

Question 57

What are the two 0= _______ + _____________?

Answer 57

0 = energy lost + energy gained 0 = ΔH(reaction) + ΔH(solution)