Download Energy Practice

1
ENERGY WORKSHEETS
1. What is the SI unit for energy?
□ Calculate the heat transferred in or out of a system using Q =mct
□ Explain kinetic energy changes the occur during chemical reactions
KINETIC ENERGY CHANGES
2. Define:
a. Kinetic Energy (Ek):
To calculate the kinetic energy change (Q) for any substance when its temperature
changes we use the following relationship:
Q = mc∆t
Q = Energy transferred to another substance as heat: Change in kinetic energy:
(usually J or kJ)
m = mass of substance to be considered (usually g or kg)
c = the specific heat of the substance (the amount of energy required or released when
one kilogram of the substance changes in temperature by 1°C (usually expressed in J/g•°C).
∆T = change in temperature (°C): tf - ti (final temperature minus initial temperature)
QUESTIONS:
3. What is the relationship between temperature and heat?
4. What is the Second Law of thermodynamics?
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5. How is energy transferred from a hotter substance to a cooler substance according to the
particle theory of matter?
6. A hot flame is use to raise the temperature of 500 g of water from 20.0°C to 40.0°C? How
much energy is transferred from the flame to the water?
7. A 100 g sample of copper is placed in cold water causing the copper to cool from 55.3°C to
12.8°C. How much energy is released by the copper?
8. If 250 g aluminum pot cools, after it was used to boil water on the stove, from 100°C down
to 55.0°C. Find the amount of energy released to the surroundings.
9. It required 50.0 kJ of heat energy to heat 500 g of a solid from 5.0°C to 50°C. Calculate the
specific heat capacity of the solid.
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10. A red hot iron bar is lowered into a 500 g sample of water which is at a temperature of
25.0°C. If the water sample absorbs 5.00 kJ of energy calculate the final temperature of
the water.
11. If 200 g of water at 10.0°C absorbs 18.0 kJ of energy what is the temperature change of
the water?
□ Define: enthalpy
□ Define: molar enthalpy
□ Predict the enthalpy change for chemical equations using standard enthalpies of formation
□ Calculate energy changes in chemical reactions
POTENTIAL ENERGY CHANGES (ENTHALPY/MOLAR ENTHALPY)
12. Define:
a. Potential Energy (Ep):
Molar Enthalpy change (kJ/mol) and Enthalpy change (kJ) are related by multiplying the
Molar Enthalpy change by the amount of substance in moles:
13. The molar enthalpy of combustion of butane, in an open air environment, is -2657.3 kJ/mol.
How much energy would be transferred to the surroundings (i.e. what is the enthalpy
change) if 20.0 mol of butane were burned?
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14. The molar enthalpy of combustion of heptane ( C7H16(l)) is -4464.7 kJ/mol. What mass of
heptane must be burned for an enthalpy change of -6000 kJ.
15. What is the molar enthalpy of reaction for 4.00 moles of substance XY which reacts with
substance Z and absorbs 300 kJ of heat energy from the surrounding environment?
16. A reference gives a value of +35.21 kJ/mol for the molar enthalpy of vaporization for
methanol. What enthalpy change occurs in the evaporation of 10.0 g of methanol?
17. Given ΔvapHm is +1.37 kJ/mol for NH3(g), find the mass of ammonia that can be condensed
from vapor to liquid (with no temperature change) during an enthalpy change of 10.0 kJ.
18. An experiment produces evidence that the evaporation of 4.00 g of liquid butane, C4H10(l),
requires a gain in enthalpy of +1.67 kJ. Find the molar enthalpy of vaporization for butane
from this evidence.
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□ Use calorimetry data to determine the enthalpy changes in combustion reactions
COMBUSTION CALORIMETRY
19. Briefly describe what a calorimeter is.
20. Define the first and second law of thermodynamics and explain how they form the basis for
calorimetry.
21. List three assumptions made when using a simple calorimeter



22. Calculate the quantity (in moles) of methane gas that must be burned to increase the
temperature of water in a 120 L hot water tank from 5.00°C to 54.50°C. (The molar
enthalpy change of combustion of methane is – 802.7 kJ/mol)
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23. A 2.50 g sample of sucrose (C12H22O11(s)) was burned in excess oxygen in a bomb
calorimeter which contained 2.19 kg of water. The temperature of the water increased from
20.50°C to 25.01°C. Determine the molar enthalpy change of combustion for sucrose.
24. Calculate the final temperature of a 500 g iron ring (c = 0.449 J/g•°C) that is initially at
25.0°C and is heated by combusting 4.95 g of ethanol ( ΔcH = -1234.8 kJ/mol), in an open
system.
25. How much propane (in grams) would have to be burned in an open system to raise the
temperature of 300 mL of water from 20.00°C to its boiling point? The molar enthalpy of
combustion of propane is -2043.9 kJ/mol.
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26. A 12.7 g sample of sulfur (S8(s)) is placed in a bomb which is then filled with oxygen under
pressure. The bomb is placed in the calorimeter which is filled with 2.20 kg of water at
21.08°C. The reaction mixture is ignited and the temperature of the water rises to a high of
33.88°C. From this data, calculate the molar enthalpy change of combustion of sulfur.
27. In a calorimetry experiment the burning of 5.08 g of benzene (C6H6(l)) released enough
heat to raise the temperature of 5.00 kg of water from 10.0°C to 19.6°C. Calculate the
molar enthalpy change of combustion of benzene.
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28. A chemist wants to determine empirically the enthalpy change for the following reaction as
written:
Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)
The chemist uses a simple calorimeter to react 0.50 g of magnesium ribbon with 100mL
of 1.00 mol/L HCl(aq). The initial temperature of the HCl(aq) is 20.40°C. After the
reaction the highest recorded temperature is 40.70°C.
a. Calculate the molar enthalpy change of Mg(s).
b. State all of the assumptions that you made in order to determine the enthalpy
change in Question “a”.
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□ Use calorimetry data to determine the enthalpy changes in chemical reactions occurring in
solution
SOLUTION CALORIMETRY
In these problems we assume the volume of calorimeter water is the two solutions added
together! We also assume the specific heat of the solution is the same as the specific heat of
water.
29. A student mixed 100.0 mL of 1.50 mol/L sulfuric acid with 200.0 mL of 1.50 mol/L sodium
hydroxide. Both solutions were at 19.67oC initially and the highest temperature reached by
the reaction mixture was 34.06oC. Calculate the molar enthalpy of neutralization for
sulfuric acid.
30. A laboratory technician initially adds 45.0 mL of concentrated, 11.6 mol/L hydrochloric acid
to 355.0 mL distilled water to form a dilute solution. The final temperature of the solution
shows an increase of 2.6oC. Calculate the molar enthalpy of dilution of hydrochloric acid.
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31. Lab Exercise
Problem:
What is the molar enthalpy change of neutralization for hydrochloric acid in the following
experiment?
Experimental Design:
Barium hydroxide and hydrochloric acid are mixed and the maximum temperature increase is
recorded.
Evidence:
Barium hydroxide solution
Hydrochloric acid
Volume: 50.0 mL
Volume: 200.0 mL
Concentration: 2.00 mol/L
Concentration: 1.00 mol/L
Initial Temperature: 21.0°C
Initial Temperature: 21.0°C
Final Temperature of the mixture: 32.5°C
Analysis:
32. In order to neutralize 50.0 mL of 0.400 mol/L KOH(aq) in a simple calorimeter, 0.630 mol/L
H2SO4(aq) is added. The initial temperature of the acid and the base before they are
combined is 25.0°C. Once the reaction has come to completion the highest temperature
reached is 28°C.
a. Calculate the volume of sulfuric acid needed to perfectly neutralize the potassium
hydroxide solution. (Hint: Use simple stoichiometry)
b. Calculate the molar enthalpy of neutralization for the potassium hydroxide.
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□ Write balanced equations for chemical reactions that include energy changes
□ Use thermochemical equations to communicate and calculate energy changes
□ Predict the enthalpy change for chemical reactions using standard enthalpies of formation
THERMOCHEMICAL EQUATIONS
33. In each of the following examples the enthalpy change for the reaction is given. Find the
molar enthalpy change of reaction for the underlined substance.
a. 2 C(s) + 2 H2O(g)  2 CO(g) + 2 H2(g)
∆H = +262.6 kJ
b. 2 Al(s) + 3/2 O2(g)  Al2O3(s)
∆H = -1680 kJ
c. 3 C(s) + 2 Fe2O3(s) + 466 kJ  4 Fe(s) + 3 CO2(g)
d. 2 Mg(s) + O2(g)  2 MgO(s) + 1203.2 kJ
e. 3 X(s) + 4 Y(g) + 1200 kJ  X3Y4(s)
34. Write out thermochemical equations for the following…
(a) The formation of nitrogen monoxide. Use ∆H notation.
(b) The decomposition of magnesium carbonate. Include energy in the equation.
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(c) The combustion of copper(II) sulfide. The molar enthalpy of copper(II) sulfide is
-800 kJ/mol. Write this in two ways, energy in the equation and using ∆H notation.
35. Boron reacts with hydrogen to form diboron hexahydride (diboron) gas. The molar enthalpy
of reaction for boron is +15.7 kJ/mol.
a. Write the balanced chemical equation using whole number coefficients and including
the energy using ΔH notation.
b. Write the balanced thermochemical equation using whole number coefficients.
36. The molar enthalpy of combustion for ethane (C2H6(g)), in an open air environment,
is -1428.4kJ/mol.
a. Write the thermochemical equation
b. What is the molar enthalpy of reaction for CO2(g) in this equation?
c. What mass of CO2(g) will be produced if 30.0 MJ of heat is released from this
reaction?
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37. Methane is the main component in natural gas used to heat our homes. The molar
enthalpy of combustion for methane in an open air environment is -802.5 kJ/mol.
a. Write the thermochemical equation
b. If heating the average Edmonton home requires 7.50 GJ of energy, each month,
what is the monthly mass of oxygen removed from the environment?
38. Referring to the equation below, answer the following:
2 C2H2(g) + 5 O2(g)  4 CO2(g) + 2 H2O(g) + 2600 kJ
a. What is the enthalpy change for the reaction?
b. What is the molar enthalpy change of the reaction for acetylene?
c. What is the molar enthalpy change of the reaction for carbon dioxide?
d. If 520 kJ of heat are released, how many grams of acetylene were burned?
e. If 10.4 MJ of heat are released, how many moles of CO2(g) were produced?
f.
What would be the enthalpy change for the reaction if one mole of acetylene were
represented?
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39. Write thermochemical equations for the following reactions:
a. Formation of methanol
b. Decomposition of methanol
40. Given the following reaction:
3X + 4Y  2Q + 5P
∆H = -520 kJ
a.
Is the above reaction endothermic or exothermic?
b.
What is the molar enthalpy change of the reaction for Q?
c. How much heat would be involved if 1.00 mol of Y reacted?
d. What is the molar enthalpy change for P?
e. How many moles of X would be required to produce 26.0 kJ of heat?
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□ Classify chemical reactions as endothermic or exothermic
□ Explain potential energy changes that occur during chemical reactions
□ Draw potential energy diagrams and label reactants, products and enthalpy changes
□ Use potential energy diagrams to calculate energy changes
ENTHALPY (POTENTIAL ENERGY CHANGES)
41. During a chemical reaction energy is absorbed in order to ____________ chemical bonds,
while energy is released when bonds are _____________ .
42. During an exothermic reaction the net energy change is ____________(+/-) and energy
is released. During an endothermic reaction the net energy change is__________(+/-)
and energy is absorbed.
43. What is the difference between the enthalpy change during a chemical reaction and the
molar enthalpy change? What are the units for each?
44. For each of the following reactions decide whether the reaction is endothermic or
exothermic, then sketch a potential energy diagram.
a. NaCl(s) + 411.2 kJ  2 Na(s) + Cl2(g)
b. 8 Ba(s) + S8(s) + 16 O2(g)  8 BaSO4(s)
ΔH = -11 785.6 kJ
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c. The formation of carbon disulfide (see textbook).
d.
The decomposition of silver iodide.
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□ Explain and use Hess’ Law to calculate energy changes for a net reaction from a series of
reactions
HESS’S LAW OF ADDITION
(Principle of Additivity of Reaction Heats)
When a reaction can be expressed as the algebraic sum of two or more other reactions,
the heat of the reaction is the algebraic sum of the heats of these reactions.
Hess’s law enables us to determine heats of reaction that cannot be easily determined
experimentally.
45. The standard molar enthalpy change of combustion of liquid ethyl alcohol is
-1370.1 kJ/mol and that of acetic acid is -875.7 kJ/mol. The equations are:
∆H = -1370.1 kJ
∆H = -875.7 kJ
C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(l)
HC2H3O2(l) + 2 O2(g)  2 CO2(g) + 2 H2O(l)
What is the heat of reaction for the oxidation of ethyl alcohol to acetic acid?
C2H5OH(l) + O2(g)  HC2H3O2(l) + H2O(l)
46. Given:
1/2 N2(g) + 1/2 O2(g)  NO(g)
1/2 N2(g) + O2(g)  NO2(g)
∆H = ?
∆H = +90.5 kJ
∆H = +33.9 kJ
Calculate the standard enthalpy change for the following reaction:
NO(g) + 1/2 O2(g)  NO2(g)
∆H = ?
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47. Use the following information to write the thermochemical equation, balanced using the
lowest whole number coefficients, for the oxidation of ammonia.
∆H = +46.1 kJ
∆H = +33.9 kJ
∆H = -242.0 kJ
NH3(g)  1/2 N2(g) + 3/2 H2(g)
1/2 N2(g) + O2(g)  NO2(g)
H2(g) + 1/2 O2(g)  H2O(g)
48. Find the heat of formation of one mole of SO3(g) from the following data:
S8(s) + 8 O2(g)  8 SO2(g) + 2378.4 kJ
SO2(g) + 1/2 O2(g)  SO3(g) + 99.0 kJ
49. Given:
N2(g) + 5/2 O2(g) + 15 kJ  N2O5(g)
H2(g) + 1/2 O2(g)  H2O(l) + 286 kJ
1/2 H2(g) + 1/2 N2(g) + 3/2 O2(g)  HNO3(l) + 173 kJ
What is the heat of the reaction for:
N2O5(g) + H2O(l)  2 HNO3(l)
ΔH = ?
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50. Given:
∆H
∆H
∆H
∆H
Ag2O(s) + 2 HCl(g)  2 AgCl(s) + H2O(l)
Ag(s) + 1/2 Cl2(g)  AgCl(s)
1/2 H2(g) + 1/2 Cl2(g)  HCl(g)
H2(g) + 1/2 O2(g)  H2O(l)
= -324.7 kJ
= -127.0 kJ
= -92.3 kJ
= -285.9 kJ
From the above information calculate the molar enthalpy change of formation of Ag2O(s).
51. Given the following equations:
Cl2(g) + 2 KBr(aq)  Br2(aq) + 2 KCl(aq)
H2(g) + Cl2(g)  2 HCl(g)
HCl(aq) + KOH(aq)  KCl(aq) + HOH(l)
HBr(aq) + KOH(aq)  KBr(aq) + HOH(l)
HCl(g)  HCl(aq)
Br2(l)  Br2(aq)
HBr(g)  HBr(aq)
∆H
∆H
∆H
∆H
∆H
∆H
∆H
=
=
=
=
=
=
=
-96.2 kJ
-184 kJ
-57.3 kJ
-57.3 kJ
-77.0 kJ
-4.2 kJ
-79.9 kJ
Calculate the molar enthalpy change of formation of HBr(g) from its elements.
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□ Predict the enthalpy change for chemical reactions using standard enthalpies of formation
□ Explain the origin for the potential energy containained within the chemical bonds of
hydrocarbons
□ Identify the reactants and products of photosynthesis, cellular respiration and complete
combustion reactions and classify them as endothermic or exothermic
HESS’ LAW
The enthalpy changes of reaction can be determined for a more complex reaction that is not
a formation reaction or decomposition reaction. The formula below summarizes Hess’ law:
∆H = ∑ ΔfH products - ∑ ΔfH reactants
52. Calculate the molar enthalpy change of combustion of benzene.
53. Calculate the molar enthalpy change of combustion of ethanol.
54. Given the following reaction:
4 NH3(g) + 3 O2(g)  2 N2(g) + 6 H2O(g)
a. Calculate the molar enthalpy change of reaction for ammonia.
b. What is the molar enthalpy change of reaction of oxygen?
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55. Determine the molar enthalpy change of reaction of hydrogen sulfide in the following
reaction below used to remove sulfur from sour gas plants in Alberta.
16 H2S(g) + 8 SO2(g)  3 S8(s) + 16 H2O(g)
56. The combustion of methyl benzene (CH3C6H5(l)) in a calorimeter causes the temperature of
the water in the calorimeter to increase. The molar enthalpy change of combustion of
methyl benzene is –3915.6 kJ/mol. Calculate the molar enthalpy change of formation of
methyl benzene.
57. The molar enthalpy change of combustion of carbon disulfide liquid is –1076.7 kJ/mol. If the
reaction produces carbon dioxide and sulfur dioxide gas, calculate the molar enthalpy of
formation of carbon disulfide.
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58. The equation for the combustion of octene is:
C8H16(l) + 12 O2(g)  8 CO2(g) + 8 H2O(g) + 4902.4 kJ
The molar enthalpy of formation of octene is?
59. Nonene, C9H18(l), burns according to the equation:
C9H18(l) + 27/2 O2(g) 
9 CO2(g) + 9 H2O(g)
∆H = -5615 kJ
The molar enthalpy of formation of C9H18(l) is?
60. Calculate the molar enthalpy change per mole of carbon dioxide in the following reactions:
a. Photosynthesis
b. Cellular Respiration
c. Combustion of Glucose
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d. Compare and contrast these reactions (What is similar? What is different?)
e. Where does the energy contained within the bonds of hydrocarbons originate?
Explain.
61. Which of the following fuels releases the most energy per mole of hydrocarbon burned?
CH4 C2H6 C3H8 C4H10 C5H12 or C6H14 ?
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COMBINATION PROBLEMS
62. Calculate the mass of ethanol that must be burned to heat 2.00 L of water from 25.0°C to
65.0°C.
63. 10.0 g of methanol is burned to heat 1.00 kg of water at 20.0°C. What is the final
temperature of the water?
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64. Methanol is often the main component in fondue fuel. What mass of methanol burning
would be required to heat 1.20 kg of vegetable oil from 24.0°C to 98.0°C? (The specific
heat of the vegetable oil is 8.60 J/g°C)
65. How much energy could we get from eating 10.0 g of sucrose? (Assume the sucrose is
oxidized in the body cells to produce carbon dioxide gas and liquid water).
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66. What mass of lime (CaO(s)) could be produced by decomposing limestone (CaCO3(s))
into lime and carbon dioxide by the application of 100 MJ of heat?
67. How much energy is released in the decomposition of 50.0 g of ethylene?
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68. Consider the reaction below for the next question:
3 C(s) + 2 Fe2O3(s) + 46.9 kJ  4 Fe(s) + 3 CO2(g)
a. How much heat is required to produce 100 g of Fe(s)?
b. What mass of iron(III) oxide could be reduced using 200 kJ of heat?
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□ Explain changes in bond energy that occur during chemical reactions
THERMOSTABILITY
69. Consider the following reactions:
I
BX2  B
+
X2
∆H = -500 kJ
II
P2Q3  P
+
3Q
∆H = +500 kJ
III
R4P  4 R
P
∆H = -200 kJ
IV
LS2  L
+
+ 2S
∆H = +200 kJ
a. In which of the above reactions is the potential energy of the products greater than
the potential energy of the reactants?
b. Which of the above compounds are the most stable?
70. Draw the a potential energy diagram for the simple decomposition of butane
(C4H10(g))and one for the simple decomposition of sodium chloride (NaCl(s)). Compare the
amount of energy that must be absorbed by each compound to decompose into elements;
which is more stable?
71. Rank the following substances in order of decreasing stability: methane, aluminum oxide,
ethene, and sulfur dioxide.
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□ Define: activation energy
□ Label activation energy on potential energy diagrams
ACTIVATION ENERGY
72. Describe two criteria that must be met in order for a reaction to take place according to
collision theory.
73. How does an increase in temperature affect the rate of a chemical reaction? Explain your
answer in terms of activation energy.
74. Describe the energy conversions during a chemical reaction. Use the terms kinetic energy,
potential energy, bonds breaking and bonds forming in your answer.
75. Describe what is meant by activated complex.
76. Consider the following reaction:
A2(g) + B2(g)  2 AB(g)
The energy of activation (Ea) in the forward direction is 143 kJ. The energy of activation
(Ea) for the reverse direction is 75 kJ.
a. Draw the potential energy diagram for the forward direction. Include ΔH.
b. Is this reaction endothermic or exothermic?
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□ Explain the effects of catalysis on chemical reactions
CATALYZED REACTIONS
77. Explain why a catalyst will never affect the enthalpy change of a chemical reaction. Use a
potential energy diagram in your explanation.
78. How do catalysts speed up a reaction?
79. Are catalysts consumed in a reaction? (Yes or No)
80. How do CFCs (chlorofluorocarbons) deplete the ozone layer?
81. The broken line on the energy diagram indicates that:
A.
B.
C.
D.
the temperature of the reaction has increased
the pressure of the system has decreased
more reactants have been added
a catalyst has been added
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82. One of the byproducts of the cracking process used at Novacor is ethyne (C2H2(g)). In presence
of a palladium catalyst, the ethyne forms ethene and ethane. This equation is represented by the
unbalanced equation
C2H2(g) + H2(g)  C2H4(g) + C2H6(g) + energy
The energy diagram that represents both the catalyzed (-----) and uncatalyzed reactions (
) is
83. Consider the diagram above.
a. The potential energy change for this forward reaction is
b. The energy of activation for this forward reaction is?
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c. The potential energy change for this reverse reaction is?
d. The energy of activation for this reverse reaction is?
e. Draw a line for the catalyzed reaction if it decreases the activation energy by
50 kJ.
Use the following information to answer the next question:
How do they get the caramel into the Caramilk bar?
Solid sucrose C12H22O11(s) will turn into liquid sugars C6H12O6(l) and C6H10O5(l), but at an
extremely slow rate. If a small amount of invertase catalyst is added, the rate is greatly
increased.
84. With the invertase (catalyst) present, the ∆H is
A.
B.
C.
D.
larger and the peak is lower
smaller and the peak is lower
the same and the peak is lower
the same and the peak is higher
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