Which is true about chemical energy changes?
| Reaction type | Energy flows | Temperature of surroundings |
A | exothermic | to the surroundings | increases |
B | exothermic | from the surroundings | decreases |
C | endothermic | to the surroundings | decreases |
D | endothermic | from the surroundings | increases |
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Which statement is true about exothermic and endothermic reactions?
The products have more energy than the reactants in an exothermic reaction
The products have more energy than the reactants in an endothermic reaction
The activation energy for an exothermic reaction is always larger than the activation energy for an endothermic reaction
Most reactions are endothermic
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2 marksA student uses this apparatus to investigate the temperature change that occurs when ammonium nitrate is dissolved in water.
She uses this method.
put 100 cm3 of water into the polystyrene cup and measure the initial temperature of the water
add 8.00 g of ammonium nitrate and stir
record the lowest temperature reached by the solution
The table shows her results.
Initial temperature of water in oC | 20.0 |
Lowest temperature of solution in oC | 14.2 |
Use the results of the experiment to explain what type of reaction is taking place when ammonium nitrate is added to water.
3 marksShow that the heat energy change, Q, is about 2400 J.
[mass of 1.00 cm3 of solution = 1.00 g]
[for the solution, c = 4.185 J / g / °C]
Q = .............................. J
4 marksUse your answer to part (b) to calculate the enthalpy change, ΔH, in kilojoules per mole of ammonium nitrate.
[Mr of ammonium nitrate = 80.0]
Include a sign in your answer.
ΔH = .............................. kJ / mol
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Sodium oxide reacts with water to form sodium hydroxide.
A scientist plans to measure the temperature change of the reaction using calorimetry in a lab.
This is the scientist's basic method:
use 50 g of water
record the temperature of the water
add sodium hydroxide
stir
record the maximum temperature reached
Which apparatus list is the most appropriate for this experiment?
Electronic balance, measuring cylinder, glass beaker, thermometer
Measuring cylinder, glass beaker, polystyrene cup, thermometer
Electronic balance, glass beaker, polystyrene cup, thermometer
Measuring cylinder, glass beaker, glass rod, thermometer
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A student uses this apparatus to investigate the heat energy released when a liquid fuel is burned.
This is the student’s method.
measure the mass of the spirit burner and fuel
add 100 cm3 of water to the copper can
record the temperature of the water
use the spirit burner to heat the water until the temperature rises by 30 °C
immediately measure the new mass of the spirit burner and fuel
Suggest why the student measures the mass of the spirit burner and fuel immediately after heating the water.
When the fuel is burned, the student notices that a black solid forms on the bottom of the copper can.
i) Identify the black solid.
(1)
ii) Explain why the black solid forms.
(2)
6 marksi) Show that the heat energy change, Q, to raise the temperature of 100 cm3 of water by 30 °C is approximately 13 kJ.
[mass of 1.0 cm3 of water = 1.0 g] [c for water = 4.2 J / g / °C]
(3)
ii) The student burns 0.96 g of methanol, CH3OH Calculate the molar enthalpy change, ΔH, in kJ / mol, for the combustion of methanol. Include a sign in your answer. [Mr of methanol = 32]
(3)
ΔH = ...................................................................... kJ / mol
The table shows data book values for the molar enthalpy change, ΔH, for the combustion of some alcohols with different numbers of carbon atoms per molecule.
Number of carbon atoms per molecule | 1 | 2 | 3 | 4 | 5 |
Molar enthalpy change, ΔH, in kJ/mol | -730 | -1370 | -2020 | -2680 | -3320 |
i) Plot the data values from the table on the grid. Draw a straight line of best fit.
(2)
ii) Deduce the value of ΔH for an alcohol with six carbon atoms per molecule. Show on the graph how you obtained your answer.
(2)
ΔH = ...................................................................... kJ/mol
iii) State the relationship between ΔH and the number of carbon atoms per molecule.
(1)
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1 markWhen solid sodium oxide is added to water, an exothermic reaction occurs:
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2NaOH (aq)
A student uses the apparatus below to measure the temperature change of the reaction:
The student records the following results for the reaction:
mass of sodium oxide used = 3.0 g
volume of water used = 50 cm3
starting temperature of mixture = 19.5 oC
maximum temperature of mixture = 30.5 oC
What is the heat energy, Q, released in this experiment, in J?
c = 4.2 J/g/ oC
mass of 1 cm3 of water = 1 g
Q = 2,310 J
Q = 2,450 J
Q = 139 J
Q = 126 J
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This question is about lithium oxide.
The diagram shows the electron configurations of an atom of lithium and an atom of oxygen.
Describe the changes in electronic configuration when lithium and oxygen react to form lithium oxide, Li2O
10 marksLithium oxide reacts with water to form lithium hydroxide as the only product. A scientist uses this apparatus to measure the temperature change of the reaction.
This is the scientist’s method.
pour 100 g of water into a polystyrene cup
record the temperature of the water
add the lithium oxide and stir the mixture
record the maximum temperature reached
The diagram shows the thermometer readings before and after adding the lithium oxide.
i) Complete the table, giving all values to the nearest 0.1 °C.
(2)
temperature in °C after adding the lithium oxide |
|
temperature in °C before adding the lithium oxide | 17.3 |
temperature rise in °C |
|
ii) Calculate the heat energy change in the reaction. Give your answer to two significant figures. [c = 4.2 J / g / °C]
(4)
heat energy change = .............................. J
iii) In another experiment the scientist obtains these results.
amount of lithium oxide in mol | 0.0580 |
heat energy change in J | 5210 |
Calculate the molar enthalpy change (ΔH) in kJ / mol. Include a sign in your answer.
(3)
ΔH = ..............................kJ / mol
iv) Give a reason why the scientist does the experiment in a polystyrene cup.
(1)
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1 markAnother student carries out the experiment to find the heat energy released in the reaction of sodium oxide with water:
Na2O (s) + H2O (l) 2NaOH (aq)
They found that 10,800 J of heat energy was released when 0.060 mol of sodium oxide reacts.
What is the value of ΔH, in kJ/mol, for the reaction of sodium oxide with water?
0.0056 kJ/mol
180 kJ/mol
1,800 kJ/mol
8,000 kJ/mol
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A student investigates the reaction between zinc and copper(II) sulfate solution. The equation for the reaction is
Zn + CuSO4 → Cu + ZnSO4
This is his method.
add exactly 25.0 cm3 of copper(II) sulfate solution to a polystyrene cup
record the temperature of the solution
add about 5 g of zinc powder (an excess) and stir the mixture
record the highest temperature reached
i) Suggest why it is not important to add an exact mass of zinc powder.
(1)
ii) State the colour change of the solution. from ........................................................... to ...........................................................
(2)
7 marksThe table shows the student’s results
volume of copper(II) sulfate solution in cm3 | 25.0 |
initial temperature of copper(II) sulfate solution in °C | 19.0 |
final temperature of solution in °C | 31.5 |
i) Show that the heat energy change (Q) is about 1300 J. [for the solution, c = 4.18 J/g/°C] [mass of 1.00 cm3 of solution = 1.00 g]
(3)
ii) The mass of anhydrous copper(II) sulfate (CuSO4) used to make 25.0 cm3 of solution is 2.00 g. Calculate the amount, in moles, of CuSO4 in 2.00 g. [Mr of CuSO4 = 159.5]
(1)
iii) Calculate the value of the enthalpy change (ΔH), in kilojoules per mole, for the reaction between zinc and copper(II) sulfate. Include a sign in your answer.
(3)
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The actual molar enthalpy change, ΔH, for the reaction of sodium oxide with water, is -238 kJ/mol.
Experimental results from calorimetry in labs give less exothermic results.
Which of the following does not explain this difference?
The reaction may have been incomplete
Heat energy also transfers to the surroundings
The calorimetry equipment also has a specific heat capacity, and so affects the final temperature reading
The student measured the initial temperature of the water as lower than the actual value.
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When aqueous solutions of potassium hydroxide and nitric acid are mixed together, an exothermic reaction occurs. The diagram shows the apparatus used in an experiment to measure the temperature increase.
This is the student’s method:
use the larger measuring cylinder to add 25cm3 of aqueous potassium hydroxide to the polystyrene cup
record the steady temperature
use the smaller measuring cylinder to add 5cm3 of dilute nitric acid to the cup, stir the mixture with the thermometer
record the highest temperature of the mixture
continue adding further 5cm3 portions of dilute nitric acid to the cup, stirring and recording the temperature, until a total volume of 35cm3 has been added.
A teacher advises the student to use a 50 cm3 burette instead of the 10 cm3 measuring cylinder. Suggest two reasons why it would be better to use a burette instead of a measuring cylinder to add the acid in this experiment.
1 markThe diagram shows the thermometer readings at the start and at the end of one experiment.
Complete the table to show:
the thermometer reading at the start of the experiment
the temperature rise in the experiment.
thermometer reading at end / °C | 26.8 |
thermometer reading at start / °C |
|
thermometer rise / °C |
|
Another student uses the same method, adding the dilute nitric acid from a burette. The table shows his results.
This is the student’s graph.
The point where the lines cross represents complete neutralisation.
i) Identify the maximum temperature reached during the experiment.
(1)
maximum temperature = .............................................................. °C
ii) Identify the volume of dilute nitric acid that exactly neutralises the 25 cm3 of aqueous potassium hydroxide.
(1)
volume = .............................................................. cm3
Another student records these results.
volume of aqueous potassium hydroxide | 20.0 cm3 |
starting temperature of aqueous potassium hydroxide | 18.5 °C |
maximum temperature of mixture | 30.0 °C |
volume of dilute nitric acid | 20.0 cm3 |
Calculate the heat energy released in this experiment.
c = 4.2 J /g/ °C |
mass of 1 cm3 of mixture = 1 g |
heat energy = .............................................................. J
In another experiment, the heat energy released is 1600 J when 0.040 mol of potassium hydroxide is neutralised. Calculate the value of ΔH, in kJ/mol, for the neutralisation of potassium hydroxide.
ΔH = .............................................................. kJ/mol
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A student uses this apparatus to investigate the heat energy change when a salt dissolves in water to form a solution.
This is the student’s method.
Add 50 cm3 of distilled water to a polystyrene cup
Record the initial temperature of the water
Add a known mass of solid anhydrous copper(II) sulfate to the polystyrene cup and stir the solution with the thermometer until all the solid has dissolved
Record the maximum temperature of the copper(II) sulfate solution
i) Name the piece of apparatus the student should use to add the distilled water to the polystyrene cup.
(1)
ii) The student stirs the solution to help the solid dissolve more quickly.
Suggest another reason why the student stirs the solution.
(1)
iii) State the colour of the copper(II) sulfate solution.
(1)
3 marksThe diagram shows the temperatures in one experiment.
Complete the table, giving all values to the nearest 0.1 °C.
maximum temperature in °C |
|
initial temperature in °C |
|
increase in temperature in °C |
|
6 marksIn a second experiment, when a student dissolves the anhydrous copper(II) sulfate in 50 cm3 of distilled water, the increase in temperature is 3.3 °C.
i) Show that the heat energy change (Q) in this second experiment is approximately 700 J.
[for water, c = 4.2 J / g / °C]
[mass of 1.0 cm3 of water = 1.0 g]
(2)
ii) In this experiment, the student uses 1.70 g of the anhydrous copper(II) sulfate.
Calculate the molar enthalpy change (ΔH) in kJ / mol.
Include a sign in your answer.
[Mr of CuSO4 = 159.5]
(4)
ΔH = ................................ kJ / mol
2 marksAnother student does a similar experiment but uses hydrated copper(II) sulfate instead of anhydrous copper(II) sulfate.
The table shows his results.
initial temperature in °C | 23.8 |
final temperature in °C when all solid dissolves | 22.7 |
Explain what the results show about the type of energy change that occurs when hydrated copper(II) sulfate dissolves.
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A student investigates the reaction between magnesium and hydrochloric acid.
He uses this method:
Step 1: add 25 cm3 of dilute hydrochloric acid to a polystyrene cup
Step 2: record the temperature of the acid
Step 3: find the mass of a 10 cm strip of magnesium ribbon
Step 4: add the magnesium ribbon to the hydrochloric acid
Step 5: when all the magnesium has reacted, record the highest temperature reached
Complete the word equation for the reaction.
magnesium + hydrochloric acid → ......................... + ...........................
8 marksThe thermometer shows the temperature of the acid at the start of the experiment.
i) Complete the table by giving the temperatures to the nearest 0.1 °C.
(2)
temperature of the acid at the start in °C |
|
highest temperature reached in °C |
|
temperature rise in °C | 20.8 |
ii) Show that the heat energy change (Q) for this reaction is about 2200 J.
[mass of 1.0 cm3 of solution = 1.0 g]
[for the solution, c = 4.2 J / g / °C]
(2)
iii) The mass of magnesium used by the student was 0.12 g.
Calculate the value of the enthalpy change (ΔH), in kilojoules per mole of magnesium, for the reaction between magnesium and hydrochloric acid.
Include a sign in your answer.
(4)
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The reaction between magnesium and copper(II) sulfate solution is exothermic. This apparatus is used to measure the temperature increase when excess magnesium is added to 100 cm3 of copper(II) sulfate solution.
i) State why a reaction occurs when magnesium is added to copper(II) sulfate solution.
(1)
ii) Complete the word equation for this reaction.
(1)
magnesium + copper(II) sulfate .............................................................. + ..............................................................
The temperature at the start of the reaction is 20.2 °C. The maximum temperature recorded is 56.3 °C.
i) Calculate the heat energy change, in joules, for the reaction.
[mass of 1.00 cm3 of solution = 1.00 g] [c for the solution = 4.2 J/g/ °C]
(2)
heat energy change = .............................................................. J
ii) Explain why it is better to use a polystyrene cup rather than a glass beaker in this experiment.
(2)
The reaction between zinc and copper(II) sulfate solution is also exothermic.
i) A mass of 0.500 g of zinc is reacted with an excess of copper(II) sulfate solution. The heat energy change is 1.67 kJ. Calculate the molar enthalpy change, ΔH, in kJ/mol. Include a sign in your answer. Give your answer to three significant figures.
(3)
ΔH = .............................................................. kJ/mol
ii) The ionic equation for the reaction between zinc and copper(II) sulfate is
Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)
Explain why this is a redox reaction.
(3)
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A student uses this apparatus to investigate the reaction between potassium hydroxide solution and dilute hydrochloric acid.
This is her method.
pour 25 cm3 of potassium hydroxide solution into a polystyrene cup and record the temperature of the solution
pour 25 cm3 of dilute hydrochloric acid into a measuring cylinder and record the temperature of the acid
add the acid to the polystyrene cup and stir the mixture
record the highest temperature reached
i) Give a word equation for the reaction between potassium hydroxide and hydrochloric acid.
(1)
ii) Explain why the student needs to stir the mixture.
(2)
2 marksThe table gives the temperatures of the solutions before the student mixes them.
potassium hydroxide solution | 17.8 °C |
dilute hydrochloric acid | 18.4 °C |
Calculate the mean (average) temperature of the two solutions.
(2)
mean temperature = .............................................................. °C
8 marksThe student repeats the experiment on a different day, using 25 cm3 of potassium hydroxide solution and 25 cm3 of dilute hydrochloric acid. The thermometer shows the highest temperature reached at the end of the experiment.
i) Complete the table by giving the missing information. Give both temperatures to the nearest 0.1°C.
(2)
Mean temperature at start in °C |
|
Temperature at end in °C |
|
Temperature rise in °C | 5.2 |
ii) Show that the heat energy change, Q, in the student’s experiment is about 1100J.
[for the mixture, c = 4.2 J/g/°C]
[mass of 1.0 cm3 of mixture = 1.0 g]
(3)
iii) The student uses 0.020 mol of potassium hydroxide in his experiment. Calculate the enthalpy change (ΔH) in kJ/mol, for 1.0 mol of potassium hydroxide. Include a sign in your answer.
(3)
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A student investigates the temperature change during the reaction between zinc metal and copper(II) sulfate solution.
The student considers two different methods.
Method 1
Pour 50 cm3 of copper(II) sulfate solution into the polystyrene cup
Record the temperature of the solution
Add 3 g of zinc powder
Stir using the thermometer and record the highest temperature reached
Method 2
Record the temperature of 50 cm3 of copper(II) sulfate solution
Pour the 50 cm3 of copper(II) sulfate solution into the glass bottle
Add 3 g of zinc powder
Push the bung and thermometer into the bottle and record the highest temperature reached
Discuss the advantages and disadvantages of each method.
2 marksThe equation for the reaction is
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
50cm3 of copper(II) sulfate solution contains 0.025 mol CuSO4
A mass of 3 g of zinc is used.
Show that the zinc is in excess.
[Ar of zinc = 65]
6 marksThe student reacts a solution containing 0.025 mol CuSO4 with an excess of zinc.
These are the student’s results.
Temperature of 50 cm3 of copper(II) sulfate solution = 21.1 °C
Highest temperature reached = 40.6 °C
i) Show that the energy change Q for this reaction is about 4000 J
[mass of 1 cm3 of solution = 1.0 g]
[for the solution, c = 4.2 J / g / °C]
(3)
ii) Calculate the molar enthalpy change (ΔH), in kJ / mol, for the reaction.
(3)
ΔH = .............................. kJ / mol
The ionic equation for the reaction is
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
Explain what is oxidised and what is reduced in this reaction.
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