OCR Gateway GCSE Chemistry

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GCSEChemistryOCR GatewayTopic Questions3. Chemical Reactions3.2 Energetics

3.2 Energetics

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1a4 marks

A student investigates some exothermic and endothermic reactions.

He measures the temperature changes during some chemical reactions.

Look at his table of results.

Reaction Temperature at start (°C) Temperature at end (°C) Temperature change (°C)
1 15 25 +10
2 15 15 0
3 18 15 -3
4 15 20 +5

What can you conclude about the type of energy change in each reaction?

Explain your answer.

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1b2 marks
A student does an experiment with an acid and an alkali.

q16b-paper-1-specimen-2016-ocr-gcse-chemistry
1
He adds the acid to a beaker and measures its temperature.
2.
He then adds the alkali to the beaker and stirs the mixture.
3.

At the end of the reaction, he removes the thermometer from the beaker and measures the temperature.

How should he improve his method? Explain your answer.

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1c
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A student adds water to calcium oxide. A vigorous exothermic reaction takes place forming calcium hydroxide.

Calcium hydroxide has the formula Ca(OH)2.

Show that the relative formula mass (Mr) of calcium hydroxide is 74.1.

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1a4 marks

Look at the energy profile for a reaction.

q18-paper-3-specimen-2016-ocr-gcse-chem

Explain what can be concluded about this reaction?

Include the quantities A and B in your answer.

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1b
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Higher Only

Look at the equation 

q18b-paper-3-specimen-2016-ocr-gcse-chem

The table shows the bond energies of the bonds involved.

Bond Bond energy (kJ/mol)
C–H 435
O=O 498
C=O 805
O–H 464

i)
What type of energy change happens when bonds are broken and when bonds are made?

Bonds broken ..........................................................

Bonds made ............................................................
[2]
ii)
Calculate the energy change for this reaction.

Answer = …………………………………. kJ/mol [3]

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1c5 marks

When propane reacts with oxygen, energy is given out.

  • Propane gives out 50 kJ/g.
  • A propane burner is used to boil 200 g of water to make a cup of tea.
  • The initial temperature of the water is 15 °C.

What mass of propane (in g) is needed to heat this water?

Use the following equation:

 
Energy transferred in J = 4.2 J / g°C × mass of water in g × temperature change in °C.
 
Answer = …………………………………. g

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2a3 marks

A student is investigating chemical reactions that produce heat.

She adds zinc to hydrochloric acid, HCl.

Zinc chloride, ZnCl2, and hydrogen gas are made.

i)
Write the balanced symbol equation for this reaction.
[2]
ii)
What term is used to describe a reaction that produces heat?
[1]

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2b1 mark

The student draws the reaction profile for this reaction, as shown in Fig. 18.1.

fig-18-1-paper-3-nov-2020-ocr-gcse-chem

Explain what is meant by the term activation energy.

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2c
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Higher Only 

Combustion reactions produce a lot of heat energy.

Bond energies can be used to calculate the energy change in combustion reactions.

Bond Bond energy
(kJ / mol)
C–H 413
O=O 498
C=O 805
O–H 464

Methane is a common fuel used in combustion reactions.

Methane reacts with oxygen. Carbon dioxide and water are made, as shown in Fig. 18.2.

fig-18-2-paper-3-nov-2020-ocr-gcse-chem

i)
The reaction of methane with oxygen produces heat.

Explain why.

Use ideas about bond breaking and bond making in your answer.

[3]

ii)
Calculate the total energy transferred to break the bonds in the reactants in Fig. 18.2.

Total energy transferred = ............................................... kJ / mol [1]

iii)
Calculate the total energy transferred to make the bonds in the products in Fig. 18.2.

Total energy transferred = ............................................... kJ / mol [1]

iv)
Use your answers to parts (ii) and (iii) to calculate the energy change for the reaction in Fig. 18.2.

Energy change = ............................................... kJ / mol [1]

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3a3 marks

Anhydrous copper sulfate reacts with water to make hydrated copper sulfate.

CuSO4  +  5H2O  →  CuSO4.5H2O
anhydrous copper sulfate   water    hydrated copper sulfate

                        
The reaction is exothermic.

Draw and label a reaction profile for this reaction

q21a-paper-3-june-2018-ocr-gcse-chem

How did you do?

3b
Sme Calculator3 marks

Higher Only 

Two students investigate the burning of methane in oxygen.

q21b-paper-3-june-2018-ocr-gcse-chem

Look at the table of bond energies.

Bond Bond energy
(kJ / mol)
O–H 459
C=O 799
O=O 494
C–H  

The reaction is exothermic and 802 kJ of energy are given out when 1 mole of methane burns.

The students have looked up the bond energies. They have different values for the C–H bond energy.

Student A thinks the C–H bond energy is 432 kJ / mol. Student B thinks the C–H bond energy is 411 kJ / mol.

Who is correct?

Use the bond energies and the energy given out in the reaction to calculate the C–H bond energy.

Answer = ...................................... kJ / mol

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1a4 marks

A student investigates the reaction of different metals with hydrochloric acid.

q19-paper-1-june-2019-ocr-gcse-chem

i)
The student notices that the temperature rises when a metal is added to the acid.

What name is given to this type of reaction when the temperature rises?
[1]
ii)
The metal magnesium reacts with hydrochloric acid, HCl, to form magnesium chloride, MgCl2, and hydrogen.

Write a balanced symbol equation for this reaction.
[2]
iii)
The experiment is repeated with aluminium and hydrochloric acid.

Write down the name of the salt produced in this reaction.
[1]

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1b
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The student repeats the experiment with different metals.

She repeats the experiment three times for each metal.

She measures the temperature change in each experiment.

Look at her results.

Metal Temperature change (°C)
Test 1 Test 2 Test 3
Magnesium 10.3 10.5 10.2
Zinc 8.6 8.7 7.6
Iron 5.2 4.9 5.1

i)
One of the student’s results is anomalous.

Put a circle enclose ring around the anomalous result in the table.
[1]
ii)
Suggest a reason why the result could be anomalous.
[1]
iii)
Calculate the mean temperature change for magnesium.

Give your answer to 1 decimal place.

Mean temperature change = ...................................................... °C

[2]

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1c4 marks
i)
The student wants to improve her experiment to get more accurate results.

Suggest an improvement to her experiment, which will give more accurate results.

Give a reason for the improvement.

Improvement ......................................................

Reason ........................................................

[2]

ii)
The student concludes that the more reactive the metal is, the higher the temperature rise.

Describe further tests the student can do to confirm her conclusion.

[2]

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1d3 marks

A reaction profile shows the energy involved in a reaction.

Draw the reaction profile for the reaction between magnesium and hydrochloric acid.

Label the:

  • products
  • energy change
  • activation energy.
q19d-paper-1-june-2019-ocr-gcse-chem

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2a6 marks

A student investigates the energy changes in different reaction mixtures, X, Y and Z.
Fig. 19.1 shows the apparatus she uses.

fig-19-1-paper-1-nov-2020-ocr-gcse-chem

Look at the table of the student’s results.

i)
Add the temperature change for each reaction mixture.
Include a + or – sign where relevant.

Reaction
mixture		 Temperature at start
(°C)		 Temperature at end
(°C)		 Temperature change
(°C)
X 20.0 25.5 ..............................
Y 19.0 8.0 ..............................
Z 20.0 20.0  ..............................

[1]

ii)
Which reaction mixture has an endothermic reaction?

Tick (✔) one box.

X   square
Y   square
Z   square

Explain your answer.

[2]

iii)
Complete the reaction profile for an endothermic reaction in Fig. 19.2.

The products have already been added.

Label the:

• reactants
• energy change
• activation energy.
fig-19-2-paper-1-nov-2020-ocr-gcse-chem

[3]

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2b2 marks

The burning of methane gas is an exothermic reaction.

When methane, CH4, burns in oxygen, O2, carbon dioxide, CO2, and water, H2O, are made.

Write the balanced symbol equation for this reaction.

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2c2 marks

A student investigates the combustion of ethanol.

Fig. 19.3 shows the apparatus the student uses. 

fig-19-3-paper-1-nov-2020-ocr-gcse-chem

The temperature change is much lower than the textbook value.

Suggest two ways that the student could improve the set-up of the apparatus in Fig. 19.3, so the temperature change is closer to the textbook value.

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3a
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A student investigates the energy changes during some chemical reactions.

She measures the temperature at the start and end of each reaction.

Look at her results.

Reaction Temperature at
start (°C)		 Temperature at
end (°C)		 Temperature
change (°C)		 Type of reaction
A 20 25 ........................... Exothermic
B 18 10 ........................... ...........................
C 21 35 ........................... ...........................
D 20 20 ........................... No reaction

Complete the table.

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3b1 mark

Which reaction has the largest energy change?

Answer = ...................................... 

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3c
Sme Calculator2 marks

Look at the reaction profile for reaction A.

q16c-paper-1-june-2018-ocr-gcse-chem

i)
Calculate the energy change in this reaction.

Answer = ...................................... kJ [1]

ii)
Calculate the activation energy.

Answer = ...................................... kJ [1]

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4a6 marks

Simple distillation can be used to separate mixtures of liquids.

A scientist is using simple distillation to separate a mixture of alcohols.

Look at the table. It shows the boiling points of three alcohols.

Alcohol Boiling point (°C)
Methanol 65
Ethanol 78
Propanol 97

i)
Which alcohol will be distilled first?

Tick (✓) one box.

Methanol    square

Ethanol       square

Propanol     square

Explain your answer.

[2]

ii)
Simple distillation uses evaporation and condensation to separate mixtures.

ethanol space open parentheses straight l close parentheses space rightwards arrow over leftwards arrow from condensation to evaporation of space ethanol space left parenthesis straight g right parenthesis

Describe the change in the arrangement of particles as substances evaporate.

[1]

iii)
Describe the change in the movement of particles as substances evaporate.
[1]
iv)
The scientist wants to improve the separation of the mixture of alcohols.

Suggest a piece of equipment he could use.

Explain how this will improve the separation of the mixture of alcohols.
[2]

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4b1 mark

Ethanol can be used as a biofuel. The combustion of ethanol is an exothermic reaction.

Explain why combustion is an exothermic reaction.

Use ideas about bond breaking and bond making in your answer.

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4c
Sme Calculator5 marks
i)
Methanol is another biofuel that can be used in combustion reactions.

CH3OH + 2O2 → CO2 + 2H2O

Look at the table. It shows some bond energies.

Bond Bond energy
(kJ/mol)
C−H 413
O=O 498
C−O 358
C=O 805
O−H 464

Calculate the energy transferred to break all the bonds in the reactants.

Energy transferred = .............................................. kJ/mol [2]

ii)
Calculate the energy transferred when all the bonds form in the products.

Energy transferred = .............................................. kJ /mol [2]

iii)
Use your answers to parts (i) and (ii) to calculate the energy change for this reaction.

Energy change = .............................................. kJ /mol [1]

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