Energetics (OCR GCSE Chemistry A (Gateway))

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

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, HC, to form magnesium chloride, MgC₂, 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]

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.

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

Put a  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]

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]

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.

A student investigates the energy changes in different reaction mixtures, X, Y and Z.

Fig. 19.1 shows the apparatus she uses.

Look at the table of the student’s results.

i) Add the temperature change for each reaction mixture.

Include a + or – sign where relevant.

[1]

ii) Which reaction mixture has an endothermic reaction?

Tick (✔) one box.

X   
Y   
Z   

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.

[3]

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.

A student investigates the combustion of ethanol.

Fig. 19.3 shows the apparatus the student uses. 

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.

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.

Complete the table.

Which reaction has the largest energy change?

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

Look at the reaction profile for reaction A.

i) Calculate the energy change in this reaction.

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

ii) Calculate the activation energy.

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

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.

i) Which alcohol will be distilled first?

Tick (✓) one box.

Methanol    

Ethanol       

Propanol     

Explain your answer.

[2]

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

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]

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.

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.

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]

Look at the energy profile for a reaction.

Explain what can be concluded about this reaction?

Include the quantities A and B in your answer.

Higher Only

Look at the equation 

The table shows the bond energies of the bonds involved.

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]

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

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]

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

Explain what is meant by the term activation energy.

Higher Only 

Combustion reactions produce a lot of heat energy.

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

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.

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]

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

The reaction is exothermic.

Draw and label a reaction profile for this reaction

Higher Only 

Two students investigate the burning of methane in oxygen.

Look at the table of bond energies.

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

Higher Only 

Lead is most commonly extracted from an ore called galena, PbS.

Extracting lead from the galena ore involves two steps.

Step 1: The galena ore is roasted in air to produce lead oxide, PbO.

Step 2: The lead oxide is heated in a blast furnace with carbon.

i) The reaction in step 1 is an exothermic reaction.

Draw a labelled reaction profile diagram for an exothermic reaction.

Label the activation energy and the energy change on your diagram.

[4]

ii) In step 2 the lead oxide is reduced by carbon.

PbO + C → Pb + CO

Explain, in terms of electron transfer, why carbon is called a reducing agent in this reaction.

[1]

Higher Only 

i) Solid lead reacts with nitric acid, HNO₃.

Lead nitrate, Pb(NO₃)₂, nitrogen oxide, NO, and water are made.

Write a balanced symbol equation for this reaction.

[2]

ii) How many moles of lead nitrate would be produced if 20.7 g of lead reacts with nitric acid?

Give your answer to 2 significant figures.

Moles of lead nitrate = ......................................................... [4]