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Photosynthesis is an endothermic reaction.

Energy from Reactions

Burning fuels releases energy you can measure and compare. This GCSE Chemistry quiz explores energy from reactions, including combustion, temperature change, and choosing the best fuel.

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Fascinating Fact:

Fuels are compared by how much energy they release per gram or per mole when burned. This is called the enthalpy of combustion.

In GCSE Chemistry, energy from reactions often focuses on combustion and the idea that energy transfers to the surroundings. You may calculate energy changes using temperature rise data, compare fuels fairly, and link results to bond breaking and bond making.

Key Terms

Combustion: A reaction where a fuel burns in oxygen, releasing energy and usually forming carbon dioxide and water.
Enthalpy Change: The overall energy change for a reaction, usually measured in kJ per mole.
Calorimetry: A method of measuring energy transfers by recording temperature changes in a known mass of water.

Frequently Asked Questions

How do you calculate energy released from burning a fuel in GCSE Chemistry?

You can calculate energy using q = m c ΔT, where m is the mass of water, c is the specific heat capacity, and ΔT is the temperature change. This estimates energy transferred to the water.

Why is combustion usually an exothermic reaction?

Combustion is usually exothermic because forming new bonds in carbon dioxide and water releases more energy than is needed to break the bonds in the fuel and oxygen, so energy is released overall.

What makes a fair comparison between different fuels?

A fair comparison uses the same method and compares energy per gram or per mole. You should also consider energy lost to the surroundings, incomplete combustion, and keeping conditions consistent.

Try These Related Quizzes

1 .

Pick the correct combination for the correct information regarding bond energies.

Bond breaking - Exothermic - Takes in energy
Bond making - Endothermic - Gives out energy

Bond breaking - Endothermic - Takes in energy
Bond making - Exothermic - Gives out energy

Bond breaking - Endothermic - Gives out energy
Bond making - Exothermic - Takes in energy

Bond breaking - Exothermic - Gives out energy
Bond making - Endothermic - Takes in energy

Remember EXothermic EXit and ENdothermic ENter

2 .

The overall energy change in a chemcial reaction is -24kJ/mol. What does this tell us about the reaction?

It is endothermic

More energy was taken in than given out

It is exothermic

The temperature decreased

A negative energy change shows the reaction is exothermic and so heat and/or other energy has been transferred into the surroundings, often increasing the temperature

3 .

Using the bond energies provided, calculate the energy change for this reaction.
2CH₄ + O₂ → 2CH₃OH

-1471kJ/mol

-329kJ/mol

-89kJ/mol

+2567kJ/mol

When carrying out these calculations, always subtract the reactants energy from the products

4 .

In the Haber process, nitrogen and hydrogen are reacted together to produce ammonia. Use the bond energies provided to calculate the overall energy change in the reaction.

N₂ + 3H₂ ⇌ 2NH₃

-93kJ

-4599kJ

1401kJ

93kJ

945 + (3x436) - (6x-391) = -93kJ

5 .

Use the bond energies provided to calculate the energy change in the following reaction.

H₂ + Cl₂ → 2HCl

-247kJ/mol

-1543kJ/mol

-185kJ/mol

-158kJ/mol

Bond breaking = 436+243kJ/mol; bond making 2x432kJ/mol

6 .

Use the bond energies provided to calculate the energy change in the following reaction.

oxygen + hydrogen → water

1856kJ/mol

1370kJ/mol

442kJ/mol

486kJ/mol

Add together the energy of each bond of the reactants, do the same for the bonds in the products. Then subtract the total bond energy of the reactants from that of the products to get the answer

7 .

Use the bond energies provided to calculate the amount of energy given out when propane is burnt.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

2054kJmol^-1

-628kJmol^-1

1958kJmol^-1

-2054kJmol^-1

Remember that CO₂ has 2 C=O double bonds not single bonds

8 .

If ΔH for a reaction is negative, what does this tell us about the reaction?

It is endothermic

More energy is taken in to break bonds than is given out when bonds form

It is exothermic

The temperature of the reaction mixture decreases

Remember to always subtract reactants from products - the total bond energy of the products must be less than the reactants to end up with a negative answer

9 .

Use the bond energy values provided to calculate ΔH for the reaction of photosynthesis.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Hint: For glucose (cyclical) there are the following bonds formed: 7xC-H, 5xC-C, 7xC-O and 4xO-H.

3253kJ/mol

-3201kJ/mol

-6999kJ/mol

6501kJ/mol

Photosynthesis is an endothermic reaction and therefore ÄH is positive. LHS total = 15228kJ RHS total = 11976 LHS - RHS = ?H

10 .

What is the value for ΔH for respiration?

3210kJ/mol

-3253kJ/mol

-6999kJ/mol

6501kJ/mol

Respiration is the same as photosynthesis, but in the opposite direction!!

You can find more about this topic by visiting BBC Bitesize - Calculations in chemistry

Author: Kate Gardiner (Chemistry Educator & GCSE Quiz Writer)

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