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CfE Higher Chemistry Unit 3 > Chemical Energy > Flashcards

Flashcards in Chemical Energy Deck (16)
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Why is it important for chemists to be able to predict the quantity of heat energy taken in or given out?

Endothermic reactions may incur costs in supplying heat energy in order to maintain the reaction rate.

Exothermic reactions may require removal of heat to prevent the temperature rising


What is meant by an enthalpy change?

Enthalpy change is the difference in energy between the products and the reactants.


What equation is used to calculate the enthalpy change for a reaction?

Eh = cmΔT


Define the enthalpy of combustion of a substance.

The enthalpy of combustion of a substance is the enthalpy change (energy released) when one mole of the substance burns completely in oxygen.


A spirit burner and a metal beaker can be used to experimentally measure the energy released by a fuel when burned.

Describe how to set up the experiment.


What measurements need to be taken when determining the energy released when a fuel burns?

  1. the start and the final temperature of the water which allows you to calculate the temperature change
  2. the mass of the burner before and after the experiment which allows you to calculate the mass of alcohol burned.


Give a reason why your experimental enthalpy value differs from the textbook value.

  1. heat loss to the surroundings
  2. incomplete combustion


What does Hess's law tell us about the enthalpy change of a reaction?

Hess's law states that the enthalpy change for a chemical reaction is independent of the route taken.


When rearranging equations to calculate an enthalpy change, what two rules have to be followed?

  1. The enthalpy change (ΔH) is proportional to the quantities of reactants and products. For example, burning twice as much fuel will result in twice the enthalpy change for the process.
  2. If a reaction is reversed then the sign of the enthalpy change must also be reversed.


ΔHT = Ca(s) + 2H2O(ℓ) → Ca(OH)2(s) + H2(g)

Calculate the enthalpy change, in kJ mol−1 , for the reaction above by using the data shown below.

H2(g) + ½O2(g) → H2O(ℓ) ΔH1 = −286 kJ mol−1

Ca(s) + O2(g) + H2(g) → Ca(OH)2(s) ΔH2 = −986 kJ mol−1

ΔH= ΔH− 2ΔH1

ΔH= −984 −​ (2 x −286)

= −986 + 572 

= −414 kJ mol−1



The energy required to break a covalent bond between two atoms is called the ​                       

Bond enthalpy


Is energy required or released when bonds are broken?

Is breaking bonds exothermic or endothermic?

Energy is required to break bonds.

Bond breaking is an endothermic process.


Is energy required or released when bonds are made?

Is making bonds exothermic or endothermic?

Energy is released when new chemical bonds are formed.

Bond making is an exothermic process.


Bond enthalpies quoted in the data book are the energies required to break what.

The bond enthalpies quoted in the data book are the energies required to break one mole of a particular bond between a pair of atoms in the gaseous state.


Why can the values in the databook only be used to calculate an approximate enthalpy change?

Some values are mean molar bond enthalpies which are average values quoted for bonds which occur in different molecular environments.

Energy to break C-C in ethane will be different to the energy needed to break C-C in decane.


The mean bond enthalpy of a C – F bond is 484 kJ mol−1.

In which of the processes is ΔH approximately equal to +1936 kJ mol-1?

A CF4(g) → C(s) + 2F2(g)

B CF4(g) → C(g) + 4F(g)

C CF4(g) → C(g) + 2F2(g)

D CF4(g) → C(s) + 4F(g)

ΔH comes from 4 x C – F bonds being broken (1936 = 4 x 484) and no bonds being made.

Rules out A and C

Changing the physical state of any reactant (or product) will involve an enthalpy change which rules out D.

Answer is B