SmarterEd

Aussie Maths & Science Teachers: Save your time with SmarterEd

CHEMISTRY, M4 EQ-Bank 10 MC

Using Hess’s Law, if the enthalpy changes for the following reactions are known:

Determine the enthalpy change for the reaction:


 

Show Answer Only

Show Worked Solution

→ Reverse equation 2, making    a product and it changes the sign for .

Equation 2*:

→ Combine equation 2* with equation 1 which cancels out and adds the enthalpy values together.

→ Thus, the enthalpy for the final reaction

CHEMISTRY, M4 EQ-Bank 9 MC

Which statement best describes Hess's Law?

  1. The total enthalpy change for a reaction depends on the pathway taken to reach the final products.
  2. The total enthalpy change for a reaction is zero if the reaction is reversible.
  3. The total enthalpy change for a reaction is independent of the pathway taken, as long as the initial and final states are the same.
  4. The enthalpy change for a reaction is the same regardless of whether it occurs in one step or multiple steps, provided heat is lost to the surroundings.
Show Answer Only

Show Worked Solution

→ Hess’s Law states that the total enthalpy change for a reaction is independent of the pathway taken, as long as the initial and final states are the same.

→ This means that whether a reaction happens in one step or multiple steps, the overall enthalpy change will be the same if the reactants and products are the same.

CHEMISTRY, M4 EQ-Bank 2

  1. Identify Hess' Law.   (2 marks)

  2. Given these heats of formation, :

  1. Calculate for the combustion of:

i.  1 mole of ethane () to produce carbon dioxide and water.   (2 marks)

ii. 1 mole of ethane () to produce carbon monoxide and water.   (2 marks)

Show Answers Only

a.    Hess’s Law:

→ The total enthalpy change for a reaction is the same, regardless of the route by which the chemical reaction occurs, provided the initial and final conditions are the same.

b.i.   

b.ii.   

Show Worked Solution

a.    Hess’s Law:

→ The total enthalpy change for a reaction is the same, regardless of the route by which the chemical reaction occurs, provided the initial and final conditions are the same.
 

b.i.   Enthalpy change:

→ complete combustion of 1 mole of ethane () to produce carbon dioxide and water.


 

b.ii.  Enthalpy change:

→ incomplete combustion of 1 mole of ethane () to produce carbon monoxide and water.

CHEMISTRY, M4 EQ-Bank 2 MC

Three equations and their values are shown.

Equation 1:        
Equation 2:      
Equation 3:      

 
Using this information, what is the for the following reaction?

Show Answers Only

Show Worked Solution

Reverse Equation 3 to express as a product:

Equation 3*:

Double the coefficients of Equation 2 which also doubles .

Equation 2*:

Combine the corresponding enthalpies of Equation 1, 2* and 3* to find the total :

CHEMISTRY, M4 2016 VCE 17*

The combustion of hexane takes place according to the equation

Consider the following reaction.

  1. Calculate the value of , in kJ mol, for this reaction   (2 marks)

  2. Is the reaction exothermic or endothermic?   (1 mark)

Show Answers Only

a.   

b.    Endothermic

Show Worked Solution

a.   

→ Reverse equation:

→ Double equation:

→

b.    Endothermic

CHEMISTRY, M4 EQ-Bank 35

The enthalpies of reaction of a number of chemical reactions are as follows:

Reaction 1:      

Reaction 2:        

Reaction 3:                                            

Calculate the enthalpy change for the reaction below using the enthalpies of reaction above.

              (3 marks)

Show Answers Only

 

Show Worked Solution

→ Reverse equation 1 to make a product (changes the sign of the enthalpy).

     
 

→ Add the equations and their values and cancel any reactants and products.

CHEMISTRY, M4 EQ-Bank 32

The enthalpies of reaction of a number of chemical reactions are as follows:

Reaction 1:      

Reaction 2:                                    

Reaction 3:                                

Calculate the enthalpy change for the reaction below, stating whether the reaction is exothermic or endothermic: 

                (4 marks)

Show Answers Only

 

Show Worked Solution

→ Reverse equation 1 to make a product (changes the sign of the enthalpy).

     

→ Add a multiplier to Reaction 2 as this will allow the to cancel when applying Hess’s Law.

             
 

→ Add the equations and their values and cancel any reactants and products.

   
 

 

→ The reaction is endothermic.

CHEMISTRY, M4 EQ-Bank 31

Define Hess's Law with reference to reaction pathways and changes in enthalpy.   (2 marks)

Show Answers Only

→ The enthalpy change in a chemical reaction between the reactants and products is constant, regardless of the number of pathways or chemical reactions undertaken to get from the reactants to the products.

Show Worked Solution

→ The enthalpy change in a chemical reaction between the reactants and products is constant, regardless of the number of pathways or chemical reactions undertaken to get from the reactants to the products.