Understanding Hess's Law and Enthalpy Changes

This video tutorial explains how to use Hess's Law to calculate the enthalpy change for a reaction. It provides the standard enthalpy changes for three combustion reactions and demonstrates how to manipulate these equations to find the enthalpy change for the formation of ethene and hydrogen from ethane. The process involves adjusting coefficients, reversing equations, and summing enthalpy changes to derive the final result.

What is the main purpose of using Hess's Law in this video?

• To find the equilibrium constant of a reaction.

• To determine the rate of a chemical reaction.

• To calculate the enthalpy change of a reaction using known reactions.

• To measure the temperature change in a reaction.

In the first reaction, what is the enthalpy change for the combustion of two moles of ethane?

• +137 kJ

• -1411 kJ

• -572 kJ

• -3120 kJ

What is produced in the second reaction involving hydrogen?

• Carbon dioxide and water

• Two moles of water

• Ethene and oxygen

• Ethane and hydrogen

What is the enthalpy change for the combustion of two moles of hydrogen?

• -1411 kJ

• -3120 kJ

• +137 kJ

• -572 kJ

In the third reaction, what is the enthalpy change for the combustion of one mole of ethene?

• -572 kJ

• -3120 kJ

• -1411 kJ

• +137 kJ

Why do we need to reverse the second reaction involving hydrogen?

• To balance the oxygen molecules

• To decrease the number of moles of water

• To increase the enthalpy change

• To match the products in the final equation

What is the final product when combining the three reactions?

• One mole of ethane, one mole of hydrogen, and one mole of ethene

• Two moles of hydrogen and ethene

• Two moles of carbon dioxide and water

• Three moles of oxygen

What is the final enthalpy change for the formation of ethene and hydrogen from ethane?

• -572 kJ

• -1411 kJ

• -3120 kJ

• +137 kJ

What happens to the enthalpy change when a reaction is reversed?

• The enthalpy change remains the same.

• The enthalpy change is doubled.

• The sign of the enthalpy change is reversed.

• The enthalpy change is halved.

What is the significance of cancelling out the same substances on opposite sides of the equations?

• It reduces the number of reactants.

• It balances the chemical equation.

• It increases the enthalpy change.

• It helps in simplifying the overall reaction.