Enthalpy Changes - Thermodynamics Pt A (2.1)

Question 1

Explain the first law of thermodynamics

Answer 1

• Energy cannot be created or destroyed, only converted from one form to another
• In a closed system, the total energy of the system before the chemical change must equal the total energy of the system after the chemical change

Question 2

Define enthalpy

Answer 2

The total energy associated with a system at a constant pressure

Question 3

How do you measure enthalpy?

Answer 3

You cannot measure it directly but you can measure the enthalpy change of a reaction (ΔH)

This energy is usually released as heat energy

Question 4

Give the enthalpy change equation

Answer 4

ΔH = Hproducts – Hreactants

Question 5

Is the enthalpy change of an exothermic reaction positive or negative?

Answer 5

Positive - there is a rise in temperature of the surroundings since energy is lost to the surroundings (give out heat)

Question 6

Is the enthalpy change of an endothermic reaction positive or negative?

Answer 6

Negative - there is a fall in temperature since energy is taken in (absorbed) from the surroundings

Question 7

What is standard enthalpy change?

Answer 7

It is the enthalpy change under standard conditions

Question 8

Give the 3 standard conditions

Answer 8

• Pressure: 100kPa (1atm)
• Temperature: 298K
• Substance is in its standard physical state at these conditions

Question 9

Define activation energy

Answer 9

The minimum energy required for the reaction to occur

Question 10

What is an exothermic reaction?

Answer 10

A reaction when energy is released to the surroundings (usually as heat)

Question 11

What is an endothermic reaction?

Answer 11

A reaction when energy is taken in from the surroundings