R1.1 - measuring enthalpy changes

Question 1

What is a chemical reaction?

Answer 1

A chemical reaction involves the transfer of energy between the system and the surroundings, with total energy being conserved.

Question 2

What is the difference between heat and temperature?

Answer 2

Heat - process of energy transfer that occurs due to a temperature difference.
Temperature - measure of the average kinetic energy of particles.

Question 3

What is energy?

Answer 3

Energy - measure of the ability to do work.

Question 4

What is the system and the surroundings?

Answer 4

System - reaction mixture.
Surroundings - everything else

Question 5

What is enthalpy?

Answer 5

Enthalpy (H) - is a measure of the chemical potential energy stored in a system. The enthalpy changes as chemical bonds and intermolecular forces change during a reaction, resulting in a heat transfer which can be observed.

Question 6

What are the characteristics of endothermic reactions?

Answer 6

• ΔH is positive
• Surroundings –> System
• Bond breaking
• Reactants more stable than products
• E.g) Thermal decomposition

Question 7

What are the characteristics of exothermic reactions?

Answer 7

• ΔH is negative
• System –> Surroundings
• Bond making
• Products more stable than reactants
• E.g) Combustion

Question 8

What is an open system?

Answer 8

Open system - system where both energy and matter can be exchanged.

Question 9

What is a closed system?

Answer 9

Closed system - a system where only energy can be exchanged, but not matter.

Question 10

What is an isolated system?

Answer 10

Isolated system - system in which neither energy or matter can be exchanged.

Question 11

What is activation energy?

Answer 11

Activation energy - minimum amount of energy required for particles to collide and for a reaction to occur.

Question 12

What is standard enthalpy change?

Answer 12

Standard enthalpy change (ΔH) - heat transferred at constant pressure under standard conditions and states, determined by a change in temperature of a pure substance, measured in kJ mol-1

Question 13

What are the standard conditions?

Answer 13

• Pressure of 100kPa
• Concentrations of 1 mol dm-3
• All substances should be in their standard states

Question 14

What is the standard state of a substance?

Answer 14

Standard state - the pure form of the substance under standard conditions of 100kPa and 298K.

Question 15

What is specific heat capacity?

Answer 15

Specific heat capacity (c) - heat needed to increase the temperature of a unit mass of a material by 1K.

Question 16

How do you calculate specific heat capacity?

Answer 16

Specific heat capactity = (heat added (q)) / mass (m) x temperature change (ΔT)

Question 17

What are 3 assumptions about enthalpy change?

Answer 17

1) Heat is lost as soon as zinc is added and the reaction starts.
2) All heat generated was absorbed by water.
3) Volume of CuSO4 = Volume of water (1g cm-3 density)

Question 18

Why might enthalpy change be less than expected?

Answer 18

1) Not investigated under standard conditions
2) Heat lost to the surroundings