Enthalpy changes

Question 1

What does “system” refer to in a reaction?

Answer 1

The substances in a chemical reaction. i.e. the reactants and products.

Question 2

What does “surroundings” refer to in a reaction?

Answer 2

Everything that is not the system (could be the reaction container, air, ect)

Question 3

What is an exothermic reaction? (give an example)

• definition
• what is recorded
• an example
• the enthalpy graph

Answer 3

An exothermic reaction releases heat energy from the system into the surroundings. A rise in temperature is recorded.
Examples include combustion and acid/base neutralisations.
Enthalpy change is negative.
Graph (with enthalpy on Y axis)
_______reactants

                           \_\_\_\_\_\_\_\_ products

Question 4

What is an endothermic reaction?

• definition
• what is recorded
• an example
• the enthalpy graph

Answer 4

An endothermic reaction takes heat energy from the surroundings into the system. A fall in temperature is recorded.
Examples include the thermal decomposition of limestone.
Enthalpy change is positive.
Graph (with enthalpy on Y axis)
________products

_________reactants

Question 5

What are the four standard conditions for thermochemical measurements?

Answer 5

a) temperature at 298K or 25 degrees Celsius
b) pressure at 1 atm or 101325 Pa
c) concentration: any solution used must be 1 moldm-3
d) substances: must be in standard physical states (l,s,g) appropriate to standard conditions. i.e. ice could not be used because H20 is a liquid at 298K

Question 6

What is the standard enthalpy change of a reaction?

Answer 6

The energy change at constant pressure when molar quantities of reactants stated in the balanced equation react together all under standard conditions.

Question 7

Give the symbol for standard enthalpy change of a reaction.

Answer 7

triangle (representing change/difference), small r (reaction), capital H (enthalpy), london underground symbol in top right (standard conditions)

Question 8

What must you remember when writing out balanced equations for reactions?

Answer 8

State symbols must always be shown

Question 9

What is enthalpy change measured in, and explain what these units mean.

Answer 9

kJmol-1

Refers to the energy released per mole of the simplest chemical equation