Energetics

Question 1

enthalpy change

Answer 1

is the heat energy transferred in a reaction at constant pressure

Question 2

exothermic reactions

Answer 2

give out energy to their surroundings. the products of the reaction end up with less energy than reactants, the enthalpy change is negative.

Question 3

endothermic reactions

Answer 3

take in energy from their surroundings. this means that the products of the reaction have more energy than the reactants, so the enthalpy change is positive

Question 4

bond enthalpies

Answer 4

they are the energy which is given out or taken in when bonds form or break.

Question 5

bond breaking is ___________?

Answer 5

endothermic

Question 6

bond forming is ______________?

Answer 6

exothermic

Question 7

mean bond enthalpy

Answer 7

the energy required to break an individual bond can change depending on where it is

Question 8

enthalpy change of reaction

Answer 8

bond breaking - bond forming

Question 9

standard enthalpy change of reaction

Answer 9

standard enthalpy change of reaction is the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states

Question 10

standard enthalpy change of formation

Answer 10

standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditionns

Question 11

standard enthalpy change of combustion

Answer 11

standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states.

Question 12

Exothermic and endothermic reactions

Answer 12

When a chemical reaction takes place, the products and reactants have different stabilities and thus there is a change in potential energy. However since total energy is always conserved, any change in potential energy must be balanced by an equal and opposite change in kinetic energy.
The lower the chemical potential energy of a given chemical species, the more stable it is. This means that stable species have a lower potential energy than unstable species.

In some reactions, the products are more stable than the reactants. The products therefore have less potential energy than the reactants, and the potential energy of the reacting species decreases.

Since the total energy is always conserved, it follows that the kinetic energy of the species must increase. The particles thus move faster and the temperature increases. Reactions in which the products are more stable than the reactants thus involve a transfer of energy from potential to kinetic and an increase in temperature.

PE –> KE