energetics 1 p1

Question 1

what does system mean in a chemical reaction?

Answer 1

the atoms and bonds involved in the chemical reaction

Question 2

explain the law of conservation

Answer 2

the amount of energy in an isolated system remains the same. energy cannot be destroyed or created, it can only be transferred from one form to another

Question 3

what energy change is breaking bonds associated with?

Answer 3

energy is taken in to break bonds –> endothermic reaction

Question 4

what energy change is making bonds associated with?

Answer 4

energy is released to make bonds –> exothermic reaction

Question 5

what is an endothermic reaction?

Answer 5

a reaction with an overall positive enthalpy change (+ΔH) –> enthalpy of products > enthalpy of reactants

Question 6

what is an exothermic reaction?

Answer 6

a reaction with an overall negative enthalpy change (-ΔH) –> enthalpy of products < enthalpy of reactants

Question 7

explain an enthalpy change diagram for an endothermic reaction

Answer 7

reactants lower
products higher
activation energy points upwards
enthalpy on y axis and reaction progress on x axis

Question 8

explain an enthalpy change diagram for an exothermic reaction

Answer 8

reactants higher
products lower

Question 9

what is meant by activation energy?

Answer 9

the minimum energy required for a reaction to take place

Question 10

what are the standard conditions?

Answer 10

100kPa
298 K

Question 11

what does ‘in standard state’ mean?

Answer 11

the state an element/compound exists at in standard conditions

Question 12

enthalpy change of formation

Answer 12

the energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions
H2(g) + 1/2O2(g) –> H2O(l)

Question 13

enthalpy change of combustion

Answer 13

the energy change that takes place when 1 mole of a substance is completely combusted
C(s) + O2(g) –> CO2(g)

Question 14

enthalpy of neutralisation

Answer 14

the energy change that takes place when 1 mole of water is formed from a neutralisation reaction

Question 15

enthalpy change of reaction

Answer 15

the energy change associated with a given reaction

Question 16

enthalpy change formula for experimental data

Answer 16

Q = mcΔT
m is mass of substance (usually water)
c is specific heat capacity
ΔT is change in temperature

Question 17

why might experimental methods for enthalpy determination not be accurate?

Answer 17

• heat lost to the surroundings
• not in standard conditions
• reaction may not go to completion

Question 18

average bond enthalpy

Answer 18

mean energy required to break 1 mole of bonds in gaseous molecules

Question 19

why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 19

bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate

Question 20

enthalpy change of reaction using bond enthalpies formula

Answer 20

ΔH = ∑(bond enthalpies of reactants/bonds broken) - ∑(bond enthalpies in products/bond made)

Question 21

what is Hess’ Law?

Answer 21

states that the enthalpy change for a reaction is the same regardless of the route taken

Question 22

which way do the arrows go on a Hess cycle for formation and for combustion?

Answer 22

formation - upwards
reactants and products
elements
combustion - downwards
reactants & oxygen and products & oxygen
combustion products