Energetics

Question 1

what is energy?

Answer 1

the ability of a system to do work

Question 2

what is enthalpy? what is its symbol?

Answer 2

name used for chemical energy, symbol is H

Question 3

what is an enthalpy change?

Answer 3

the amount of heat energy taken in/given out during any change in a system providing the pressure is constant

Question 4

what happens when an enthalpy change occurs?

Answer 4

energy is transferred between system and surroundings (the system = the chemicals, the surroundings is everything other than the chemicals)

Question 5

what happens during an exothermic reaction?

Answer 5

• energy transferred from system to surroundings
• products have less energy than reactants

Question 6

list 2 examples of exothermic reactions

Answer 6

• combustion
• neutralisation

Question 7

what happens during an endothermic reaction?

Answer 7

• energy transferred from surroundings to system
• products have more energy than reactants

Question 8

what are standard conditions?

Answer 8

• 100kPa
• 298K (25°c)
• solutions at 1 mol/dm³

Question 9

define standard enthalpy change of formation

Answer 9

energy transferred when 1 mole of the compound is formed from its elements under standard conditions, all reactants are in their standard states

Question 10

what is the symbol for standard enthalpy change of formation?

Answer 10

ΔHf

Question 11

define standard enthalpy change of combustion

Answer 11

enthalpy change that occurs when 1 mole of a substance is combusted completely under standard conditions, all reactants and products being in their standard states

Question 12

what is the symbol for standard enthalpy change of combustion?

Answer 12

ΔHc

Question 13

what are the products of incomplete combustion? will it be more/less exothermic than complete combustion?

Answer 13

• soot (carbon), carbon monoxide and water
• it will be less exothermic than complete combustion

Question 14

equation to calculate the energy change for an experimental reaction

Answer 14

• energy change (J) = mass of solution (thing being heated) (g) x specific heat capacity (J/g/K) x temperature change (K)
• Q=M x Cp x ΔT

Question 15

how do you convert Celsius to Kelvin?

Answer 15

add 273 to the Celsius value

Question 16

what is the SHC of a solution when the reactants are dissolved in water?

Answer 16

SHC = 4.18
because in any reaction where the reactants are dissolved in water we assume the SHC is the same as pure water

Question 17

what are the general steps to calculate the enthalpy change of a reaction

Answer 17

1. Using Q = m x SHC x ΔT calculate energy change for quantities used
2. Work out the moles of the reactants used moles=mass/mr
3. Divide q by the number of moles of the reactant not in excess to give ΔH
4. Add a sign and unit (if temp inc then add a negative sign)

Question 18

what are some potential errors when measuring enthalpies of combustion using calorimetry

Answer 18

• Energy losses from calorimeter
• Incomplete combustion of fuel
• Incomplete transfer of energy
• Evaporation of fuel after weighing
• Heat capacity of calorimeter not included
• Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment

Question 19

describe a method of a practical using the calorimetric method to measuring enthalpies of combustion

Answer 19

1) wash equipment (cup and pipettes etc) with the solutions used. dry cup after washing
2) put polystyrene cup in a beaker for insulation and support
3) measure out desired volumes of solutions with volumetric pipettes + transfer to insulated cup
4) clamp thermometer into place making sure thermometer bulb is immersed in solution
5) measure initial temps of the solution (or both solutions if 2 are used) repeat every minute for 2-3 minutes
6) after 3 mins transfer 2nd reagent to cup if a solid reagent is used then add solution to cup first and then add the solid weighed out on a balance. If using a solid reagent then use ‘before and after’ weighing method
7) stir mixture (ensures that all of the solution is at same temp)
8) record temp every minute after addition for several minutes

Question 20

if the reaction is slow, why may it be difficult to obtain accurate results of enthalpy change? how can this be counteracted?

Answer 20

• exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction
• take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together
• take temp of the reactants for a few mins before they are added together to get a better avg temp. If the two reactants are solutions then temp of both solutions need to be measured before addition + an average temperature is used

Question 21

what potential errors are there when carrying out a method of a practical using the calorimetric method to measuring enthalpies of combustion

Answer 21

• energy transfer from surroundings (usually loss)
• approximation in SHC of solution (method assumes all solutions have SHC of water)
• neglecting SHC of calorimeter- we ignore any energy absorbed by apparatus
• reaction or dissolving may be incomplete or slow
• density of solution is taken to be the same as water

Question 22

what is Hess’s law?

Answer 22

• Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place
• it is a version of the first law of thermodynamics, which states that energy is always conserved

Question 23

is standard enthalpy change of formation reaction - products or products - reactants?

Answer 23

products - reactants

Question 24

is standard enthalpy change of combustion reactants - products or products - reactants?

Answer 24

reactants - products

Question 25

why do elements have a ΔHc but not a ΔHf?

Answer 25

- elements are already in their standard states: they are not formed from anything so their ΔHf is just 0 - when calculating ΔHc don't forget to multiply by the number of moles

Question 26

does breaking bonds require or release energy?

Answer 26

requires energy

Question 27

does making bonds require or release energy?

Answer 27

release

Question 28

what is the energy change equation?

Answer 28

energy change = energy change of reactants - energy change of products