Chapter 3- Enthalpy

what does system mean?

• all the chemicals- both reactants and products

what does surroundings mean?

• everything else in the universe, outside of the system.

  • energy can be transferred between the system and surroundings in both directions

energy meaning?

• the ability of a system to do work

work meaning?

the energy transfer that is the result of a force moving an object through a distance

what is heat?

the energy transfer that is a result of a temperature difference between system and surroundings

what is enthalpy change?

the amount of heat energy taken in or given out during any change in a system

• provided the pressure is constant

bond enthalpy meaning?

• the enthalpy change needed to break that covalent bond into gaseous atoms

  • bond enthalpies are always positive because it will always require energy to overcome the attractive forces in the bond

exothermic reaction?

• heat energy is transferred from the system (chemicals) to the surroundings

  • the products have less enthalpy than the reactants

  • in an exothermic reaction, the enthalpy change is negative

why do exothermic reactions release heat?

• because more energy is released forming new bonds in products than is absorbed breaking bonds in the reactants

exothermic diagram?

endothermic reaction?

• heat energy is transferred from the surroundings to the system (chemicals)

  • the products have more enthalpy than the reactants

  • in an endothermic reaction, the enthalpy change is positive

why do endothermic reactions absorb energy (take in energy)?

• because more energy is needed to break reactant bonds than is released forming product bonds

endothermic reaction image?

examples of exothermic reactions?

• combustion of fuels

• oxidation of carbs such as glucose in respiration

example of endothermic reaction?

• thermal decomposition of calcium carbonate

how to remember which reaction breaks/makes bonds?

BENDO MEXO

• Breaking bonds- ENDOthermic

• Making bonds- EXOthermic

definition of the standard enthalpy change of formation?

the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298k and 100kPa)

• and all reactants and products are in their standard states

symbol for enthalpy change of formation?

definition of standard enthalpy of combustion?

the enthalpy change that occurs when 1 mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kPa),

• alll reactants and products being in their standard states

symbol for enthalpy change of combustion

definition of the enthalpy change of reaction?

the enthalpy change when the number of moles of reactants as specified in the balanced equation react together

definition of the enthalpy change of neutralisation?

the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

• enthalpy changes of neutralisation are always exothermic

what are the standard conditions?

• 298Kelvin

• 1 mol dm-3

• 101 kPa

specific heat capacity equation:

energy transferred (J) = mass of water (g) x specific heat capacity (4.18J g -1 K -1) x temperature difference (°C)

• q = mc t

how to calculate enthalpy change?

energy transferred / moles in reaction

what is activation energy?

the minimum energy required for a reaction to take place

what is hess law?

the enthalpy change of a reaction is independent of the route it takes

exam question: Explain why this value of Δ_cH is different from the data book value and suggest how the experimental design could be modified to improve the accuracy of the Δ_cH value obtained. 

difference from data book value:

• Heat losses 

• Incomplete combustion 

• Data book uses standard values 

• Evaporation of alcohol from wick 

• Evaporation of water from beaker

how to improve:

• burn in plentiful oxygen

• draft shield

• add lid to beaker

• heat for longer to reduce percentage uncertainty in mass/temp measurements

• use standard conditions

• digital thermometer OR 3 DP balance