Enthalpy

Define bond enthalpy

Energy required to break the bond of 1 mole of gaseous atom. Always endothermic because bond breaks.

Explain why enthalpy of formation of elements in standard state is 0

In elements in standard state, there is no change in enthalpy as the reactants and products are the same.

Define standard enthalpy of formation

Change in enthalpy when one mole of compound is formed from its elements in their standard states

What are the assumptions of a calorimetry lab?

Substance is pure (no impurities)
No heat loss to surroundings (perfect insulation)
All heat released in reaction is absorbed by solution
Specific heat capacity and density of the solution are the same as those of water
The greater the mass, the more heat it can absorb.

Why is the bond enthalpy in data booklet different from lab bond enthalpy?

The bond enthalpy in the data booklet is the average values taken from a range of compounds containing that bond. Experimental values of bond enthalpy depends on the specific environment and molecules.

How does the bond enthalpy data relate to bond length and polarity?

Shorter bonds tend to have higher bond enthalpies as the atoms are held more tightly. Polar bonds are usually stronger than nonpolar bonds due to electrostatic attractions between partial charges.

How is the enthalpy change of formation and combustion measured?

Hf = Products - reactants
Hc = reactants - products
If the reactants’s enthalpy is higher than the products = exothermic = negative enthalpy
If reactant’s enthalpy is lower than the products = endothermic = positive enthalpy

Define enthalpy density

Enthalpy per unit volume of system (kJ/m3)

Define enthalpy

Measurement of total energy in a thermodynamic system at constant pressure. Internal energy + pressure and volume.

Define standard enthalpy change

Heat transferred at constant pressure under standard conditions (298K or 25˚C, 105 Pa, 1 mol/m3) and states. It can be determined from the change in temperature of a pure substance.

State the difference of heat and temperature

Heat is the total energy transferred between substances between substances due to temperature difference (J or kJ). Temperature is the average kinetic energy of particles. Heat depends on mass and specific heat capacity while temperature is not.

What does it mean when products are at a lower energy level than reactants?

Products at a lower energy level than the reactants mean that the total average bond enthalpy of formation of bonds is greater than the total average bond enthalpy of reactants.

What is the activation energy in terms of reactants and products

The minimum energy that reactants particles must have to break bonds and form the transition state before new bonds can form in the products.

What are the sources of error of a calorimetry lab?

Heat loss to surroundings
Incomplete combustion
Evaporation of liquid fuel
Inaccurate temperature or mass measurements

Why is Q of system = -q of surroundings because heat of system is negative sign of surroundings

Energy is conserved. The heat loss by the system equal the heat gained by surroundings or vice versa. The reason it is opposite signs is so that net energy is zero (no energy lost or gained).

How does heat transfer occur between a cold object and a hot object?

Thermal energy transfers from the hot object to the cold object via conduction (or convection, radiation).
Eventually, both objects reach thermal equilibrium. The final temperature is closer to the hotter object if it has greater mass or heat capacity.

What can be deuced form the temperature change that accompanies chemical or physical change?

The size of the change can be used to calculate enthalpy change if mass and specific heat.

What is the relationship between temperature and kinetic energy in particles?

It is a linear proportional relationship. As temperature increases, the kinetic energy between particles also increase.

What does it mean that a reaction is exothermic?

Bonds are formed, energy is released to surroundings from system, products have a lower bond enthalpy than the reactants, negative enthalpy,

How does the bonding in N2 explain that combustion is endothermic?

Why do calorimetry experiments typically measure a smaller change in temperature than is expected from theoretical values?

How can enthalpy change for combustion reactions such as alcohols or food be investigated experimentally?

Explain why no reaction takes place between substances in room temperature unless the reactants are sparked, exposed to UV light or heated

Requires energy to overcome activation energy, molecules have insufficient energy to react at room temperature

How to know if the ∆H is negative or positive?

If the reactants bond enthalpy > products = exothermic. Products > reactants = endothermic