Energy Change Flashcards
What is energy often defined as?
The ability to do work.
Name different forms of energy.
Potential (stored)
Are the various forms of energy interconvertible?
Yes.
Give an example of energy conversion in the human body.
Chemical energy in food is converted to heat energy and kinetic energy.
What does an electric bulb convert electrical energy into?
Heat energy and light energy.
What is the Law of Conservation of Energy?
Energy can neither be created nor destroyed
What is an exothermic reaction?
A reaction that releases heat to the surroundings.
What is the sign of ΔH for an exothermic reaction?
Negative (-).
Give examples of exothermic reactions.
Combustion reactions
What is an endothermic reaction?
A reaction that absorbs heat from the surroundings.
What is the sign of ΔH for an endothermic reaction?
Positive (+).
Give examples of endothermic reactions.
Melting of ice
What is enthalpy (H)?
The heat content of a system at constant pressure.
What is enthalpy change (ΔH)?
The difference between the enthalpy of the products and the enthalpy of the reactants.
What occurs during chemical reactions?
Energy changes.
Why do energy changes occur in chemical reactions?
Because reactants and products possess different amounts of chemical energy.
Can we measure the amount of chemical energy in a substance?
No.
What forms can energy changes in chemical reactions be observed as?
Heat
Give an example of a reaction that produces heat.
Mixing a strong acid and a strong base.
Give an example of a reaction that produces light and heat.
Magnesium burning in air.
Give an example of a reaction that produces sound and heat.
Ignition of a hydrogen and oxygen mixture.
Which energy change is easiest to measure?
Heat energy changes.
What is standard enthalpy change (ΔH⁰)?
The enthalpy change when reactants are converted to products in their standard states under standard conditions.
What are standard conditions?
100 kPa pressure and a specified temperature (usually 298 K or 25°C).
What is the standard state of a substance?
The pure form of the substance at 100 kPa and the specified temperature.
What is the standard enthalpy change of formation (ΔH⁰f)?
The enthalpy change when one mole of a substance in its standard state is formed from its constituent elements in their standard states.
What is the standard enthalpy change of combustion (ΔH⁰c)?
The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions.
What type of reaction is neutralization?
Exothermic.
What is the heat of neutralization?
The amount of heat evolved during a neutralization reaction in which one mole of water is formed.
What is the standard heat of neutralization (ΔH⁰n)?
The amount of heat evolved when 1 mole of hydrogen ions (H+) from an acid reacts with 1 mole of hydroxide ions (OH-) from an alkali to form 1 mole of water under standard conditions.
Approximately what value is the heat of neutralization for any strong acid and strong base?
-57.4 kJ/mol.
Why is the heat of neutralization less when weak acids or weak bases are involved?
Because the reactants are only partially dissociated into ions in solution
What is combustion?
A chemical process that is always accompanied by the evolution of heat.
What is the heat of combustion?
The amount of heat evolved when one mole of a substance is burned completely in oxygen.
What is the standard heat of combustion (ΔH⁰c)?
The heat evolved when one mole of the substance is burned completely in oxygen under standard conditions.
Why is the heat of combustion an important quantity?
Because the combustion of fuels is the main source of energy for industrial and domestic purposes
What is calorific value?
The amount of heat evolved per kilogram of the substance.
What occurs in the first step when an ionic solid dissolves in water?
Water molecules split up the ions in the crystal into free ions.
What is the energy needed for splitting up ions in a crystal called?
Lattice energy.
What occurs in the second step when an ionic solid dissolves in water?
Free ions are hydrated by water molecules.
What is the energy evolved when free ions are hydrated by water molecules called?
Hydration energy.
What is the heat of solution?
The resultant heat change of the two steps (lattice energy and hydration energy) when an ionic solid dissolves in a solvent.
Under what condition is dissolution exothermic?
If lattice energy < hydration energy.
Under what condition is dissolution endothermic?
If lattice energy > hydration energy.
Why is the dissolution of hydrated salts endothermic?
Because the ions are already extensively hydrated
