enthalpy changes (chapter 6) Flashcards
definitions
What are exothermic reactions?
Chemical reactions that transfer heat energy to the surroundings, causing an increase in temperature.
Example: Magnesium reacting with sulfuric acid releases energy, increasing the temperature of the mixture.
What are the surroundings in an exothermic reaction?
The surroundings include the solvent (water), the air around the test-tube, the test-tube itself, and anything dipping into the test-tube (e.g., a thermometer).
What are some examples of exothermic reactions?
Examples include the combustion of fuels, oxidation of carbohydrates (respiration), and the reaction of water with quicklime.
What are endothermic reactions?
Chemical reactions that absorb heat energy from the surroundings, causing a decrease in temperature.
Example: Sodium hydrogencarbonate reacting with citric acid absorbs heat, lowering the temperature.
What are some examples of endothermic reactions?
Examples include the decomposition of limestone by heating, photosynthesis, and dissolving certain ammonium salts in water.
What is enthalpy change?
The energy exchange between a chemical reaction and its surroundings at constant pressure, denoted by the symbol ΔH.
What are the units of enthalpy change?
The units of enthalpy change are kilojoules per mole (kJ mol−1).
What do reaction pathway diagrams show?
They illustrate enthalpy changes, with the enthalpy of reactants and products on the y-axis and the reaction pathway on the x-axis.
What is activation energy?
The minimum energy that colliding particles must possess for a reaction to occur, represented as a hump in reaction pathway diagrams.
What indicates an exothermic reaction in a reaction pathway diagram?
For exothermic reactions, the enthalpy of the reactants is greater than that of the products, resulting in a negative ΔH.
What indicates an endothermic reaction in a reaction pathway diagram?
For endothermic reactions, the enthalpy of the products is greater than that of the reactants, resulting in a positive ΔH.
What are standard conditions for measuring enthalpy changes?
Standard conditions are a pressure of 101 kPa, a temperature of 298 K, and each substance in its normal physical state.
What is the standard enthalpy change of reaction?
The enthalpy change when the amounts of reactants in the stoichiometric equation react to give products under standard conditions.
What is the standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions.
What is the standard enthalpy change of combustion?
The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.
What is the standard enthalpy change of neutralisation?
The enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions.
What is calorimetry?
A technique used to measure the enthalpy change of reactions using a calorimeter.
What is the specific heat capacity of water?
The specific heat capacity of water is 4.18 J g−1 °C−1.
What is the formula for calculating heat transferred?
The formula is q = mcΔT, where q is heat transferred, m is mass, c is specific heat capacity, and ΔT is temperature change.
What does a rise in temperature indicate in terms of ΔH?
A rise in temperature is positive, indicating that ΔH is negative for exothermic reactions.
What does the law of conservation of energy state?
Energy cannot be created or destroyed.
This is also called the first law of thermodynamics.
What is Hess’s law?
The total enthalpy change in a chemical reaction is independent of the route by which the reaction takes place as long as the initial and final conditions are the same.
What are enthalpy cycles?
Enthalpy cycles, also called energy cycles or Hess cycles, illustrate Hess’s law.
What is the standard enthalpy change of formation of an element in its standard state?
It is zero.
What is the procedure to calculate the enthalpy change of reaction using an enthalpy cycle?
- Write the balanced equation at the top. 2. Draw the cycle with elements at the bottom. 3. Draw in all arrows, ensuring correct directions. 4. Apply Hess’s law, accounting for the number of moles.
What is bond breaking and bond making?
Bond breaking is endothermic and bond making is exothermic.
What is bond dissociation energy?
The amount of energy needed to break a specific covalent bond.
What is the reaction for the Haber process?
The reaction for the Haber process is N2(g) + 3H2(g) ⇌ 2NH3(g).
How can enthalpy changes be calculated using bond energies?
Enthalpy changes can be calculated by setting out a balance sheet for bonds broken and formed.
What does a positive value indicate in bond energy calculations?
A positive value indicates that the bond breaking is endothermic.
What does a negative value indicate in bond energy calculations?
A negative value indicates that the bond forming is exothermic.
