Chemistry Unit 7 (Lessons 1-2)

Lesson 1

Thermochemistry

Find the ΔHr0 for the reaction of two moles of potassium iodide and one mole of lead (II) nitrate to form 2 moles of potassium nitrate and one mole of lead (II) iodide. 2KI(aq) + Pb(NO3)2(aq)→2KNO3(aq) + PbI2(s)

ΔHr0=-53.1kJ

You are conducting a calorimetry experiment using a calorimeter with 500mL of water in its outer chamber to determine the enthalpy of reaction of a chemical reaction. The initial temperature of the water is 25°C. After the reaction, the temperature of the water is 32.5°C. Disregarding any heat loss to the walls of the container, calculate the ΔHr0. The specific heat of water is 4.18 J/(g•K).

ΔHr0=15.7 kJ

One mole of hydrogen gas (H2), reacts with one mole of bromine Br2(g) to produce 2 moles of hydrogen bromide gas (HBr). The reaction releases 72.58 kJ of heat. Which equation is the correct form to show the standard molar enthalpy of formation?

1/2 H2(g) + 1/2 Br2(l) → HBr(g)ΔHf0= -36.29 kJ

Place in order(1-4) the steps followed to produce carbon monoxide by the combustion of carbon graphite in a limited oxygen environment.

4 - the liberated oxygen combines with carbon to form more CO2 that breaks down into CO and O2 again, and the cycle continues 1 - carbon and oxygen combine to form CO2 3 - the hot carbon then acts as a catalyst to break down some of the CO2 into CO and O2 2 - as the carbon continues to burn, it uses up all of the oxygen

How does a calorimeter measure the enthalpy change of a chemical reaction?

By multiplying the specific heat of the water by the mass of the water and the change in temperature of the water, you can calculate the amount of heat released or absorbed by the reaction. The heat of the reaction raises the temperature of the water. The heat of the reaction is transferred through the walls of the reaction chamber to the water in the outer chambers.

Lesson 2

Calorimetry and Hess's Law

How does a cube of ice melting in a glass of water in a warm room demonstrate the law of entropy?

Heat flows from the water to melt the ice and increases the entropy of the system.

______________ law states that the overall enthalpy change in a reaction is the same whether it occurs in one step or a series of steps.

Hess's

How does the law of entropy apply to chemical reactions?

If there is a difference in the amount of energy between the reactants and the products, the reaction will proceed in the direction that produces the highest entropy.

Hess's law can be applied in the following rules:

Known enthalpy changes can be combined to calculate unknown enthalpies of reaction. To find the enthalpy of formation of a compound, you can use the enthalpy changes of reactions that involve the elements that make up the compound, even if those reactions are not necessarily intermediate steps in the production of that compound. Thermochemical equations may be reversed, if necessary, to calculate unknown enthalpies, as long as the sign of the enthalpy value also changes. If a reaction contains intermediate steps, the overall enthalpy change of the reaction is equal to the sum of the enthalpy changes of the intermediate steps.

Which statement is true about the thermochemical equation below: 2H2(g) + O2(g) → 2H2O(g) + 285.83 kJ

The reaction is exothermic and releases 285.83 kJ of heat.

Which of the following statements would be true about a system in which the total enthalpy of the reactants is greater than the enthalpy of the products?

The ΔHr0 would be negative and the reaction would be exothermic.

The entropy of a substance will never be zero because:

a value of S=0 could only occur if molecular motion completely stopped at a temperature of absolute zero

An apparatus used to measure changes in enthalpy is a __________.

calorimeter

The measurement of enthalpy changes caused by chemical reactions, changes of state, or the formation of a solution is called ___________.

calorimetry

A bomb calorimeter is used to measure the enthalpy of ______.

combustion

The standard molar enthalpy of _______________ is the amount of heat transferred during the complete combustion of one mole of a reactant in its standard state.

combustion

reactions: a substance burns in oxygen

combustion

The first law of thermodynamics, also called the law of ________ of energy, states that the total amount of energy in a thermodynamic system remains constant.

conservation

Changes in enthalpy demonstrate the law of _________ because each loss of energy in one place results in an equal gain of energy in another place.

conservation of energy

To the chemist, the law of _______ is interpreted as the amount of energy lost by a chemical reaction is equal to the amount of energy gained.

conservation of energy

_________ reactions: a single compound undergoes a chemical reaction to produce two or more component substances

decomposition

Physical processes that chemical thermodynamics study are _________ and changes of _________.

dissolution, state

The total amount of energy in a thermodynamic system is called ___________.

enthalpy

Another way to describe __________ is the measure of disorder in a system.

entropy

The second law of thermodynamics, also called the law of ________, states that for any thermodynamic system not in thermodynamic equilibrium, energy will move to increase entropy.

entropy

______________, which is a measure of the energy that is unavailable to perform work, is also the measure of randomness, or disorder, of a thermodynamic system.

entropy

The standard molar enthalpy of _______________ is the amount of energy that is transferred when one mole of a compound is formed from its component elements in their standard states.

formation

Thermochemistry can help a chemist do the following: a. tell how much ________ is transferred in chemical reactions and physical changes. b. predict whether certain substances will ______ with each other. c. predict the _______ of reactions. d. predict what kinds of _________ will occur.

heat, react, rates, reactions

The standard molar enthalpy of formation of an unstable compound is _____.

positive

The amount of heat transferred when the number of moles of reactants specified in a chemical equation is completely converted into products in their standard states is called the standard molar enthalpy of ________________.

reaction

_________ reactions: a substance ionizes and recombines to become another substance

single-displacement & double-displacement

The ___________ state of a substance is the state (solid, liquid, or gas) of that substance at room temperature (25°C or 298 K) and a pressure of 100 kPa.

standard

The result of addition of one or more terms is a ______________.

summation

_________ reactions: two or more substances combine to form a new compound

synthesis

A(n) ______________ equation is a balanced chemical equation that includes the amount of heat energy involved in a chemical reaction.

thermochemical

The branch of chemistry that studies the heat transfers that accompany chemical or physical changes is ________________.

thermochemistry

A precisely defined portion of the universe that is being studied according to the amount of energy that is being transferred is a(n) _______________ system, which also refers to the reactants, products, and chemical processes that accompany a chemical reaction.

thermodynamic

In thermochemistry, a ___________ refers to the reactants, products, and chemical processes that accompany a chemical reaction.

thermodynamic system

The study of the relationship between heat and work within chemical reactions or physical changes of state is called chemical ________________.

thermodynamics

The enthalpy of formation of an uncombined element is __________.

zero

Find the ΔHr0 for the reaction of one mole of solid magnesium carbonate that decomposes into one mole of solid magnesium oxide and one mole of carbon dioxide gas. MgCO3(s)→MgO(s) + CO2(g)

ΔHr0=+117.0kJ