Chem Unit 6 Chemical Reactions

unit for energy

joules, J, are the standard unit for energy in chemistry. KJ is used to show the energy change in a reaction.

1 Kj = 1000 J

heat reaction (enthalpy change)

the amount of heat absorbed or released during a reaction that occurs at constant pressure.

• a change of energy occurs as reactants interact, bonds break apart, and when products form

• we determine the heat of a reaction, ∆H, as the difference in the energy of the products and the reactants.

∆H= H_products - H_reactants

exothermic reactions

when heat is released alongside the formed products. meaning the energy of the products is lower than that of the reactants.

• the heat of reaction can be written as one of the products, but it can also

  be written as a ∆H value with a negative sign (-).

endothermic reactions

when heat is required to convert the reactants to products; meaning the energy of the products is higher than that of the reactants.

• the heat of reaction can be written as one of the reactants, and it can also be written as a ∆H value with a positive sign (+).

hess’s law

heat can be absorbed or released in a single chemical reaction or in several steps.

when there are two or more steps in the reaction, the overall enthalpy change is the sum of the enthalpy changes of those steps, provided they all occur at the same temperature.

1. if you reverse a chemical equation, you must also reverse the sign of ∆H.

2. If a chemical equation is multiplied by some factor, then ∆H must be multiplied by

   the same factor.

guide to hess’s law

STEP 1

arrange the given equations to

place reactants on the left and

products on the right.

STEP 2

if an equation is multiplied to

balance coefficients, multiply

the ¢H by the same number.

STEP 3

combine the equations and

cancel any substances that are

common to both sides. Add

the ¢Hs.

how do you go from grams to moles?

g —> mols = g / molar mass

limiting reactants vs. excess reactant

the reactant that is completely used up is the limiting reactant. the reactant that does not completely react and is left over is called the excess reactant.

actual yield

The actual amount of product produced by a reaction.

heat of reaction

The heat (symbol AH) absorbed or released when a

reaction takes place at constant pressure.

limiting reactant

The reactant used up during a chemical reaction, which limits the amount of product that can form

law of conservation of mass

In a chemical reaction, the total mass of the reactants is equal to the total mass of the products; matter is neither lost nor gained

mole-mole factor

A conversion factor that relates the number of

moles of two compounds in an equation derived from their coefficients.

percent yield

The ratio of the actual yield for a reaction to the

theoretical yield possible for the reaction.

theoretical yield

The maximum amount of product that a reaction

can produce from a given amount of reactant.