Chemical Reactions and Heat
2 examples how heat is transferred from one object to another
1) cooking 2) holding hands
Fact
All combustion reactions are Exothermic
Explain why Hess's law is useful in the chemistry laboratory
Allows delta H (heat energy) to be calculated for reactions that can not be performed directly
Fact
An endothermic reaction absorbs heat from the environment
Fact
Bond forming in chemical reactions releases energy
Fact
Conditions such as temperature, pressure, and the physical states of the substances in a reaction can affect delta H
What does Hess's law say about the enthalpy change for a net reaction?
Delta H for a net reaction can be determined by summing delta H's for the individual steps
What happens to the sign of delta H if a reaction is run in the the reverse direction from the way it is written
Delta H sign reversed
How can delta H be calculated for an equation in which the coefficients have been multiplied by a factor of two?
Delta H x 2
When ammonium chloride dissolves in a beaker of water, the beaker becomes cold to touch. Explain this
Endothermic reaction. Heat is absorbed from the beaker by the reactants and the transfer of heat causes a temperature drop
Fact
Energy can be stored in the chemical bonds of a substance
Provide an analogy that explains why bond breaking requires energy
Energy is always required to break things 1) holding hands requires energy
Fact
Every substance has a specific heat which tells you how much heat is necessary to raise the temperature of 1 gram of the substance by 1 Celsius degree
Combustion
Example of an Exothermic reaction
Compare the enthalpy of the reactants and the products in both Exothermic and endothermic reactions
H of the products is less then that of the reactants for Exothermic reactions
Fact
Heat is transferred between 2 objects that are at different temperatures
Why is the joule, the SI unit for energy, also appropriate for measuring heat?
Heat=transfer of energy
Fact
If a reaction is endothermic the amount of heat appears on the left side of the arrow in the balanced equation
Fact
In an Exothermic reaction h products will always be less than h reactants
Fact
It is the energy that maintains your body temperature close to 37° C
What is meant by the standard state of an element?
Most stable form of an element under standard conditions
Endothermic reactions
Reactions that absorb heat
Exothermic reactions
Reactions that release heat
Can a reaction proceed more than one direction? Explain your answer.
Sometimes adding or removing energy can reverse a reaction
Joule
The SI unit of energy and heat
Fact
The amount of heat needed to raise an objects temperature depends on its heat capacity
Fact
The delta h for an endothermic reaction always has a positive sign
Heat
The energy that is transferred from one object to another
Fact
The enthalpy change measured at 1 atm and 25°C when the reactants and products are in their standard states is called a standard enthalpy change
Fact
The enthalpy of a substance is similar to, but not exactly the same as, the energy of a substance.
Fact
The heat absorbed or released in a reaction depends on a quantity called enthalpt
Fact
The study of heat flow and heat measurement is called Calorimetry
Thermochemistry
The study of the changes in heat in chemical reactions
Fact
The symbol delta H literally means a change in enthalpy
Fact
The words calorimeter and Calorimetry and calorie are all derived from the Latin word Calor which means Heat
Fact
Water has one of the highest specific heats of any common substance
If you were given delta H° of a reaction, could you determine whether the reaction was Exothermic or endothermic? Explain your answer
Yes +Delta H= Endothermic -Delta H= Exothermic
Fact
You must know the number of moles of reactants involved in a reaction to calculate delta H
