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