All chemical reactions involve energy Exothermic reactions release energy

Energy will appear on the product side of the equation

Ex. Fast freddie Endothermic reactions require energy to

be put into the reaction Energy will appear on the reaction side of the

equation Lab/ demo

How do you measure heat in an reaction?

Calorimeter If a reaction is exothermic, heat is

released and the temperature of the calorimeter will increase

If a reaction is endothermic, heat is absorbed and the temperature of the calorimeter will decrease

Specific heat (c)- is the amount of energy required to raise the temperature of a 1g substance by 1oC

Water’s specific heat is 4.184 J/goC See chart for other values

Q= mc T Q= change in heat; in kJ M= mass of substance, in grams T= temperature; in oC c= specific heat (find in table)

By convention an increase in temp. is positive where a decrease is negative

Therefore a change from 50oC to 40oC will be a change of -10oC

2,000.0 g of water in calorimeter has its temperature raised 3.0°C by an exothermic chemical reaction. How much heat was transferred?

Q = mcΔT note: since the temperature increased,

ΔT will be a positive number = (2000.0 g) × 4.184 J g ·°C ×3.0°C  =25,000 J or

25 kJ There was a net release of 25 kJ of energy

released by the reaction.

A 1000.0-g mass of water whose temp was 50°C lost 33,600 J of heat over a 5-min period. What was the temperature of the water at the end of the 5-min period?

This time, we know the value of Q and are asked to find the final temperature. We will be able to calculate final temperature once we find the change in temperature, ΔT.

Q = mcΔT ΔT =  Q/ m × c;  ΔT =  -33,600 J

/1000.00 g× 4.184 J /(g ·°C)=-8°C During the reaction, the temperature

decreased 8°C. Therefore, the final temperature was 42°C.

(50 - 8 = 42°C)

Specific heat assignment Calorimetry lab

Whenever a chemical reaction occurs, there is always a change in energy. Where does this energy come from?

Energy is stored in the chemical bonds that hold atoms together. During a chemical change, these bonds are rearranged – (broken, and formed.)

Energy is requiredto break chemical bonds

Energy is releasedwhen new bonds form

Almost all chemical reactions involve bonds that are both broken and formed.

An equation shows the net difference in energy change.

During exothermic reactions there is a net release of energy.

Consider what happens when hydrogen and fluorine gas combine to produce hydrogen fluoride:

As atoms rearrange, both H—H and F—F bonds are broken while H—F bonds form.  In this reaction, more energy is released when HF bonds form than is needed to break bonds. The net difference is a release of 546 kJ of energy so the energy term is on the product side of the equation:

H2(g) + F2(g) → 2 HF(g) + 546 kJ

To help you understand the concept of net energy change consider this analogy:

Say you have a lemonade stand. On Day 1, you had to spend $8.00 to buy

your supplies - lemonade, sugar, cups. At the end of the day you had $10.00. What is your profit, or net difference for the day? You had a profit of $2.00. This would be like an exothermic reaction - there is more money at the end of the day than was initially put in.

On Day 2, the supplies cost $15.00, but you only sold $10.00 worth of lemonade. Not such a good day - a net loss of $5.00. This is like an endothermic reaction - more was put in then was gained at the end.

Assignment 1.1.1