<< Chapter < Page Chapter >> Page >

Interesting fact

Lightsticks or glowsticks are used by divers, campers, and for decoration and fun. A lightstick is a plastic tube with a glass vial inside it. To activate a lightstick, you bend the plastic stick, which breaks the glass vial. This allows the chemicals that are inside the glass to mix with the chemicals in the plastic tube. These two chemicals react and release energy. Another part of a lightstick is a fluorescent dye which changes this energy into light, causing the lightstick to glow!

Endothermic and exothermic reactions

  1. In each of the following reactions, say whether the reaction is endothermic or exothermic, and give a reason for your answer.
    1. H 2 + I 2 2 H I + 21 k J . m o l - 1
    2. C H 4 + 2 O 2 C O 2 + 2 H 2 O Δ H = -802 kJ.mol - 1
    3. The following reaction takes place in a flask: B a ( O H ) 2 . 8 H 2 O + 2 N H 4 N O 3 B a ( N O 3 ) 2 + 2 N H 3 + 10 H 2 O Within a few minutes, the temperature of the flask drops by approximately 20C.
    4. 2 N a + C l 2 2 N a C l Δ H = -411 kJ
    5. C + O 2 C O 2
  2. For each of the following descriptions, say whether the process is endothermic or exothermic and give a reason for your answer.
    1. evaporation
    2. the combustion reaction in a car engine
    3. bomb explosions
    4. melting ice
    5. digestion of food
    6. condensation

Spontaneous and non-spontaneous reactions

Demonstration : spontaneous and non-spontaneous reactions

Apparatus and materials:

A length of magnesium ribbon, thick copper wire and a bunsen burner.

Method:

  1. Scrape the length of magnesium ribbon and copper wire clean.
  2. Heat each piece of metal over the bunsen burner, in a non-luminous flame. Do Not look directly at the flame. Observe whether any chemical reaction takes place.
  3. Remove the metals from the flame and observe whether the reaction stops. If the reaction stops, return the metal to the bunsen flame and continue to heat it.

Results:

  • Did any reaction take place before the metals were heated?
  • Did either of the reactions continue after they were removed from the flame?
  • Write a balanced equation for each of the chemical reactions that takes place.

In the demonstration above, the reaction between magnesium and oxygen, and the reaction between copper and oxygen are both non-spontaneous . Before the metals were held over the bunsen burner, no reaction was observed. They need energy to initiate the reaction. After the reaction has started, it may then carry on spontaneously. This is what happened when the magnesium reacted with oxygen. Even after the magnesium was removed from the flame, the reaction continued. Other reactions will not carry on unless there is a constant addition of energy. This was the case when copper reacted with oxygen. As soon as the copper was removed from the flame, the reaction stopped.

Now try carefuly adding a solution of dilute sulfuric acid to a solution of sodium hydroxide. What do you observe? This is an example of a spontaneous reaction because the reaction takes place without any energy being added.

Spontaneous reaction

A spontaneous reaction is a physical or chemical change that occurs without the addition of energy.

Activation energy and the activated complex

From the demonstrations of spontaneous and non-spontaneous reactions, it should be clear that most reactions will not take place until the system has some minimum amount of energy added to it. This energy is called the activation energy . Activation energy is the 'threshold energy' or the energy that must be overcome in order for a chemical reaction to occur.

Questions & Answers

if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand
Syamthanda Reply
hey , can you please explain oxidation reaction & redox ?
Boitumelo Reply
hey , can you please explain oxidation reaction and redox ?
Boitumelo
for grade 12 or grade 11?
Sibulele
the value of V1 and V2
Tumelo Reply
advantages of electrons in a circuit
Rethabile Reply
we're do you find electromagnetism past papers
Ntombifuthi
what a normal force
Tholulwazi Reply
it is the force or component of the force that the surface exert on an object incontact with it and which acts perpendicular to the surface
Sihle
what is physics?
Petrus Reply
what is the half reaction of Potassium and chlorine
Anna Reply
how to calculate coefficient of static friction
Lisa Reply
how to calculate static friction
Lisa
How to calculate a current
Tumelo
how to calculate the magnitude of horizontal component of the applied force
Mogano
How to calculate force
Monambi
a structure of a thermocouple used to measure inner temperature
Anna Reply
a fixed gas of a mass is held at standard pressure temperature of 15 degrees Celsius .Calculate the temperature of the gas in Celsius if the pressure is changed to 2×10 to the power 4
Amahle Reply
How is energy being used in bonding?
Raymond Reply
what is acceleration
Syamthanda Reply
a rate of change in velocity of an object whith respect to time
Khuthadzo
how can we find the moment of torque of a circular object
Kidist
Acceleration is a rate of change in velocity.
Justice
t =r×f
Khuthadzo
how to calculate tension by substitution
Precious Reply
hi
Shongi
hi
Leago
use fnet method. how many obects are being calculated ?
Khuthadzo
khuthadzo hii
Hulisani
how to calculate acceleration and tension force
Lungile Reply
you use Fnet equals ma , newtoms second law formula
Masego
please help me with vectors in two dimensions
Mulaudzi Reply
how to calculate normal force
Mulaudzi

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 11 physical science' conversation and receive update notifications?

Ask