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  • Chemical reactions >  Chemical reactions
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      Chemical reactions
      Chemical reactions
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      What is a chemical reaction?

      I will know

      • How to identify a chemical reaction.
      • What a chemical reaction is, how it takes place and a theory that explains why.
      • How we can increase the speed of a chemical reaction.

      A chemical reaction is a chemical change. That is, a change in which substances disappear and new substances appear. For instance, when butane is burned, butane disappears and carbon dioxide and water appear.

      Often when a chemical reaction occurs, a noticeable change takes place indicating that something new is being formed. Look at what happens during these reactions:

      A bright light is emitted
      Heat is emitted
      Gases are released
      Coloured smoke forms
      A solid forms
      A solid dissolves

      When a strip of magnesium (Mg) is burned, it reacts with the oxygen (O2) in the air and is converted into a powdery substance called magnesium oxide, MgO. A bright light is emitted during this reaction. 

      Pour some hydrochloric acid (HCI) into a beaker containing sodium hydroxide (NaOH). Place a thermometer in the beaker and observe how the temperature increases.

      When you add a strong acid to some metals, they decompose and release hydrogen gas, H2.

      Place a beaker containing an aqueous ammonia solution, NH3, next to another beaker containing a concentrated hydrochloric acid solution, HCI. Ammonium chloride will form, which you will observe as white smoke appearing in the air.

      A transparent solution of lead(II) nitrate, Pb(NO3)2, is mixed with a transparent potassium iodide solution, KI. After a while, a yellow solid forms at the bottom of the container. This is lead(III) iodide, PbI2.

      Marble is calcium carbonate, CaCO3. If you pour acid onto marble, you will see fizzing bubbles which eventually disappear. In this reaction the solid has dissolved, releasing a gas, carbon dioxide, CO2.

       

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      Chemical reactions
      Chemical reactions
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      A solid forms

      Evidence will show that a chemical reaction has taken place. For example, in the following video, a solid is formed as a result of a chemical reaction.

      /useruploads/ctx/a/218785110/r/s/73516966/ESPM000305783.mp4?idcurso=3445286
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      PAY ATTENTION

      • Describe what you see.
      • What evidence is there that a chemical reaction occurs?
      Chemical reactions
      Chemical reactions
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      Burning iron

      Watch the following video of a chemical reaction that affects iron wool. 

      What is the evidence showing a chemical reaction has occurred?

      /useruploads/ctx/a/218785110/r/s/73519402/ESPM000305784.mp4?idcurso=3445286
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      Chemical reactions
      Chemical reactions
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      Formation of a gas

      Watch the following video. How do we know that a chemical reaction has occurred?

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      Chemical reactions
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      Formation of carbon dioxide

      The following video shows a chemical reaction in which carbon dioxide (CO2) is formed.  

       

      /useruploads/ctx/a/218785110/r/s/48199254/ESPM000305786.mp4?idcurso=3445286
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      Chemical reactions
      Chemical reactions
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      How are chemical reactions produced?

      In a chemical reaction, one set of substances is transformed into a new set of substances.

      The substances that react are called the reactants.

      The new ones are called the products.

       

      Reactions can occur with any type of substance, but for now we will focus on what happens to molecular substances. Crystals react in a similar way.

      Look at the diagram below showing the reaction between chlorine gas and hydrogen gas to form hydrogen chloride:

      estilo tamaño 26px pila pila Cl subíndice 2 más normal H subíndice 2 con llave horizontal de abajo debajo con Reactants debajo flecha derecha espacio pila pila 2 HCl con llave horizontal de abajo debajo con Products debajo fin estilo

      In order to form the H-Cl molecule, the Cl-Cl and H-H molecules must first be broken apart. This will only happen if the Cl2 and H2 molecules collide with enough energy and in the right orientation to make the collision effective.

      In a chemical reaction the molecules of the reactants are destroyed, but their atoms are not. They rearrange themselves to form new molecules. For this to happen the molecules first have to collide with each other.

      Chemical reactions
      Chemical reactions
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      The theory of chemical reactions

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      According to collision theory, chemical reactions occur when the particles of the reactants collide with each other and are broken. The loose atoms rearrange themselves, forming the new molecules.

       

      But not all collisions break the molecules apart; a collision is only effective if the molecules collide in the right orientation and with sufficient speed to be able to break the molecules of the reactants apart and form the products.

      PAY ATTENTION

      The speed of the particles is a very important factor in determining whether a reaction occurs or not. The greater the speed, the greater the kinetic energy. Therefore, high-speed collisions are more effective.

       

      Chemical reactions
      Chemical reactions
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      How can we increase the speed of a reaction?

      We can increase the speed of a reaction by:

      Increasing the temperature
      subtitulo

      The higher the temperature, the faster the particles will move.

      Splitting the substance into smaller pieces
      subtitulo

      Solid reactants react at a faster rate when the particle size is smaller, because the contact surface is greater. Similarly, a solid will react faster when it is in a solution because the particles that form the solid have already separated.

      The speed of a reaction and pressure
      subtitulo

      In a gas, the higher the pressure, the greater the speed of the reaction. This is because the particles will be much closer to each another and therefore more collisions will occur.

       

      t7-multimedia
      Chemical reactions
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      What changes and what is conserved in a reaction?

      In a chemical reaction, the atoms of the different elements are not lost, they just rearrange themselves in a different way.

      Look at what happens when hydrogen gas and oxygen gas react to form water. We can see from the formula for water that for this reaction to be possible, two hydrogen molecules must combine with one oxygen molecule to form two water molecules:

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      Chemical reactions
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      Water formation reaction

      t9-multimedia
      Reactants: 2 H2 (g) + O2 (g) Products:   2 H2O (g)

      2 hydrogen molecules, each with 2 atoms.

      4 H atoms in total.

      2 water molecules, each with 2 hydrogen atoms.

      4 H atoms in total.

      1 oxygen molecule, with 2 atoms.

      2 O atoms in total.

      2 water molecules, each with 1 oxygen atom.

      2 O atoms in total
      3 molecules in total  (2 H2 + 1 O2). 2 molecules in total (2 molecules H2O).
      The mass in the reactants is the mass of 4 H atoms and the mass of 2 O atoms. The mass of the products is the mass of 4 H atoms and the mass of 2 O atoms.

       

      In a chemical reaction:

      What is conserved What changes
      •  The atoms involved.
      •  The mass.
      • The chemical formulas of the substances.
      • In many cases, the number of molecules.

       

      Chemical reactions
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      Law of conservation of mass

      In 1772, the French chemist Antoine-Laurent Lavoisier established the law of conservation of mass, following a series of experiments.

      In a chemical reaction, matter is neither created nor destroyed, only transformed. Therefore, the total mass remains the same.

      The total mass of the reactants is equal to the total mass of the products.

       

      Antoine-Laurent Lavoisier (1743-1794)

      Lavoisier is considered to be the founder of modern chemistry. He systemised the experimental method and the use of scales in all experimentation. He is known for having said: "Nothing is created, nothing is destroyed, everything transforms".

       

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      Chemical reactions
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      Activities

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      Chemical reactions
      Chemical reactions
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      What is a chemical reaction?

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      Indicate whether the following statements about chemical reactions are true or false.

      • A chemical reaction is any change in matter.
      • Correct answer
        Wrong answer
      • In a chemical reaction, reactants disappear and products appear.
      • Correct answer
        Wrong answer
      • In a chemical reaction, products are added to reactants.
      • Correct answer
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        1 attempt
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      Chemical reactions
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      Reactants and products

      0
      t13-multimedia

      Identify the reactants and products in the following reaction: "Ammonia can be obtained by the reaction of nitrogen gas and hydrogen gas".

      • Ammonia is a .
      • Nitrogen is a .
      • Hydrogen is a .
        product reactant

        3 attempts
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      Collision theory

      0

      According to collision theory, what is necessary for a reaction to occur?

      • It is only necessary that the molecules of the reactants collide with each other.

      • Correct answer
        Wrong answer

      • It is enough that the molecules of the reactants collide in the right orientation.

      • Correct answer
        Wrong answer

      • The bonds that unite the atoms in the reactants must be destroyed and new bonds must be formed.

      • Correct answer
        Wrong answer

      • The substances forming the reactants must be at a high temperature.

      • Correct answer
        Wrong answer
      t14-multimedia

        3 attempts
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      Chemical reactions
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      Changing the speed of a reaction

      0

      Classify the following actions according to whether they increase or decrease the speed of a chemical reaction.

      • Decreasing the temperature.

      • Splitting the substances to be reacted into small pieces.

      • If the reactants are gases, putting them in a larger container.

      • If one of the reactants is oxygen, blowing in a blast of air.

      Increases the speed of reaction

        Increases the speed of reaction

        Decreases the speed of reaction

          Decreases the speed of reaction

            /*%%SmartyNocache:50663232568301b1ea90e40_45575157%%*/smarty->registered_plugins[Smarty::PLUGIN_FUNCTION]['textweb'][0], array( array('name'=>"slide_classify_initial_group",'value'=>"Ninguno",'value_en'=>"Reset"),$_smarty_tpl ) );?> /*/%%SmartyNocache:50663232568301b1ea90e40_45575157%%*/ Increases the speed of reaction Decreases the speed of reaction

            3 attempts
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          Chemical reactions

          0

          Mark the correct option or options.

          In any chemical reaction, what always remains constant is...

          • the number of molecules.

          • Correct answer
            Wrong answer

          • the number of atoms.

          • Correct answer
            Wrong answer

          • the mass.

          • Correct answer
            Wrong answer

          • the volume.

          • Correct answer
            Wrong answer
          t16-multimedia

            3 attempts
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          Chemical reactions
          Chemical reactions
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          Conducting an experiment at home

          P

          Gather the necessary materials and carry out the following experiment at home:

          1. Put some sodium bicarbonate in a bowl.
          2. Add a generous splash of vinegar.

          What happens? Do some research and write a brief explanation. Attach a picture of your experiment.

          My files
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            1 attempt
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          Chemical reactions
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          What is a chemical reaction?

          0

          Mark the correct answer in each case.

          • One of these processes is NOT a chemical change. Which one?
          • Correct answer
            Wrong answer
          • The speed of a chemical reaction may vary if we modify one of these factors. Which one?
          • Correct answer
            Wrong answer
          • Which of the following magnitudes is conserved in a chemical reaction?
          • Correct answer
            Wrong answer
          • It always changes in a chemical reaction.
          • Correct answer
            Wrong answer
          • According to the collision theory, for a collision to be effective, the molecules must have:
          • Correct answer
            Wrong answer

            3 attempts
          Done
          Chemical reactions
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          Now I know

          t19-multimedia
          How do we know that a chemical reaction has occurred?
          subtitulo
          • A bright light is emitted.
          • Heat is emitted.
          • Gases are released.
          • Coloured smoke forms.
          • A solid forms.
          • A solid dissolves.
          The theory of chemical reactions
          subtitulo
          • In a chemical reaction, one set of substances is transformed into a new set of substances. The substances that react are called the reactants and the new ones are called the products.
          • According to collision theory, chemical reactions occur when the particles of the reactants collide with each other and are broken. The loose atoms rearrange themselves, forming the new molecules.
          • But not all collisions break the molecules apart; a collision is only effective if the molecules collide in the right orientation and with sufficient speed to be able to break the molecules of the reactants apart and form the products.
          What changes and what is conserved in a reaction
          subtitulo

          In a chemical reaction:

           

          What is conserved

          What changes
          •  The atoms involved.
          •  The mass
          • The chemical formulas of the substances.
          • In many cases, the number of molecules
          Law of conservation of mass or Lavoisier’s law
          subtitulo

          In a chemical reaction, matter is neither created nor destroyed, only transformed. Therefore, the total mass remains the same. The total mass of the reactants is equal to the total mass of the products.

           

           

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