Chemistry_Grade_10-12 (1).pdf

Results did any reaction take place before the metals

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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 en- ergy. This was the case when copper reacted with oxygen. As soon as the copper was removed from the flame, the reaction stopped. Now try adding a solution of dilute sulfuric acid with 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. Definition: Spontaneous reaction A spontaneous reaction is a physical or chemical change that occurs without the addition of energy. 14.6 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 261
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14.6 CHAPTER 14. ENERGY CHANGES IN CHEMICAL REACTIONS - GRADE 11 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. Definition: Activation energy Activation energy or ’threshold energy’ is the energy that must be overcome in order for a chemical reaction to occur. It is possible to draw an energy diagram to show the energy changes that take place during a particular reaction. Let’s consider an example: H 2 ( g ) + F 2 ( g ) 2 HF ( g ) [H 2 F 2 ] ( activated complex) 2HF products H 2 + F 2 reactants activation energy ΔH = 268 k.J.mol 1 Time Potential energy Figure 14.1: The energy changes that take place during an exothermic reaction The reaction between H 2 ( g ) and F 2 ( g ) (figure 14.1) needs energy in order to proceed, and this is the activation energy. Once the reaction has started, an in-between, temporary state is reached where the two reactants combine to give H 2 F 2 . This state is sometimes called a transition state and the energy that is needed to reach this state is equal to the activation energy for the reaction. The compound that is formed in this transition state is called the activated complex . The transition state lasts for only a very short time, after which either the original bonds reform, or the bonds are broken and a new product forms. In this example, the final product is HF and it has a lower energy than the reactants. The reaction is exothermic and Δ H is negative.
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