# Method 1 put the thermometer through the plastic lid

This preview shows pages 271–273. Sign up to view the full content.

Method: 1. Put the thermometer through the plastic lid, cover the beaker and record the temperature in the empty beaker. You will need to leave the thermometer in the beaker for about 5 minutes in order to get an accurate reading. 2. Take the thermometer out of the jar. 3. Soak a piece of steel wool in vinegar for about a minute. The vinegar removes the protective coating from the steel wool so that the metal is exposed to oxygen. 4. After the steel wool has been in the vinegar, remove it and squeeze out any vinegar that is still on the wool. Wrap the steel wool around the thermometer and place it (still wrapped round the thermometer) back into the jar. The jar is automatically sealed when you do this because the thermometer is through the top of the lid. 5. Leave the steel wool in the beaker for about 5 minutes and then record the temperature. Record your observations. Results: You should notice that the temperature increases when the steel wool is wrapped around the thermometer. Conclusion: The reaction between oxygen and the exposed metal in the steel wool, is exother- mic , which means that energy is released and the temperature increases. 14.3 The heat of reaction The heat of the reaction is represented by the symbol Δ H , where: Δ H = E prod E react In an exothermic reaction, Δ H is less than zero because the energy of the reactants is greater than the energy of the product. For example, H 2 + Cl 2 2 HCl ΔH = -183 kJ In an endothermic reaction, Δ H is greater than zero because the energy of the reactants is less than the energy of the product. For example, C + H 2 O CO + H 2 ΔH = +131 kJ Some of the information relating to exothermic and endothermic reactions is summarised in table 14.1. 257

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
14.3 CHAPTER 14. ENERGY CHANGES IN CHEMICAL REACTIONS - GRADE 11 Table 14.1: A comparison of exothermic and endothermic reactions Type of reaction Exothermic Endothermic Energy absorbed or re- leased Released Absorbed Relative energy of reac- tants and products Energy of reactants greater than energy of product Energy of reactants less than energy of product Sign of Δ H Negative Positive Definition: Enthalpy Enthalpy is the heat content of a chemical system, and is given the symbol ’H’. Important: Writing equations using Δ H There are two ways to write the heat of the reaction in an equation For the exothermic reaction C ( s ) + O 2 ( g ) CO 2 ( g ), we can write: C ( s ) + O 2 ( g ) CO 2 ( g ) ΔH = -393 kJ.mol 1 or C ( s ) + O 2 ( g ) CO 2 ( g ) + 393 kJ.mol 1 For the endothermic reaction, C ( s ) + H 2 O ( g ) H 2 ( g ) + CO ( g ), we can write: C ( s ) + H 2 O ( g ) H 2 ( g ) + CO ( g ) ΔH = +131 kJ.mol 1 or C ( s ) + H 2 O ( g ) + 131 kJ.mol 1 CO + H 2 The units for ΔH are kJ.mol 1 . In other words, the ΔH value gives the amount of energy that is absorbed or released per mole of product that is formed. Units can also be written as kJ, which then gives the total amount of energy that is released or absorbed when the product forms.
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern