CHEMISTRY Lecture Notes - Chemical Change, Stopwatch, Hydrogen Peroxide

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astrodabber

11 Mar 2023

School

Delhi Technological University

Department

Chemistry

Course

CHEMISTRY

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Physical changes (such as melting or evaporating) do not produce any new chemical substances. These changes are often easy to reverse and mixtures produced are usually relatively easy to separate. In chemical reactions, new chemical products are formed that have very different properties to the reactants. Most chemical reactions are impossible to reverse. Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic). The majority of chemical reactions are exothermic with only a small number being endothermic. Increase in the concentration of a solution, the rate of reaction will increase. This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second, increasing the rate of reaction. Increasing the concentration means there are more particles per cm3, so there is less space between the particles. Since there are more particles then it follows that there are more collisions, hence the rate of reaction increases.

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Chemical Reactions and Equations Learning objectives To observe physical and chemical properties associated with chemical changes To be able to give evidence for the occurrence of a chemical reaction To write a balanced chemical equation To identify areaction as a combination, decomposition, precipitation, single or double replacement, or combustion reaction ntroduction When a substance undergoes a physical change, it changes its appearance but not its composition. For example, when ice melts to form water, it undergoes a physical change from solid to liquid. In a chemical change, the composition of a substance is changed to give a new substance with different physical and chemical properties. For example, when silver tarnishes, the shiny silver (Ag) changes to a dull gray silver sulfide (AgaS). Not only has the chemical formula changed from Ag to Agus, but also the physical properties (shininess) of the substance have changed. Some examples of physical and chemical changes are listed in Table 1 below. Table 1. Comparison of Physical and Chemical Changes Examples of Physical changes Examples and Evidence of Chemical Changes Change in state (melting, freezing, evaporating Formation of a gas (bubbles) subliming) Formation of a solid (precipitate) Change in size Tearing Change in Color Heat is given off or absorbed A solid dissolving in a solvent In a chemical reaction, atoms in the reactants are rearranged to produce new combinations of atoms in the products. However, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. This is true according to the Law of Conservation of Mass, which states that the total mass remains constant during a chemical reaction. In other words, matter cannot be created or destroyed in a chemical reaction. In a chemical equation, the reactants are shown on the left and the products are on the right. An arrow in between indicates that a chemical reaction occurs. Finally, the physical state of each compound is designated in parentheses after the chemical formula. Solid, liquid, gas, and aqueous (dissolved in water) are shown as(s,(n,(g), and (aq), respectively.

3. The process of water freezing to form ice at 0 degrees C isa. neither exothermic nor endothermic.b. an endothermic phase change.c. an exothermic chemical reaction.d. an exothermic phase change. e. an endothermic chemical reaction.

True or False: When a chemical reaction has reached equilibrium, the concentrations of reactants and products are equal.

True or False: When a chemical reaction has reached equilibrium, the rate of the foward reaction = the rate of the reverse reaction.

True or False: Most chemical reactions are reversible to some extent

True or False: When a chemical reaction has reached equilibrium the concentration of reactants and products remains unchanged.

CHEMISTRY Lecture Notes - Chemical Change, Stopwatch, Hydrogen Peroxide

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