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Tutor profile: Sarah U.

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Sarah U.
Experienced Chemistry and English Tutor
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Questions

Subject: Writing

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Question:

What makes a great thesis statement?

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Sarah U.
Answer:

A thesis statement is a claim made early in an essay, usually in the introduction, which outlines the main objective of the essay. A good thesis statement has several requirements. It must be recognizable to the reader as the thesis statement and directly answer the essay question. A good thesis statement is comprised of three distinct parts: the topic, the position, and the support. A great thesis statement will be insightful and show critical thinking. It may do that in a variety of ways, such as addressing an competing view or showing a larger understanding of themes, symbolism or character development. Every line in the essay must pertain in some way to the thesis statement, so it is arguably the most important line(s) in the essay.

Subject: Chemistry

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Question:

Which of the following substitution/redox reactions with halogens (F, Cl, Br, I) can take place? 1. NaI(aq) + Cl2(g) 2. NaF(aq) + Cl2(g) 3. NaCl(aq) + Br2(aq) 4. NaF(aq)+Br2(aq)

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Sarah U.
Answer:

The reactions you're looking at are between halogens (F, Cl, Br, I), which exist primarily in their diatomic forms (F2, Cl2, Br2, and I2) and halide ions (F-, Cl-, Br-, and I-). To determine which halogen reacts with which halide ion, you need to look at the periodic table. Electronegativity increases to F. Therefore: F > Cl> Br> I electronegativity The higher the electronegativity, the stronger the oxidizing power of oxidizing agents. Therefore: F2> Cl2> Br2> I2 oxidizing agent strength The oxidizing agent gains electrons (is reduced) in the reaction to produce its reduced form in the products. The reduced form is a reducing agent which can lose electrons (be oxidized). The strength of the corresponding reducing agents is opposite to oxidizing strength. Therefore: I- > Br- > Cl- > F- reducing agent strength For a reaction to proceed, the oxidizing agent and reducing agent in the reactants (left side of equation) must be stronger than the oxidizing and reducing agents they form in the products (right side of equation). Let's look at the reactions provided to compare oxidizing and reducing agents. 1. For NaI (aq) + Cl2 (g) If the reaction is possible, the balanced equation would be: 2NaI (aq) + Cl2 (g) -> I2 (aq) + 2NaCl (aq) I- is the reducing agent in reactants which forms I2, the oxidizing agent in the products. Cl2 is the oxidizing agent in the reactants which forms Cl-, the reducing agent in the products. I- > Cl- reducing agent strength Cl2 > I2 oxidizing agent strength The reactant agents are stronger. This reaction WILL proceed as follows: 2NaI (aq) + Cl2 (g) -> I2 (aq) + 2NaCl (aq) 2. For NaF(aq)+Cl2(g) If the reaction is possible, the balanced equation would be: 2NaF (aq) + Cl2 (g) -> F2 (aq) + 2NaCl (aq) F- is the reducing agent in reactants which forms F2, the oxidizing agent in the products. Cl2 is the oxidizing agent in the reactants which forms Cl-, the reducing agent in the products. F- < Cl- reducing agent strength Cl2< F2 oxidizing agent strength The agents are stronger in the products, so this reaction WILL NOT proceed. 3. NaCl(aq)+Br2(aq) If the reaction is possible, the balanced equation would be: 2NaCl (aq) + Br2 (aq) -> Cl2 (g) + 2NaBr (aq) Cl- is the reducing agent in reactants which forms Cl2, the oxidizing agent in the products. Br2 is the oxidizing agent in the reactants which forms Br-, the reducing agent in the products. Cl- < Br- reducing agent strength Br2 < Cl2 oxidizing agent strength The agents are stronger in the products, so this reaction WILL NOT proceed. 4. For NaF(aq)+Br2(aq) If the reaction is possible, the balanced equation would be: 2NaF (aq) + Br2 (aq) -> F2 (g) + 2NaBr (aq) F- is the reducing agent in reactants which forms F2, the oxidizing agent in the products. Br2 is the oxidizing agent in the reactants which forms Br-, the reducing agent in the products. F- < Br- reducing agent strength Br2< F2 oxidizing agent strength The agents are stronger in the products, so this reaction WILL NOT proceed. These reactions are known more as redox reactions than as substitution reactions. But, as one atom displaces another you can interpret it as substitution.

Subject: Algebra

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Question:

Jill stops at a bake sale to buy some goodies. It's late and they only have brownies and cookies left. The brownies are $2.00 each and the cookies are $1.75 each. She ends up spending $9.25 and buys 5 items. How many brownies and cookies does Jill buy?

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Sarah U.
Answer:

In order to solve this problem, you need to translate this word problem into math equations. We are told Jill buys some number of brownies at $2.00 each and some number of cookies at $1.75 each and she spends $9.25 total. If we make the number of brownies equal to x and the number of cookies equal to y, we can make the following equation: 2.00x + 1.75y = 9.25 We are also told that the number of brownies (x) and the number of cookies (y) equals 5 total items. So, we can make the following equation: x + y = 5 We can use substitution to solve this problem, because we have 2 unknowns (x and y) and two equations: 2x + 1.75y = 9.25 x + y = 5 The way you do this is to solve for one variable in one equation and substitute that variable in the other equation. Because it is easier to solve for one variable in the second equation, I will manipulate the second equation: x+ y = 5, subtract y from both sides x= 5- y Now, we can put that expression (5-y) in our first equation to replace x. 2x +1.75y=9.25 2(5-y) + 1.75y = 9.25, distribute the 2 10 - 2y +1.75y = 9.25, combine like variables 10 - 0.25y = 9.25, subtract 10 form both sides -0.25y = -0.75, divide both sides by -0.25 y= 3 Jill bought 3 cookies. To find the value for x, the number of brownies, we have to substitute the value of y into either equation. The second equation is easier, so we can use that one. x+ y = 5, substitute 3 for y x+ 3 = 5, subtract 3 from both sides x=2 Jill bought 2 brownies. The answer is Jill bought 3 cookies and 2 brownies.

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